often calledpure lines. Some of the surviving lines may be characterized by reduced vigour
and fertility, a condition known asinbreeding depression. If pure lines originating from
different parental stocks are crossed together,hybrid vigor(i.e.,heterosis) may be demon-
strated. Outcrossing thus avoids the deleterious effects of inbreeding depression, and pro-
motes heterozygosity, genetic variability, and genetic exchange. Plants species have
therefore evolved a wide variety of natural mechanisms that favor cross-pollination; and
scientists have needed to invent an alphabet soup to describe the myriad of mating syn-
dromes observed (see Richards 1986, Stuessy 1989). Several of these, including protandry;
protogyny; chasmogamy; heterostyly; imperfect flowers on monoecious, dioecious, or
polygamous plants; and incompatibility, are discussed in somewhat greater detail below.
2.4.2.1.2. Sex Distribution within a Flower and within a Plant. Plants are the ulti-
mate hermaphrodites—most are bisexual with male and female organs together in one
flower (also referred to as a “perfect flower”), but there are many ways in which sex
organs are distributed within a flower, within a plant, and within a plant population.
Some plants have separate male (staminate) flowers and female (pistillate) flowers on a
single plant and are termed monoecious(e.g., maize) In other species the male and
female flowers occur on separate plants (known asdioecy), or can have a mixture of
male, female, and perfect flowers on the same plants (termedmixed polygamous). Sex
determination in such plants is under genetic control, with monoecy in maize, for
example, under the control of a set of genes known as thetasselseedloci. A number of
different mechanisms have been identified that establish the sexuality of dioecious
plants, including the presence of heteromorphicsex chromosomeswith males having XY
and females XX chromosomes, or varying X : autosome ratios similar to that found in
Drosophila(Bridges 1925). Even when both male and female organs occur in the same
flower, the timing of sexual expression can vary. Sometimes pollen is shed before the
stigma is receptive in a process known asprotandry, or a stigma can mature and cease to
be receptive before pollen is shed (protogyny).
2.4.2.1.3. Self-Incompatability Genetic Systems. Many plant species have a genetic
self-incompatibility (SI) mechanism that promotes outcrossing and is defined as “the
inability of a fertile hermaphrodite seed plant to produce zygotes after self-pollination.”
SI mechanisms are estimated to occur in more than half of all angiosperm species. The
Figure 2.11.Cleistogamous flowers (b) are fertilized prior to the opening of sepals and petals, which
ensures that the plant is self-pollinated. A noncleistogamous flower is shown in (a). (Adapted from
Briggs and Walters 1997).
36 MENDELIAN GENETICS AND PLANT REPRODUCTION