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Sex Determination
Citation for published version:
Bachtrog, D, Mank, JE, Peichel, CL, Kirkpatrick, M, Otto, SP, Ashman, TL, Hahn, MW, Kitano, J, Mayrose, I,
Ming, R, Perrin, N, Ross, L, Valenzuela, N, Vamosi, JC, Mank, JE, Peichel, CL, Ashman, TL, Blackmon, H,
Goldberg, EE, Hahn, MW, Kirkpatrick, M, Kitano, J, Mayrose, I, Ming, R, Pennell, MW, Perrin, N, Ross, L,
Valenzuela, N & Vamosi, JC 2014, 'Sex Determination: Why So Many Ways of Doing It?' PLoS Biology, vol
12, no. 7, e1001899, pp. 1-13. DOI: 10.1371/journal.pbio.1001899

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10.1371/journal.pbio.1001899

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Essay

Sex Determination: Why So Many Ways of Doing It?

Doris Bachtrog1*, Judith E. Mank2, Catherine L. Peichel3, Mark Kirkpatrick4, Sarah P. Otto5,
Tia-Lynn Ashman6, Matthew W. Hahn7, Jun Kitano8, Itay Mayrose9, Ray Ming10, Nicolas Perrin11,
Laura Ross12, Nicole Valenzuela13, Jana C. Vamosi14, The Tree of Sex Consortium"
1 University of California, Berkeley, Department of Integrative Biology, Berkeley, California, United States of America, 2 University College London, Department of Genetics,
Evolution and Environment, London, United Kingdom, 3 Fred Hutchinson Cancer Research Center, Divisions of Human Biology and Basic Sciences, Seattle, Washington,
United States of America, 4 University of Texas, Department of Integrative Biology, Austin, Texas, United States of America, 5 University of British Columbia, Department of
Zoology, Vancouver, British Columbia, Canada, 6 University of Pittsburgh, Department of Biological Sciences, Pittsburgh, Pennsylvania, United States of America, 7 Indiana
University, Department of Biology, Bloomington Indiana, United States of America, 8 National Institute of Genetics, Ecological Genetics Laboratory, Mishima, Shizuoka,
Japan, 9 Tel Aviv University, Department of Molecular Biology and Ecology of Plants, Tel Aviv, Israel, 10 University of Illinois, Department of Plant Biology, Urbana-
Champaign, Illinois, United States of America, 11 University of Lausanne, Department of Ecology and Evolution, Lausanne, Switzerland, 12 University of Oxford,
Department of Zoology, Oxford, United Kingdom, 13 Iowa State University, Department of Ecology, Evolution and Organismal Biology, Ames, Iowa, United States of
America, 14 University of Calgary, Department of Biological Sciences, Calgary, Alberta, Canada

Abstract: Sexual reproduction is variance that is otherwise hidden [2]. or female depends on the presence of a
an ancient feature of life on earth, While many unicellular organisms pro- single master sex-determining locus, the Sry
and the familiar X and Y chromo- duce gametes of equal size (isogamy, see gene, on the male-limited Y chromosome.
somes in humans and other model Box 1), sexual reproduction in most Expression of Sry early in embryonic
species have led to the impression multicellular organisms has led to the development initiates testis differentiation
that sex determination mecha- evolution of female and male gametes by activating male-specific developmental
nisms are old and conserved. In differing in size (anisogamy), and often to networks, while in its absence, ovaries
fact, males and females are deter- the evolution of two separate sexes. Even develop. The first visible signs of sexual
mined by diverse mechanisms that though the outcome of sex determina- differentiation of the ovary and testis occur
evolve rapidly in many taxa. Yet tion—whether an individual produces by the sixth week of gestation in humans
this diversity in primary sex-deter- relatively few large ova or many small [6], and sex hormones initiate further sexual
mining signals is coupled with sperm—is strongly conserved, a bewilder- differentiation in nongonadal tissues and
conserved molecular pathways that ing number of underlying mechanisms can organs [7]. When this developmental pro-
trigger male or female develop- trigger development as either a male or cess goes awry, the effects can be cata-
ment. Conflicting selection on dif- female [3,4]. strophic, causing everything from ambigu-
ferent parts of the genome and on In humans, sex is determined by sex ous external genitalia (which occurs in up to
the two sexes may drive many of chromosomes (XX females, XY males). The one in 4,500 infants) to sterility (which is
these transitions, but few systems more cryptic and difficult to diagnose but
X and Y chromosomes harbor dramatically
with rapid turnover of sex determi-
different numbers and sets of genes (about may be far more common).
nation mechanisms have been rig-
orously studied. Here we survey 1,000 genes on the X and only a few dozen Like humans and most mammals, other
our current understanding of how genes on the Y), yet they originated from genetic model systems, such as Drosophila
and why sex determination evolves ordinary autosomes during the early evolu- melanogaster flies and Caenorhabditis elegans
in animals and plants and identify tion of mammals (Figure 1). Restriction of nematodes, harbor sex chromosomes, and
important gaps in our knowledge recombination followed by gene loss on the their commonalities have led to general
that present exciting research op- Y has resulted in the morphological differ- assumptions about the conservation of sex
portunities to characterize the evo- entiation of sex chromosomes (for a review determination mechanisms. However,
lutionary forces and molecular of the molecular and evolutionary processes these model organisms present a false
pathways underlying the evolution involved in Y degeneration, see [4,5]). The impression of stability in how sex is
of sex determination. vast majority of genes on the sex chromo- determined, and their commonalities
somes are not directly involved in sex mask the diversity and turnover in sex
determination, and development as a male determination mechanisms that is readily
Introduction
Citation: Bachtrog D, Mank JE, Peichel CL, Kirkpatrick M, Otto SP, et al. (2014) Sex Determination: Why So Many
Sex—the mixing of genomes via meiosis Ways of Doing It? PLoS Biol 12(7): e1001899. doi:10.1371/journal.pbio.1001899
and fusion of gametes—is nearly universal Published July 1, 2014
to eukaryotic life and encompasses a
Copyright: ß 2014 Bachtrog et al. This is an open-access article distributed under the terms of the Creative
diverse array of systems and mechanisms Commons Attribution License, which permits unrestricted use, distribution, and reproduction in any medium,
[1]. One major role of sex is to bring provided the original author and source are credited.
together alleles carried by different Funding: The Tree of Sex Consortium was funded by NESCent. The funders had no role in study design, data
individuals, revealing beneficial genetic collection and analysis, decision to publish, or preparation of the manuscript.
Competing Interests: The authors have declared that no competing interests exist.
Essays articulate a specific perspective on a topic of
broad interest to scientists. * E-mail: dbachtrog@berkeley.edu
" Membership of the Tree of Sex Consortium is provided in the Acknowledgments.

