Lecture No: 1

MODES OF REPRODUCTION

Mode of reproduction determines the genetic constitution of crop plants, that is,
whether the plants are normally homozygous or heterozygous. This, in turn, determines the
goal of a breeding programme. If the crop plants are naturally homozygous, e.g., as in self-
pollinators like wheat, a homozygous line would be desirable as a variety. But if the plants
are heterozygous naturally, e.g., as in cross-pollinators like Maize, a heterozygous population
has to be developed as a variety. Consequently, the breeding methods have to be vastly
different for the two groups of crop plants. A knowledge of the mode of reproduction of crop
plants is also important for making artificial hybrids. Production of hybrids between diverse
and desirable parents is the basis for almost all the modern breeding programmes.
MODES OF REPRODUCTION
The modes of reproduction in crop plants may be broadly grouped into two
categories, asexual and sexual.
SEXUAL REPRODUCTION
Sexual reproduction involves fusion of male and female gametes to form a zygote, which
develops into an embryo. In crop plants, male and female gametes are produced in specialised
structures known as flowers.

Flower
A flower usually consists of sepals, petals (or their modifications), stamens and/or pistil. A
flower containing both stamens and pistil is a perfect or hermaphrodite flower. If it contains
stamens, but not pistil, it is known as staminate, while a pistillate flower contains pistil, but
not stamens. Staminate and pistillate flowers occur on the same plant in a monoecious
species, such as maize, Colocasia, castor (Ricinus communis), coconut, etc. But in dioecious
species, staminate and pistillate flowers occur on different plants, e.g., papaya, date palm
(Phoenix dactylifera), pistachio (Pistacia vera), etc. In "crop plants, meiotic division of
specific cells in stamen and pistil yields microspores and megaspores, respectively. This is
followed by mitotic division of the spore nuclei to produce gametes; the male and female
gametes are produced in microspores and megaspores, respectively.

1
Sporogenesis
Productions of microspores and megaspores is known as sporogenesis. Microspores
are produced in anthers (microsporogenesis), while megs.spores are produced in ovules
(megasporogenesis).

Microsporogenesis. Each anther has four pollen sacs, which contain numerous pollen
mother cells (PMCs). Each PMC undergoes meiosis to produce four haploid cells or
microspores. This process is known as microsporogenesis (Fig. 4.1). The microspores mature
into pollen grains mainly by a thickening of their walls.

Megasporogenesis. Megasporogenesis occurs in ovules, which are present inside the

ovary. A single cell in each ovule differentiates into a megaspore mother cell. The megaspore
mother cell undergoes meiosis to produce four haploid megaspores. Three of the megaspores
degenerate leaving one functional megaspore per ovule (Fig. 4.2). This completes
megasporogenesis.

Gametogenesis
The production of male and female gametes in the microspores and the megaspores,
respectively, is known as gametogenesis.
Microgametogenesis. This refers to the production of male gamete or sperm. During the
maturation of pollen, the microspore nucleus divides mitotically to produce a generative and
a vegetative or tube nucleus. The pollen is generally released in this binucleate stage. When
the pollen lands onto the stigma of a flower, it is known as pollination. Shortly after
pollination, the pollen germinates. The pollen tube enters the stigma and grows through the
style. The generative nucleus now undergoes a mitotic division to produce two male gametes
or sperms. The pollen, along with the pollen tube, is known as microgametophyte. The pollen
tube finally enters the ovule through a small pore, micropyle, and discharges the two sperms
into the embryo sac.

2
 Microsporogenesis and microgametogenesis (a generalized scheme)

Megagametogenesis. The nucleus of a functional megaspore divides mitotically to produce

four or more nuclei. The exact number of nuclei and their arrangement vary considerably
from one species to another. In most of the crop plants, megaspore nucleus undergoes three
mitotic divisions to produce eight nuclei. Three of these nuclei move to one pole and produce
a central egg cell and two synergid cells; one synergid is situated on either side of the egg
cell. Another three nuclei migrate to the opposite pole to give rise to antipodal cells. The two
nuclei remaining in the centre, the polar nuclei, fuse to form a secondary nucleus. The
megaspore thus develops into a mature megagametophyte or embryo sac. The development
of embryo sac from a megaspore is known as megagametogenesis. The embryo sac generally
contains one egg cell, two synergids, three antipodal cells (all haploid), and one diploid
secondary nucleus.

3
 Megasporogenesis and megagametogenesis (generalized scheme)

Significance of Sexual Reproduction

Sexual reproduction makes it possible to combine genes from two parents into a
single hybrid plant. Recombination of these genes produces a large number of genotypes.
This is an essential step in creating variation through hybridization. Almost the entire plant
breeding is based on sexual reproduction. Even in asexually reproducing species, sexual
reproduction, if it occurs, is used to advantage, e.g., in sugarcane, potato, sweet potato etc.

MODES OF POLLINATION
Pollination refers to the transfer of pollen grains from anthers to stigmas. Pollen from an
anther may fall on to the stigma of the same flower leading to self-pollination or outogamy.
When pollen from flowers of one plant are transmitted to the stigmas of flowers of another
plant, it is known as cross-pollination or allogamy. A third situation, geitonogamy, results
when pollen from a flower of one plant falls on the stigmas of other flowers of the same

4
plant, e.g., in Maize. The genetic consequences of geitonogamy are the same as those of
autogamy.
Self-pollination
Many cultivated plant species reproduce by self-pollination. Self-pollination species
are believed to have originated from cross-pollinated ancestors. These species, as a rule,
must have hermaphrodite flowers. But in most of these species, self-pollination is not
complete and cross-pollination may occur up to 5%. The degree of cross-pollination in self-
pollinated species is affected by several factors, e.g., variety environmental conditions like
temperature and humidity, and location.

Mechanisms promoting self-pollination (Do self study)

Genetic Consequences of Self-Pollination

Self-pollination leads to a very rapid increase in homozygosity. Therefore,
populations of self-pollinated species are highly homozygous, self-pollinated species do not
show inbreeding depression, but may exhibit considerable heterosis. Therefore, the aim of
breeding methods generally is to develop homozygous varieties.
Cross-Pollination
In cross-pollinating species, the transfer of pollen from a flower to the stigmas of the
others may be brought about by wind (anemophily). Many of the crop plants are naturally
cross-pollinated (Table 3.1). In many species, a small amount (up to 5-10 percent) of selfing
may occur.

Mechanisms promoting cross pollination (Do self study)

Genetic Consequences of Cross-Pollination. Cross-pollination preserves and promotes

heterozygosity in a population. Cross-pollinated species are highly heterozygous and show
mild to severe inbreeding depression and a considerable amount of heterosis. The breeding
methods in such species aim at improving the crop species without reducing heterozygosity
to an appreciable degree. Usually, hybrid or synthetic varieties are the aim of breeder
wherever the seed production of such varieties is economically feasible.

Often Cross-Pollinated Species

In many crop plants (Table 3.1), cross-pollination often exceeds 5 per cent and may
reach 30 per cent. Such species are generally known as often cross-pollinated species, e.g.,
Jowar, Cotton, arhar, safflower etc. The genetic architecture of such crops is intermediate

5
between self-pollinated and cross-pollinated species. Con-sequently, in such species
breeding methods suitable for both of them may be profitably applied. But often hybrid
varieties are superior to others.