systems in plants (Darwin 1876). Plant mating systems have continued to fascinate botanists
and geneticists since that time. Plant reproduction is clearly important to biotechnological
improvements to agriculture, as it directly or indirectly affects the quality and quantity of all
crop products.
2.4.2 Mating Systems
2.4.2.1. Sexual Reproduction. Traditional sexual reproduction is the best place to
begin the discussion of plant mating systems. Seed production by sexual reproduction
involves the transfer of pollen from an anther to the stigma of the pistil, followed by ger-
mination and growth of the pollen tube. The movement of nuclei in the pollen tube
through the style to the embryo sac and the union of functional male and female
gametes complete sexual reproduction in plants. Pollination vectors, such as insects or
wind, are responsible for the transfer of pollen, but mating systems determine whether
the pollen grain can germinate on a receptive stigma and penetrate the style. Mating
systems are classified according to the source of pollen that is responsible for fertilization.
Self-fertilization orselfing(also known asautogamy) occurs when the pollen that effects
fertilization is produced on the same plant as the female gamete with which it unites.
Cross-pollination oroutcrossing(xenogamy) occurs when the pollen of one plant is respon-
sible for fertilization of the female gamete of another plant.
The mating system of a plant species is also classified according to the relative frequency
of self- versus cross-pollination in their seed production. There is a continuum of variation
among species, ranging from complete selfing to obligate outcrossers, with those species
demonstrating both characteristics often referred to as having amixed mating system.
Most crops have been bred and selected for selfing, but can also be outcrossed. This situ-
ation enables “true” seed to be produced by selfing in which the progeny are genetically
very similar to the parent. “Homozygosity begets homozygosity.” This situation also
allows plant breeders to “shuffle” genomes from outcrossing when needed. The predomi-
nant mechanism of pollination for a species is an important factor in determining the breed-
ing method used to develop the cultivar (see Chapter 3). For example, hybrid seed
production is more readily accomplished in an outcrossing species than in a selfing
species. The formation of homozygous lines occurs naturally in a self-pollinating
species, but artificial self/sib-pollination must be practiced in outcrossing species to
obtain homozygous genotypes. Both flower morphology and development, as discussed
in more detail below, can influence rates of self- and cross-pollination.
2.4.2.1.1. Selfing (Autogamy) versus Outcrossing (Xenogamy).Some plants have
natural mechanisms that encourage self-pollination. One such mechanism, in which polli-
nation takes place while the flower is still closed, is known ascleistogamy, and is a process
that can occur even in self-incompatible species (Fig. 2.11).Homogamy, the synchronous
maturation of stamens and stigma, also facilitates self-pollination.
The effects of repeated self-fertilization, first documented in maize at the turn of the
(nineteenth–twentieth) century, has been confirmed for many crop species. Repeated
self-fertilization will yield complete homozygosity in a few generations unless the hetero-
zygous state is favored by selection. In an heterozygous diploid, the dominant allele can
shelter recessive alleles that would be deleterious in the homozygous state. Self-
fertilization quickly results in the segregation of lethal or sublethal types as homozygous
recessives are produced. Further selfings rapidly separate the material into uniform lines,
2.4. PLANT REPRODUCTIVE BIOLOGY 35