PLOS Biology | www.plosbiology.org 1 July 2014 | Volume 12 | Issue 7 | e1001899

apparent when taking a broader taxonom- [18]). Thus, sex chromosomes that are separation of male and female gonads in
ic view. In this article, we address three morphologically similar (homomorphic) the same individual, as in monoecious
common myths about sex determination must be evolutionarily young, and in time plants with separate male and female
and then deconstruct them based on a they too will degenerate. flowers (e.g., maize) and in most hermaph-
broad taxonomic survey of animals and roditic animals. Alternatively, male and
plants. The Myths Deconstructed female function can be separated in time
within an individual, as found in many
These myths do not survive a survey of plants (‘‘dichogamy,’’ [23]) and some
Myths of Sex Determination
sex determination systems across the tree animals (‘‘sequential hermaphroditism,’’
Myth 1: Sex is typically determined of life. To deconstruct these myths, we first [24]); slipper shells, for example, are born
by X and Y chromosomes provide background on the evolution of male and become female later in life.
Many biologists are habituated to think- separate sexes. We then summarize the Finally, male and female reproductive
ing about sex determination through the diversity of sex-determining mechanisms organs can be segregated into different
familiar examples of mammals and D. found among animals and plants and individuals, as in some plants (such as
melanogaster, and assume that sex determi- discuss the evolutionary forces that drive papaya and cannabis) and most animals.
nation by sex chromosomes is the norm, transitions among systems (Myth 1 revis-
that males are XY and females are XX, Separate sexes have evolved indepen-
ited). This is followed by a summary of dently many times among plants and
and that sex chromosomes are a stable more recent findings on the underlying
component of the genome. While biolo- animals, which suggests that there must
molecular genetics of sex determination be an evolutionary cost to hermaphrodit-
gists are generally aware of other modes of (Myth 2 revisited) and a deconstruction of
sex determination (such as female hetero- ism, at least in some groups. Two major
common misconceptions of sex chromo-
gamety in birds, temperature-dependent hypotheses have been proposed to explain
some evolution in humans and other
sex determination in reptiles, or develop- the evolution of separate sexes. The first
species (Myth 3 revisited). We conclude
ment of males from unfertilized eggs in hypothesis is that there are trade-offs
with an outlook for future research that
bees), these alternatives are often viewed as between male and female function, such
might improve our understanding of how
strange and aberrant [8]. as when mating displays enhance male
and why sex determination evolves so
fitness but decrease female fitness. In this
rapidly in many animals and plants.
case, individuals can gain reproductive
Myth 2: Sex is controlled by one advantages by specializing as a male or
master-switch gene The Evolution of Separate Sexes female [25]. Direct evidence for the trade-
Sex determination in model species off hypothesis is sparse [26], and observa-
While the evolution of anisogamy led to
suggests that a master-switch gene (e.g. tions consistent with it come from her-
the evolution of male and female func-
Sry in mammals, Sxl in D. melanogaster, and maphroditic great pond snails, which
tions, the evolution of separate sexes is not
xol-1 in C. elegans) acts as the main control reallocate resources to female function
inevitable across lineages. Indeed, most
element to trigger either male or female when sperm production is experimentally
flowering plants (94%, [19]) have both
sexual development. Changes in the sex abolished [27], and from strawberries, in
male and female sex organs within a single
determination pathways across taxa are which increased pollen production comes
individual and often within the same
assumed to involve adding a new master- at the cost of reduced seed set [28].
flower. By contrast, hermaphroditism is
switch gene to this molecular pathway (as Indirect evidence of a trade-off comes
rare among animals considered as a whole
in some fly taxa; [9]), with little change to from the fact that many asexual animals
(about 5% of all species), which is largely
downstream elements of the sex determi- [29] and plants [30] that still have residual
due to the absence of hermaphrodites in
nation pathway [10]. A few genes are sperm/pollen production evolve reduced
the species-rich insects, but it is common
thought to have the capacity to take on the investment in male gametes over time,
in many other animal taxa, including fish
role of sex determination genes, and these suggesting that doing so increases female
and many invertebrates (most snails,
have been co-opted as master-switch genes function. The second major hypothesis is
corals, trematodes, barnacles, and many
independently in different lineages (for that separate sexes evolve as a means to
echinoderms) [20]. Hermaphrodites can
example, dmrt1 in several vertebrates avoid self-fertilization, which can produce
mate with each other and benefit from the
[11–14] and tra in insects [15–17]). low-fitness offspring because of the expo-
advantages of sex by mixing their ge-
nomes, but when mates are difficult to sure of recessive deleterious alleles (‘‘in-
Myth 3: Sex chromosome find, hermaphrodites can also escape the breeding depression’’) [31]. Empirical
differentiation and degeneration is need for a reproductive partner by self- evidence for inbreeding depression is
inevitable fertilization (which, however, may produce widespread in animals and plants
Sex chromosomes originate from iden- low-fitness offspring due to ‘‘inbreeding [32,33]; for instance, in the Hawaiian
tical autosomes by acquiring a sex deter- depression;’’ see below). This advantage of endemic plant genus Scheidia, high in-
mination gene (for example, the origin of reproductive assurance is particularly pro- breeding depression promotes the evolu-
the Sry gene in mammals approximately nounced in sessile animals—like corals— tion of dioecy [34].
180 million years ago or Sxl in the and plants, which cannot move to find a When separate sexes are favored, the
Drosophila genus .60 million years ago). mate [21,22]. Thus there is a clear transition can occur via several evolution-
They are then thought to differentiate advantage to combining both male and ary pathways. Separate sexes may evolve
through an inevitable and irreversible female functions within an individual, from hermaphrodites either by gradual
process in which recombination between especially in taxa with low mobility. increases in sex-specific investment or
X and Y chromosomes is shut down and However, in some plants and most rapidly by the appearance of male- or
the Y degenerates (see Figure 1). Ulti- animals, species are driven to separate female-sterility mutations (Figure 2). The
mately, Y chromosomes are fated to the sexes. This can be achieved in several latter occurs regularly in plants, often
disappear entirely (‘‘born to be destroyed,’’ ways. One partial solution is the spatial generating mixed sexual systems, such as

