Apomixis
Apomixis: Another form of asexual reproduction. In apomixis,
seeds are formed but the embryos develop without
fertilization. Consequently, the plants resulting from them
are identical in genotype to the parent plant. In apomictic
species, sexual reproduction is either suppressed or absent.
When sexual reproduction also occur, the apomixis is termed
as facultative. But when sexual reproduction is absent, it is
referred to as obligateapomixis. Many crop species show
apomixis, but it is generally facultative.
Nonrecurrent apomixis: When embryos arise directly
from haploid cells.
Recurrent apomixis: embryos arise from diploid cells
and the progeny obtained from diploid embryos can
be maintained indefinitely.
A simplified classification of apomixis is given below:
a) Adventive Embryony: In this case, embryos develop directly from
vegetative cells of the ovule, such as nucellus, integument, and
chalaza. Development of embryo does not involve production fo
embryo sac. Adventive embryony occurs in mango, citrus, etc.
b) Apospory:In apospory, first diploid cell of ovule lying outside the
embryosac develops into another embryosac without reduction. The
embryo then develops directly from the diploid egg cell without
fertilization. Apospory occurs in some species of Hieraceum, Malus,
Crepis, Ranunculus, etc.
c) Diplospory: Embryo sac is produced from the megaspore,
which may be haploid or, more generally, diploid.
Generally the meiosis is so modified that the megaspore
remains diploid. Diplospory leads to parthenogenesis or
apogamy.
1) Parthenogenesis: The embryo develops from embryo sac
without pollination. It is of two types Gonial
parthenogenesis: – embryos develop from egg cell.
2) Somatic parthenogenesis: – embryos develop from any
cell of the embryo sac other than the egg cell.
d) Apogamy:In apogamy, synergids or antipodal cells develop
into an embryo. Like parthenogenesis, apogamy may be
haploid or diploid depending upon the haploid or diploid state
of the embryo sac. Diploid apogamy occurs in Antennaria,
Alchemilla, Allium and many other plant species.
Significance of Apomixis:
Apomixis is a nuisance when the breeder desires to obtain sexual
progeny, i.e., selfs or hybrids.
Apomixis is of great help when the breeder desires to maintain
varieties. Thus for breeding of apomictic species, the breeder has to
avoid apomictic progeny when he is making crosses to produce hybrids
or producing inbred lines. But once a desirable genotype has been
selected, it can be multiplied and maintained through apomictic
progeny. This would keep the genotype of a variety intact.
Asexually reproducing crop species are highly heterozygous and
show severe inbreeding depression. Therefore, breeding methods in
such species must avoid inbreeding.
