You’ll recall from discussions of Mendel’s work (see Key Concept 12.1) that some plants can reproduce sexually by both cross-pollination and self-pollination. Self-pollination increases the chances of successful pollination but leads to homozygosity, which reduces genetic diversity. Because diversity is the raw material of evolution by natural selection, homozygosity can be selectively disadvantageous. Most plants have evolved mechanisms that prevent *self-fertilization. Angiosperms prevent self-fertilization in two main ways: (1) physical separation of male and female gametophytes and (2) genetic self-incompatibility.

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SEPARATION OF MALE AND FEMALE GAMETOPHYTES Self-fertilization is prevented in dioecious species, which bear only male or female flowers on a particular plant. Pollination in dioecious species is accomplished only when one plant pollinates another. In monoecious plants, which bear both male and female flowers on the same plant, the physical separation of the male and female flowers is often sufficient to prevent self-fertilization. Some monoecious species prevent self-fertilization by staggering the development of male and female flowers so they do not bloom at the same time, making these species functionally dioecious.

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GENETIC SELF-INCOMPATIBILITY A pollen grain that lands on the stigma of the same plant will fertilize the female gamete only if the plant is self-compatible, meaning capable of self-pollination. To prevent self-fertilization, many plants are genetically self-incompatible. Self-incompatibility depends on the ability of a plant to determine whether pollen is genetically similar or genetically different from itself. Rejection of “same-as-self” pollen prevents self-fertilization. How does it occur?

Self-incompatibility in many flowering plants is controlled by a cluster of tightly linked genes called the S locus (for self-incompatibility). The S locus encodes proteins in the pollen and style that interact during the recognition process. A self-incompatible species typically has many alleles of the S locus. The pollen phenotype may be determined by its own haploid genotype or by the diploid genotype of its parent plant. In either case, if the pollen expresses an allele that matches either of the alleles expressed in the recipient pistil, the pollen is rejected. Depending on the type of self-incompatibility system, the rejected pollen either fails to germinate or is prevented from growing through the style (Figure 37.4); either way, self-fertilization is prevented.