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 Box 1. From Mating Types to Sexes into fertile males and females is a funda-
mental developmental process. Contrary
Meiotic sex likely has a single origin, which dates back to the origin of eukaryotes to Myth 1, however, diverse mechanisms
[144,145]). While most eukaryotes display some form of meiotic sex, many lack are used to determine sex [3,4] (Figure 3,
differentiated male and female gametes—a situation referred to as isogamy. Even Figure 4; Box 2). All crocodiles, most
with isogamy, however, mating is often not random but requires that joining cells turtles, and some fish exhibit temperature-
differ at a mating type (MAT) locus. Mating types might have evolved to dependent sex determination; Wolbachia
orchestrate the developmental transition from the haploid to the diploid phase of infections override existing sex determina-
the life cycle [146,147]: plus and minus gametes express complementary tion systems in many arthropod species
transcription factors, encoded by different alleles at the MAT locus; these and either kill/sterilize males or transform
combine in the zygote into heterodimers that silence the genes expressed in the them into phenotypic females; male scale
haploid phase and switch on the diploid program. insects eliminate their father’s genome
after fertilization; marine worm Bonellidae
Isogamy permits a theoretically unlimited number of mating types; high numbers
larvae develop as males only if they
increase the probability that randomly mating partners display complementarity.
Most basidiomycete fungi, for instance, present two independent MAT loci (and encounter a female; and many plants and
are therefore said to be tetrapolar, because a single meiosis can produce cells of animals—including some snails and fish—
four distinct mating types); each locus can be multiallelic, resulting in up to change sex during their lifetime in re-
thousands of different mating types. Alternatively, a low probability of sponse to environmental or social cues
encountering complementary partners might have driven a transition to [3,37].
homothallism observed in many ascomycete fungi, which refers to a mating In fact, sex determination is a rapidly
compatibility between genetically identical individuals. Homothallism evolved via evolving trait in many lineages (Figure 3),
genic capture: a single genome harbors complementary mating-type alleles, and sometimes closely related species, or
which are differentially expressed in plus and minus gametes. Mating-type populations of the same species, have
switching in yeasts allows different cells from the same clone to express different modes of sex determination
complementary mating types, and thus enter the diploid phase of their life cycle. [3,4,38]. Houseflies, for example, normal-
ly have XY sex chromosomes, but dom-
Anisogamy (small male and large female gametes) evolved independently in inant masculinizing and feminizing alleles
many eukaryotic lineages, including several different groups of protists (such as on other chromosomes exist in some
red algae, brown algae, apicomplexa, dinoflagellates, and ciliates; [148]), as well as populations that override sex determina-
most plants and animals. The transition towards anisogamy is thought to result
tion by the XY chromosomes [39]. This
from disruptive selection [1,149,150]: given opposing pressures to simultaneously
variety has stimulated investigation into
maximize the number of gametes, their encounter rate, as well as the mass and
ensuing survival of resulting zygotes, the fitness of both partners is often what evolutionary forces drive the turn-
maximized when one interacting gamete is small and mobile, while its large and over of sex determination mechanisms,
sessile partner provides the resources required for zygote development. what molecular mechanisms underlie the
Intermediate gametes do worse than small ones in terms of mobility and different modes of sex determination, and
numbers, and worse than large ones in terms of provisioning. Such constraints why sex determination is labile in some
largely explain why sexes (at the gametic level) are two and only two, and why taxa and not in others.
anisogamy independently evolved in many lineages. At the molecular level, one
route to anisogamy is by the incorporation of genes controlling gamete size into Genotypic versus environmental sex
the MAT region [151]. Further extensions of the MAT region, possibly involving determination
additional sex-antagonistic genes, led to the U and V chromosomes character- With genotypic sex determination (GSD),
izing male and female gametophytes, as found, e.g., in mosses and liverworts
which occurs in the majority of species
[152].
with known sex-determining mechanisms,
Importantly, the evolution of anisogamy does not require the evolution of genetic elements specify whether individ-
separate sexes, because hermaphrodites can produce both sperm and eggs. uals are female or male. In many animals
Similarly, several unicellular organisms that are anisogamous, such as apixom- and some plants, however, the switch to
plexa and dinoflagellates, can make cells that produce sperm as well as cells that develop into a female or male does not lie
produce eggs. The evolution of completely separate sexes, where individuals in the genes. With environmental sex determi-
cannot give rise to both sperm and egg descendants, is thought to be fairly nation (ESD), external stimuli control sex
derived and is found primarily among multicellular organisms with rare unicellular determination, such as temperature in
exceptions (e.g., the ciliate Vorticella [153] and several dioecious diatoms [154]). reptiles [40], photoperiod in marine am-
phipods and some barnacles [41,42], and
social factors in many coral-reef-dwelling
gynodioecy (mixtures of females and occur (e.g., [35,36]), indicating that the fish and limpets [43,44]. Exactly how the
hermaphrodites) and androdioecy (mix- conditions favoring the separation of male environment triggers sex development has
tures of males and hermaphrodites). and female function are not always remained an open question, although a
Figure 2 highlights the possible pathways present. recent study found that methylation pro-
for the evolution of separate sexes from a vided the link in European sea bass [45].
hermaphrodite ancestor and illustrates Myth 1 Revisited—Sex- In many species, the line between GSD
their relation to sex chromosome evolu- Determining Mechanisms Are and ESD is blurred, with certain environ-
tion. While we have emphasized the Diverse and Can Evolve Rapidly ments altering the (otherwise genetically
evolutionary transition from hermaphro- determined) sex of developing offspring
ditism to separate sexes, several examples In animals and plants that have evolved [46]. For example, tongue sole have
are known where the opposite transitions separate sexes, accurate differentiation differentiated ZW sex chromosomes, but

