ability of the pistil to discern the presence of self-pollen and to inhibit the germination or
subsequent development of self-related, but not genetically unrelated, pollen. There are two
types of SI mechanisms,gametophyticandsporophytic(Fig. 2.12); these differ in whether
the haploid pollen genotype or the diploid pollen parent genotype, respectively, determines
the success of pollination. These are important traits for controlling pollinations and are
much sought after in breeding programs.
2.4.2.1.4. Male Sterility.The ability to produce hybrid seed has been of fundamental
importance to modern agricultural practice. “Hybrid vigor” has increased the yield in
maize since the mid-1960s. The genetic approach to the production of F 1 hybrid seed
was made possible by the exploitation of various male sterility mechanisms.Male sterility
refers to the failure of a plant to produce functional pollen by either genetic or cytoplasmic
mechanisms.Cytoplasmic male sterility(CMS) is a maternally inherited trait that sup-
presses the production of viable pollen grains. It is a common trait reported in hundreds
of species of higher plants. The CMS phenotype (female parent) is used commercially in
the production of F 1 hybrid seed by preventing self-fertilization of the seed parent, in
such crops as maize, sorghum, rice, sugarbeet, and sunflower. The use of CMS lines as
female parents also requires the introduction of nuclear fertility restorer genes from the
pollen parent, so that male fertile F 1 hybrids can be produced. Novel sources of CMS
and fertility restorer genes are very important to plant breeders and the traits can be intro-
duced via biotechnological means.
2.4.2.2. Asexual Reproduction. Plants can also reproduce by asexual means,
resulting in the multiplication of genetically identical individuals. An individual reprodu-
cing asexually is referred to as acloneand the process ascloning. Potatoes and cranberries
are two agricultural plants that are propagated by asexual reproduction. Asexual reproduc-
tion in seed plants can be divided into two main classes;vegetative propagation, which
can occur through plant parts other than seed (bulbs, corms, rhizomes, stolon, tubers,
etc.), and apomixis, which can be defined as the production of fertile seeds in the
absence of sexual fusion of gametes or “seeds without sex.” Sexual fusion presupposes
a reductional meiosis if the ploidy level is to remain stable. During apomixes, the
embryo may develop from either anN(haploid) egg cell or from a 2N(diploid) egg
cell. In the latter type, known asagamospermy, a full reductional meiosis is usually
absent and chromosomes do not segregate. Another rarer form of apomixis is that in
which the embryo plant arises from tissue surrounding the embryo sac. These “adventi-
tious” embryos occur, for example, in citrus crops.
2.4.2.3. Mating Systems Summary. Having discussed the three main modes of
reproduction—selfing, outcrossing, and apomixis—we may now examine the advantages
and disadvantages of different mating systems (reviewed in Briggs and Walters 1997).
One possible advantage of repeated self-fertilization is that well-adapted genotypes can
be replicated with little change. A further advantage, especially in extreme or marginal
habitats, where relying on crossing between plants might be hazardous or even result in
total failure, is that self-fertilization is an assured method of producing progeny.
Outcrossing, on the other hand, avoids the deleterious effects of inbreeding depression,
the main disadvantage of repeated selfing, and promotes heterozygosity, genetic variability,
and genetic exchange. There are, however, costs to the plant, compared with selfers, as more
38 MENDELIAN GENETICS AND PLANT REPRODUCTION