Agriculture Reference
In-Depth Information
programme. The type of reproduction of the species (at
least in commerce) will determine the way that breeding
and selection processes can be maximized to best effect.
There are two general types of plant reproduction
sexual
and
asexual
.
will, be most adapted to cultivation. For example, most
species that can be readily used in hybrid production
are generally cross-pollinating but need to be tolerant
of inbreeding by selfing. This is because the hybrids
are effectively the cross-pollinated progeny between two
inbred genotypes.
Self-pollinating species are tolerant of inbreeding and
consequently deleterious recessive genes are not com-
mon. They tend to have flower structures and behaviour
that promote selfing. Individual lines of descent tend to
approach homozygosity, shows little heterotic advan-
tage when out-crossed and, individually, tend to have a
narrower range of adaptation.
Cross-pollinating species tend to be intolerant to
inbreeding, principally because they carry many delete-
rious recessive genes (these exist in the populations since
they can be tolerated in heterozygous form). Generally,
cross-pollinating species:
Sexual reproduction
Sexual reproduction involves fusion of male and female
gametes that are derived either from two different par-
ents or from a single parent. Sexual reproduction is, of
course, reliant on the process of
meiosis.
This involves
megaspores within the ovule of the pistil and the male
microspores within the stamen. In a typical diploid
species, meiosis involves reductional division by meiosis
of the 2
n
female cell to form four haploid megaspores,
by the process of
megasporogenesis
. This process in male
cells, to form microspores, is called
microsporogenesis
.
Fertilization of the haploid female cell by a haploid
male pollen cell results in the formation of a diploid
2
n
embryo. The endosperm tissue of the seed can result
from the union of two haploid nuclei from the female
with another from the pollen, and hence ends up as
being 3
n
.
Asexual reproduction (
mitosis
) is the multiplication
of plant parts or by the production of seeds (apomixis)
that do not involve the union of male and female
gametes. The process of mitosis will result in two cells
that are identical in genetic make-up and of the same
composition as the parental cell.
Seeds are effectively classified according to the source
of pollen that is responsible for the fertilization. In the
case of self-pollination the seeds are a result of fertiliza-
tion of female egg cells by pollen from the same plant.
Cross-pollination occurs when female egg cells are fertil-
ized by pollen from a different plant, usually one that is
genetically different. As a result plant species are usually
classified into
self-pollinating
and
cross-pollinating
species. This is of course a gross generalization. There
are species which are effectively 100% self-pollinating,
those that are 100% cross-pollinating but there exists
a whole range of species that cross-pollinate or self-
pollinate to varying degrees. From the top 122 crop
plants grown worldwide, 32 are mainly self-pollinating
species, 70 are predominantly cross-pollinating, and the
remaining 20 are cross-pollinating but do show a degree
of tolerance to successive rounds of inbreeding.
The method of pollination will be an important fac-
tor in determining the type of cultivar that can, or
Have a crossing mechanism that promotes out-
crossing
•
Show greater heterotic effect
•
Are more widely adaptable to many different envi-
ronments
•
Have individual plants that are highly heterozygous
at many loci
•
Particularly important are the outcrossing mecha-
nisms. Cross-pollinating species often have distance
barriers, time barriers or other mechanisms, which limit
or prohibit self-pollination. Plants may be monoecious,
where separate male and females flowers are located on
different parts of the plant (e.g. maize) or indeed dioe-
cious, where male and female flowers occur on different
plants. Cross-pollination is also favoured in many cases
where male pollen is shed at a time when the female
stigma on the same plant is not receptive.
Another, more clearly defined sets of mecha-
nisms are those termed as
self-incompatibility
. Self-
incompatibility occurs when a plant, which has fully
functional male and female parts, will not produce
mature seed by self-pollination. There is a set of mech-
anisms that have naturally evolved to increase cross-
pollination within plant species and hence promote
heterozygosity. Adaptation to environmental condi-
tions is greater if wider ranges of genotypes are produced
in a progeny (i.e. the progeny shows greater genetic vari-
ation). Thus the chances of survival of at least some of