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 male- or female-biased sex ratios. In
populations with a skewed sex ratio,
selection on autosomal genes typically
favors equal investment in males and
females [51,52], and a new GSD or ESD
system can become established if it restores
a more even sex ratio. An equal number of
males and females is, however, not always
favored, even among autosomal genes
(e.g., with local mate competition, [53]).
In this case, selection for biased sex ratios
can favor the establishment of a new sex-
determining system [54].
Many examples are known of sex
chromosomes, organelles, and endosymbi-
onts that bias the primary sex ratio.
Meiotic drive, where genetic elements bias
the proportion of gametes that carry them,
can create male-biased sex ratios if they
are located on the Y or Z chromosomes (as
seen in many Drosophila species [55]),
whereas driving X or W chromosomes
Figure 1. Sex chromosome differentiation. A. Reconstructed evolutionary path of sex create female-biased sex ratios (found in D.
chromosome differentiation in humans. Sex chromosomes originate from autosomes that simulans [56], stalk-eyed flies [57], and
acquired a sex-determining function (the Sry gene) after their split from monotremes.
Suppression of recombination between the sex chromosomes, associated with degeneration of rodents [58]); autosomal genes that restore
the non-recombining region of the Y chromosome, results in the morphological and genetic unbiased sex ratios are known in many
differentiation of sex chromosomes. Recombination suppression occurred in multiple episodes systems. Cyto-nuclear conflict arises be-
along the human X and Y chromosome, forming so-called evolutionary strata. The oldest stratum cause cytoplasmic factors such as mito-
is shared between eutherian mammals and marsupials, while the youngest stratum of humans is chondria or chloroplast are often inherited
primate-specific. B. The degree of sex chromosome differentiation ranges widely across species, only through the mother, and they favor
spanning the entire spectrum of homomorphic to heteromorphic sex chromosomes, from a
single sex-determining locus, as seen in pufferfish, a small differentiated region (strawberry and production of females, while autosomal
emu), most of the sex chromosomes apart from short recombining regions (humans), to the genes are inherited through both sexes and
entire sex chromosome pair, as seen in Drosophila. Note that the sex chromosomes are not drawn favor more equal sex ratios. Cytoplasmic
to scale. male sterility encoded by mitochondria
doi:10.1371/journal.pbio.1001899.g001 has been widely reported in plants,
including maize, petunia, rice, common
bean, and sunflower [59], as have nuclear-
ZW embryos develop into males when between years is low. GSD predominates at encoded male fertility restorer genes [60].
incubated at high temperatures, and sex high altitudes where there is no advan- Likewise, cellular endosymbionts are only
reversal is accompanied with substantial tage for early-born females and between- transmitted through the mother and can
methylation modification of genes in the year variance in temperature is high create maternally inherited female-biased
sex determination pathway [47]. [49]. In this situation, ESD produces sex ratios; examples include male-killing
ESD is favored over GSD when specific optimal sex ratios at low elevations, while bacteria in butterflies and Drosophila
environments are more beneficial to one GSD prevents extreme sex ratios at high [61,62]. Recurrent invasions of sex ratio
sex [3], selecting for sex-determining altitudes. Importantly, global climate distorters and their suppressors can result
mechanisms that match each sex to its change poses a threat to species with in rapid transitions among sex determina-
best environment. For example, in some temperature-dependent sex determination tion mechanisms between species, and
gobies and wrasses, nest sites are limited, if they cannot adapt rapidly enough to may be a major force contributing to the
and a male’s ability to defend his nest avoid biased sex ratios [50]. Another diversity of sex-determining mechanisms
depends on body size; individuals tend to threat to ESD systems comes from observed across the tree of life.
start life as females, and only become within: they may be prone to invasion
males once they are sufficiently large to by genetic elements that favor biased sex Turnover of sex chromosomes
successfully defend a nesting site [48]. The ratios (see below). In species with genotypic sex determi-
reverse transition, from ESD to GSD, is nation, the chromosome pair that deter-
thought to be favored when the environ- Genomic conflict and transitions in mines sex can change rapidly over time.
ment is unpredictable or not variable sex determination Transitions are particularly likely when
enough, in which case ESD could produce More generally, selection on the sex the ancestral sex chromosome exhibits
strongly skewed sex ratios or intersex ratio can trigger transitions between and little genetic differentiation, since WW or
individuals [3]. Indeed, snow skinks, which among different ESD and GSD systems YY combinations are then less likely to be
are small, live-bearing lizards, have [3]. Sex-biased inheritance patterns of lethal (Figure 5). New sex-determining
different sex-determining mechanisms in different genetic elements—such as sex genes (or copies of the original gene in a
different environments. ESD occurs at low chromosomes, organelles, or endosymbi- new location) can lead to transitions within
altitudes where early birth is advantageous onts—create a conflict over which sex is and between different XY and ZW
for females and the variance in temperature preferred, and can drive the evolution of systems (Figure 5). Invasions of sex-deter-

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Figure 2. Evolutionary pathways from hermaphroditism to separate sexes. Shown are two-step pathways involving intermediate male- and
female-sterile individuals. Loss-of-function mutations (red) are assumed to be recessive, while gain-of-function mutations (green) are assumed to be
dominant. Ancestral alleles are in black. M, male fertility allele; m, male sterility mutation; F, female fertility allele; f, female sterility mutation. Because
loss of function mutations (red) are almost 50 times more frequent than gain of function mutations (green) in flowering plants, we would expect
pathways 1 (e.g., some poplar species) or 2 (e.g., papaya) to arise earlier. Furthermore, transitions through gynodioecy, pathways 2 and 3 (e.g.,
strawberry) allow females to completely avoid inbreeding depression, while transitions through androdioecy are more costly because males must
compete with hermaphrodites for fertilization and do not have any of their own ovules to fertilize. These theoretical arguments help to account for
the prevalence of gynodioecy and the XY chromosome system (via pathway 2) observed in plants; nevertheless, all four pathways may be biologically
relevant, although no known examples for pathway 4 currently exist.
doi:10.1371/journal.pbio.1001899.g002

mining genes are facilitated when the new scale insects), males only transmit their the molecular level in D. melanogaster, C.
sex-determining gene (or a gene closely maternal set of genes (see Figure 4; Box 2: elegans, and mammals. All three involve a
linked to it) has beneficial effects on fitness Glossary). The loss of the paternal genome master-switch sex-determining gene,
[63]. in sons benefits mothers but not fathers which led to the birth of Myth 2. Although
Sexually antagonistic selection, which because these uniparental sons transmit the simplicity of a single master-switch is
occurs when a mutation is beneficial to more of a mother’s genome to grandchil- alluring, this archetype of sex determina-
one sex but detrimental to the other, can dren than do biparental sons [3]. Females tion is clearly not universal. Below we
also drive transitions between sex deter- also experience a selective advantage from discuss systems where sex is determined by
mination by different pairs of chromo- haplodiploidy (but not paternal genome multiple genes, recent molecular data on
somes [64,65]. For example, if an allele of elimination) because unfertilized eggs can the nature and evolution of sex-determin-
an autosomal gene is beneficial to males develop and contribute to fitness when ing genes, and how sex determination can
but harmful to females and becomes mating opportunities are rare. vary in different parts of the body.
linked to a dominant masculinizing muta-
tion, then chromosomes that carry both Polygenic sex determination
the male-beneficial and male-dominant Despite numerous theoretical predic- In some species, sex determination is
alleles create a novel Y that can replace tions for how and why sex determination polygenic. For example, in zebrafish
the ancestral mechanisms. Conversely, mechanisms change, many hypotheses (Danio rerio), a key developmental model
alleles that benefit females and harm males remain untested. Only a small proportion organism, sex is not controlled by a single
can create novel W chromosomes when of taxa have actually been characterized master regulator but is instead a quantita-
linked to feminizing mutations. Turnover for their sex determination mechanisms, tive threshold trait with either a male or
of sex chromosomes can also be triggered hindering the use of comparative methods female outcome, which is determined by
by the degeneration of the Y and W to assess the factors associated with multiple regions in the genome [69–71].
chromosome, which commonly follows the transitions between them. However, be- While some of those regions contain genes
cessation of recombination [66,67], and cause sex determination changes so rapid- known to play a role in sex determination
will result in the replacement of a low- ly in many clades, we can catch these in other organisms [70], there is an
fitness Y or W chromosome with a transitions in action to test theoretical enduring mystery as to how these multiple
nondegenerate one [68]. predictions in a direct, experimental loci and the environment interact to
way. control downstream sexual differentia-
Sex determination by the whole tion in zebrafish. Zebrafish gonads
genome Myth 2 Revisited—Multiple and develop as testes in the absence of
In many animals, sex determination Various Genes Can Determine signals from germ line cells, suggesting
involves the entire genome. With haplodi- Sex that the factors determining sex may
ploidy (found in about 12% of animal regulate germ cell proliferation [72].
species, including all ants, wasps, and bees) The pathways that control sexual de- Sex as a threshold trait has been
and paternal genome elimination (found in velopment have been well characterized at inferred in several other fish [73–75]

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Figure 3. Diversity of sex determination systems for representative plant and animal clades. The bubble insert graph for the plant clades
represents the relative proportion of species with documented sex chromosomes within plants with separate sexes. Vertebrates: Mammalia
(placental, marsupial, and monotreme mammals), Aves (birds), Reptilia (turtles, snakes, crocodiles, lizards), Amphibia (frogs, toads, salamanders), and
Teleostei (bony fishes). Invertebrates: Acari (mites and ticks), Crustacea (shrimps, barnacles, crabs), and Insects, which include Coccoidea (scale
insects), Coleoptera (beetles), Hymenoptera (ants, bees, and wasps), Lepidoptera (butterflies), and Diptera (flies). Plants: Gymnosperms (non-flowering
plants) and Angiosperms (flowering plants).
doi:10.1371/journal.pbio.1001899.g003

and invertebrates [76], and further chicken (Gallus gallus) [12], medaka fish No master sex determination gene has
examples of multiple interacting loci (Oryzias latipes) [78,79], and possibly the been identified in dioecious plants, but
controlling sex determination are no smooth tongue sole (Cynoglossus semilaevis) genes that affect flower sex determination
doubt waiting to be described. Indeed, [14]. In insects, paralogs of transformer (tra), have been found [86,87]. Indeed, many
in taxa where separate sexes evolved a key gene in the sex determination genes may serve as potential targets for sex
recently from a hermaphrodite ancestor, cascade of Drosophila, have evolved as the determination in plants, given that male or
as is common in plants, multiple sex- primary switch in houseflies Musca domes- female sterility can evolve in various ways
determining loci are in fact expected, tica [17], as well as the haplodiploid wasp [86]. For example, 227 male-sterility genes
since at least two independent muta- Nasonia vitripennis [15] and the honeybee have been identified in rice, with at least
tions—one suppressing male function, Apis mellifera [16]. one male-sterility gene found on each of
one suppressing female function—are These data suggest that there are rice’s 12 chromosomes—hence each auto-
necessary to produce separate sexes constraints on the types of genes that can some could, in principle, evolve a sex-
from a hermaphrodite (Figure 2). If be co-opted as master sex determination determining function [88]. This abun-
separate sexes evolve by gradual in- genes [81]. Nevertheless, there are several dance and diversity within a single species
crease in sexual investment from a cases of switch genes with no homologs in indicates that the initial evolution of
hermaphrodite, sex determination may closely related taxa. These include an separate sexes is unlikely to be limited to
also be due to polygenic inheritance. immunity-related gene in rainbow trout a scant handful of master genes.
(Oncorhynchus mykiss) [82] and Sxl in In sharp contrast with the diversity of
The nature and evolution of sex- Drosophila [83], whose ortholog has a primary sex-determining signals, some
determining genes and pathways non-sex-related function in mRNA splic- key regulatory genes play conserved
Some taxa have master-switch sex- ing in other flies [84]. The primary roles in the molecular pathways leading
determining genes that are highly con- master sex-determining gene in the to male or female gonad development
served, such as the Sry gene in nearly all silkworm Bombyx mori is a W-derived across invertebrates and vertebrates,
mammals [77]. In other lineages, such as female-specific piRNA (produced from a such as the doublesex-mab3 (DM) family
fish from the genus Oryzias [78–80], the piRNA precursor termed Fem) that genes [89,90]. Despite profound differ-
master-switch gene differs among closely- targets a Z-linked gene (named Masc), ences in the mode of sex determination
related species (Table 1). There is some and silencing of Masc mRNA by Fem and the identity of the master-switch
empirical evidence for the repeated use of piRNA is required for female develop- genes, DM genes are specifically ex-
the same master sex determination switch ment [85]. Undoubtedly, many other sex pressed in the developing gonads of
genes in animals. For example, in verte- determination genes remain to be found, almost all animals, including vertebrates
brates other than mammals, dmrt1 (a DM making it unclear at present whether (mammals [91], birds [92], turtles and
family gene) and its paralogs act as the there truly are constraints on the types of alligators [93–95], amphibians [96], and
primary sex determination signal in Afri- genes that could evolve to be master fish [97]) and invertebrates (Drosophila
can clawed frog (Xenopus laevis) [13], control switches. [98], hymenoptera [99], crustaceans

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Figure 4. Schematic overview of some sex determination (SD) mechanisms. M refers to meiosis, F to fertilization. Haploid stages (n) are
indicated as shaded areas and diploid stages (nn) are unshaded. Hermaphrodites: Most flowering plants (and gastropods and earthworms)
simultaneously contain both male and female sexual organs (simultaneous hermaphrodites). Many fish and some gastropods and plants are sequential
hermaphrodites; clownfish, for example, are born males and change into females, while many wrasses or gobies begin life as females and then change
to males. Environmental Sex Determination: In turtles and some other reptiles, sex is determined by incubation temperature of the eggs
(temperature-dependent sex determination). Social factors can act as primary sex-determining cues: sexually undifferentiated larvae of the marine
green spoonworm that land on unoccupied sea floor develop into females (and grow up to 15 cm long), while larvae that come into contact with
females develop into tiny males (1–3 mm long) that live inside the female. Genotypic Sex Determination: Almost all mammals and beetles, many
flies and some fish have male heterogamety (XY sex chromosomes), while female heterogamety (ZW sex chromosomes) occurs in birds, snakes,
butterflies, and some fish. In mosses or liverworts, separate sexes are only found in the haploid phase of the life cycle of an individual (UV sex
chromosomes). In some flowering plants and fish, such as zebrafish, sex is determined by multiple genes (polygenic sex determination). In bees, ants,
and wasps, males develop from unfertilized haploid eggs, and females from fertilized diploid eggs (haplodiploidy), while males of many scale insects
inactivate or lose their paternal chromosomes (paternal genome elimination). In some species, sex is under the control of cytoplasmic elements, such
as intracellular parasites (e.g., Wolbachia) in many insects or mitochondria in many flowering plants (cytoplasmic sex determination). In some flies and
crustaceans, all offspring of a particular individual female are either exclusively male or exclusively female (monogeny).
doi:10.1371/journal.pbio.1001899.g004

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[100,101], and mollusks [102,103]). phism in somatic tissues. Studies in Dro- Myth 3 Revisited—Sex
Thus, the evolution of sex-determining sophila have shown that only a subset of cells Chromosomes’ Eternal Youth
pathways, at least in animals, appears to express the genes of the sex determination
occur by the recruitment of new master- cascade and have a sexual identity [106]. Heteromorphic sex chromosomes
switches controlling sexual fate, while Cell-autonomous sex determination can evolve from autosomes that are initially
the downstream developmental path- result in the formation of gynandro- identical but then stop recombining and
ways that regulate gonadal differentia- morphs—individuals that contain both differentiate. Recombination suppression
tion are retained [10,81,104], although male and female characteristics, found in is favored when it links together sexually
the function of some of these down- birds and many insects, including butter- antagonistic alleles and sex-determining
stream elements appears to diverge flies and beetles. Sex determination can loci (i.e., a male-beneficial allele and a
among lineages [105]. Characterization also be regulated differently in the soma male-determining gene on a Y chromo-
of polygenic sex determination systems versus the germ line of the same species some, or a female-beneficial allele and a
and identification of master sex determi- [110,111]. In houseflies [112] and some female-determining gene on a W chromo-
nation genes across kingdoms will pro- frogs [113] and fish [114–116], transplan- some). A side effect of repressed recombi-
vide insight into the mechanistic con- tation experiments indicate that the sex of nation on Y and W chromosomes is that
straints limiting the evolution of sex germ cells depends on the surrounding natural selection is inefficient (reviewed in
determination pathways. soma, i.e., XX germ cells transplanted into [4,5]), which can result in the loss of most
male soma form sperm, and XY germ cells of their genes. Y or W degeneration has
transplanted in a female soma form oo- occurred in many animal taxa with
Sex determination: soma vs. germ cytes. In contrast, germ cells in Drosophila heteromorphic sex chromosomes, includ-
line [117] and mammals [118] receive signals ing mammals [119], many birds [120],
Sex determination can also differ with from the surrounding somatic gonad, but snakes [121], and many insects [122,123],
respect to where in the body sex is they also make an autonomous decision along with some plants, including Rumex
determined. In humans, sex is determined during germ line sexual development; this [124]. In the most extreme cases, the Y or
in the developing gonad, and gonadal sex may also be true for chickens [107]. In W is entirely lost, resulting in so-called X0
hormones in turn trigger sex determina- these animals, the ‘‘sex’’ of the soma must and Z0 systems. According to Myth 3,
tion and differentiation in nongonadal match the ‘‘sex’’ of the germ cells for proper differentiation of sex chromosomes is
tissues. By contrast, in birds, Drosophila, gametogenesis to occur. If XX germ cells evolutionarily inevitable, and the degree
and nematodes [106–109], sexual differ- are transplanted into male soma they do of heteromorphism reflects their age
entiation is a cell-autonomous process, not form sperm, and XY germ cells (Figure 5). However, as we explain below,
although secreted signaling molecules can transplanted into female soma fail to form evidence from a broad array of organisms
play a role in generating sexual dimor- oocytes. indicates that the link between sex chro-

Figure 5. Transitions versus differentiation of sex chromosomes. Transitions between homomorphic sex chromosomes result from new
masculinizing (M9) or feminizing (F9) mutations that invade an existing XY or ZW system to create a new chromosome pair (in grey) that harbors the
sex-determining gene (additional transitional karyotypes are indicated by unshaded circles). XYRXY transitions result in the loss of the ancestral Y
(and ZWRZW transitions cause loss of the ancestral W). Transitions between XY and ZW systems result in some offspring that are homozygous for
the Y (blue) or W (red) chromosome and are thus more likely if the chromosomes have similar gene content but become increasingly difficult if these
chromosomes have degenerated (side boxes on left and right), causing YY and WW individuals to be less fit.
doi:10.1371/journal.pbio.1001899.g005

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 Box 2. Glossary of Sex-Determining Mechanisms

N Hermaphrodites: individuals that contain both male and female sex organs.
N Simultaneous hermaphroditism: male and female sexual organs coexist in one individual (e.g., most flowering plants and
earthworms, many terrestrial gastropods).
N Sequential hermaphroditism: individuals change sex at some point during their life (e.g., many fish, snails, and some plants).
N Dioecy (plants) or gonochorism (animals): individuals are either male or female throughout their life.
N Environmental sex determination: sex is triggered by environmental cues, such as temperature, pH, social interactions, and
seasonality (e.g., many reptiles and some fish).
N Genotypic sex determination: an individual’s sex is established by its genotype (e.g., mammals, birds, amphibians, most
insects, some reptiles and fish, and some plants).
N Male heterogamety: type of genotypic sex determination in which males are heterozygous for the sex-determining locus
(termed X and Y, as seen in therian mammals and Drosophila).
N Female heterogamety: type of genotypic sex determination in which females are heterozygous for the sex-determining locus
(termed Z and W, as seen in birds, snakes, butterflies, and gingko trees).
N UV sex determination: separate sexes are only found in the haploid phase of the life cycle of an individual (e.g., mosses or
liverworts).
N Polygenic sex determination: sex is determined by multiple genes (e.g., some fish and flowering plants).
N Haplodiploidy: males develop from unfertilized, haploid eggs, and females from fertilized, diploid eggs (e.g., bees, ants, and
wasps).
N Paternal genome elimination: paternal chromosomes in males are inactivated or lost after fertilization, leaving males
functionally haploid (e.g., many scale insects).
N Cytoplasmic sex determination: sex is under the control of cytoplasmic elements, such as intracellular parasites (e.g.,
Wolbachia in many insects) or mitochondria (e.g., cytoplasmic male sterility in flowering plants).
N Monogeny: all offspring of a particular individual female are either exclusively male or exclusively female (e.g., some flies and
crustaceans).
N Sexual reproduction: the mixing of genomes via meiosis and fusion of gametes.
N Sex: the sexual phenotype of an individual.
N Sex determination: the mechanism by which the sexual phenotype of an individual is established in a given species.
N Sex chromosome: a chromosome involved with determining the sex of an individual.
N Autosome: a chromosome not involved with determining the sex of an individual (i.e. any chromosome that is not a sex
chromosome).
N Y degeneration: the loss of genetic information on the non-recombining Y chromosome.
N Homomorphic sex chromosomes: sex chromosomes that are morphologically indistinguishable.
N Heteromorphic sex chromosomes: sex chromosomes that are morphologically distinct.
N Sexually antagonistic selection: selection for a trait that benefits one sex to the detriment of the other sex.
N Gynodioecy: a breeding system that consists of a mixture of females and hermaphrodites.
N Androdioecy: a breeding system that consists of a mixture of males and hermaphrodites.
N Meiotic drive (also called segregation distortion): a system in which genetic elements termed segregation distorters bias the
proportion of gametes that carry them, resulting in over- or under-representation of one gametic type (i.e. non-mendelian
segregation).
N Nucleo-cytoplasmic conflict: conflict in inheritance patterns between the nuclear genome and organelle genomes that are
transmitted only maternally.
N Gynandromorphs: individuals that contain both male and female characteristics.

mosome heteromorphism and age is often million years old, respectively sexually antagonistic selection on their sex
far from direct. [121,125,126], i.e. almost as old as the chromosomes and thus avoid selection to
heteromorphic sex chromosomes of theri- suppress recombination between the X
Not all sex chromosomes become an mammals (about 180 million years old). and Y [129]. Third, sexually antagonistic
differentiated How do some ancient sex chromosomes selection can be resolved by other means,
Differentiation is often seen as the avoid differentiation? One hypothesis is such as the evolution of sex-specific
default path of sex chromosome evolution, that occasional X-Y recombination purges expression [130]. Sexually antagonistic
but contrary to Myth 3, some ancient sex deleterious alleles on the Y. A mechanism alleles can accumulate along the sex
chromosomes recombine and are undif- proposed for tree frogs is that XY embryos chromosomes, and sex-specific expression
ferentiated over most of their length. are occasionally sex-reversed, and so the X will confine the product of such alleles to
Examples are found in python snakes and Y recombine when these embryos the sex they benefit, thereby eliminating
and ratite birds, whose homomorphic sex develop into females [127,128]. Second, the selective pressure for recombination
chromosomes are about 140 and 120 some taxa may have few genes under suppression. Consistent with this last

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 Table 1. Known master sex-determining genes in vertebrates and insects, and their paralogs.

Master sex Sex-determining

Species determining gene mechanisms Gene paralog Paralog function Reference

mammals Sry sex-determining Y Sox3 HMG-box [77]

transcription factor
chicken (Gallus gallus) dmrt1 dose-dependent Z - SD pathway [12]
transcription factor
African clawed frog dmW sex-determining W dmrt1 SD pathway [13]
(Xenopus laevis) transcription factor
medaka (Oryzias latipes) dmrt1Y sex-determining Y dmrt1 SD pathway [78,79]
transcription factor
(Oryzias luzonensis) gsdfY sex-determining Y gsdf secretory protein in [80]
SD pathway
Patagonian pejerrey amhY sex-determining Y amh anti-Mullerian hormone [155]
(Odontesthes hatcheri)
rainbow trout sdY sex-determining Y Irf9 interferon [82]
(Oncorhynchus mykiss) regulatory factor
tiger pufferfish (Takifugu amhr2 dose-dependent X amhr anti-Mullerian [156]
rubripes) hormone receptor
smooth tongue sole dmrt1 dose-dependent Z - SD pathway [14]
(Cynoglossus semilaevis)
fruit flies (Drosophila) Sxl dose-dependent X CG3056 mRNA splicing, [83,84]
non-sex specific
housefly (Musca domestica) F sex-determining W tra SD pathway switch [17]
splice factor
silkworm (Bombyx mori) Fem sex-determining W - piRNA [85]
honeybee (Apis mellifera) csd haplodiploid tra SD pathway switch [16]
splice factor
wasp (Nasonia vitripennis) Nvtra haplodiploid tra SD pathway [15]
switch splice factor

doi:10.1371/journal.pbio.1001899.t001

possibility, the recombining, non-differen- genome in many species, and may even has evolved sex-essential genes, such as
tiated region along the sex chromosomes prevent turnover of sex-determining spermatogenesis genes located on the
of the emu (a ratite bird) contains an mechanisms (see below). human and Drosophila Y, sex chromo-
excess of genes whose expression is sex- some transitions are only possible if these
biased, relative to autosomes [126]. Evolutionary traps and conserved genes are moved to another chromo-
sex-determining systems some, since the old Y, along with its
Y chromosomes are not doomed In contrast to the lability of sex genes, is lost during the transition
Y chromosome degeneration has determination mechanisms in some (Figure 5). Overall, phylogenetic patterns
prompted the suggestion that the human groups, eutherian mammals, birds and in vertebrates or insects [3,139] are
Y will eventually disappear [131–133], a many insects exhibit virtually no variation consistent with the notion that hetero-
claim based on the naı̈ve assumption of a in how sex is determined (Figure 3). This morphic sex chromosomes constrain
constant rate of gene loss from the Y over stability could be due to an absence of shifts in sex determination mechanism,
time. However, theory predicts that the genetic variation, particularly when multi- but several notable exceptions exist in
rate of gene decay on the Y decreases over ple genetic steps are required for a both groups. In rodents, for example,
evolutionary time and should halt on an transition to a new sex-determining system many species with unusual sex-determin-
old, gene-poor Y chromosome [67,134]. (Figure 2). Mutations are known, however, ing systems can be found: XY females in
Recent comparative genomic studies sup- that override sex determination (Table 1) some lemming species, X0 females or
port this hypothesis as the gene content of [138], suggesting that the conservation is XX males in vole species, and X0
the primate Y chromosome has been not due to a lack of genetic variation. females and males in some Japanese
stable over the last 25 million years, Instead, evolutionary traps may stabilize spiny rats and mole voles [140]. Like-
suggesting that an equilibrium gene con- sex-determining systems for long spans of wise, some insect groups are known that
tent has been reached in humans [135]. evolutionary time. harbor variation in sex chromosome
Moreover, old gene-poor Y chromosomes Heteromorphic sex chromosomes may karyotype among species; in grasshop-
that are tens of millions of years old, such act as just such a trap. Transitions between pers, fusions between sex-chromosomes
as the Drosophila Y [136], actually show a XY and ZW systems that create YY or and autosomes combined with Y-degen-
net rate of gene gain rather than gene loss WW individuals are prevented when Y or eration and/or Y-loss have created much
[137]. Thus, the Y chromosome can be a W chromosomes lack essential genes variation in sex chromosome karyotype,
stable and important component of the (Figure 5). Also, if the Y (or W) chromosome including species with multiple X or Y

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chromosomes [141]; true fruit flies (Te- which are likely more complex than a from closely related species and to identify
phritidae) that contain both XY and ZW simple change in a master-switch sex- the evolutionary mechanisms hypothesized
species [142]; or blowfly species that determining gene. Furthermore, females to cause transitions among sex-determining
have secondarily lost their heteromorphic switching from haplodiploidy would lose systems. Finally, comparative and functional
sex chromosomes [143]. the fitness benefit associated with produc- genomic data will allow researchers to
How much sex chromosome hetero- ing uniparental sons. address how new master sex determination
morphism is required to create a trap, and Systems that involve interacting genes are incorporated into existing genetic
how strong this trap is, remains unknown. somatic and germ line sex determination networks controlling sexual development. A
To date, only one example of the reversal mechanisms may also limit transitions of full understanding of the diversity of sex
of an ancient sex chromosome back to an sex-determining mechanisms, since chang- determination mechanisms will require that
autosome has been characterized. Specif- es in either germ line sex or somatic sex we expand the taxonomic breadth of study
ically, all Drosophila species contain an alone may produce infertile individuals systems well beyond classic model organ-
autosome that was formerly an X chro- [111]. Thus, while sex determination is isms. Promising models include dipteran
mosome: the dot chromosome. This generally characterized by diversity and insects, such as houseflies or chironomids;
chromosome still has a minor feminizing turnover, some sex-determining systems teleost fish; and reptilian clades, including
role during sex determination, is targeted appear to be more evolutionarily stable turtles and lizards; as well as plant genera,
by a chromosome-specific regulatory than others [3]. such as strawberries, that show variation
mechanism similar to dosage compensa- within and between species in how sex
tion of the X, and its patterns of biased Outlook (or gender in plants) is determined. Integra-
gene expression during early embryogen- tive and interdisciplinary approaches across
esis, oogenesis, and spermatogenesis re- Studying the forces that drive the evolu-
the tree of life will illuminate the diversity of
semble that of the current X in Drosophila tion of sex determination has mainly come
sex determination and yield exciting new
[136]. The retention of the specialized from theoretical works, with little empirical
insights of how and why sex determination
genomic architecture of highly differenti- data. However, the genomic revolution has
evolves in animals and plants.
ated sex chromosomes on the dot chro- allowed researchers to address scientific
mosome illustrates the numerous barriers questions and tackle novel biological systems
to sex chromosome turnover that exist in at the molecular level. As new genomic Acknowledgments
highly heteromorphic systems, even approaches increase the pace of discovery
though there are some cases where these and characterization of sex determination Membership of the Tree of Sex Consortium
are overcome. innon-model organisms, we anticipate that (http://www.treeofsex.org/): Doris Bachtrog,
comparative phylogenetic methods will be Judith E. Mank, Catherine L. Peichel, Tia-
Haplodiploidy also appears to be an
Lynn Ashman, Heath Blackmon, Emma E.
evolutionary trap. While it has arisen a few key to examining the roles of various Goldberg, Matthew W. Hahn, Mark Kirkpa-
dozen times, the reverse transition has not ecological and genetic factors that drive trick, Jun Kitano, Itay Mayrose, Ray Ming,
been reported [3]. Transitions from or to changes in sex determination mechanisms. Sarah P. Otto, Matthew W. Pennell, Nicolas
haplodiploidy require changes in genetic Additionally, genomic data make it increas- Perrin, Laura Ross, Nicole Valenzuela, Jana C.
architecture and meiotic mechanisms, ingly possible to map sex-determining loci Vamosi.

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