Plant reproductive incompatibility

Article By:

Nasrallah, June B. Department of Plant Biology, Cornell University, Ithaca, New York.

Last reviewed:2012

DOI:https://doi.org/10.1036/1097-8542.YB120242

Most flowering plants have perfect (bisexual) flowers in which female (pistil) and male (anther) reproductive structures are found in close physical proximity, an arrangement that favors self-fertilization (Fig. 1). Self-fertilization, in which egg and sperm derived from the same plant fuse to produce viable offspring, may be advantageous in certain situations, such as when environmental conditions are stable or when mates or pollinators are scarce. However, self-fertilization can be disadvantageous under variable and unpredictable environments because the genetically identical offspring that are produced exhibit low genetic diversity and low capacity for adaptation. Consequently, flowering plants have evolved several mechanisms that promote outcrossing and allow them to avoid the potentially deleterious consequences of inbreeding. Among these, the most prevalent and best-understood genetic barriers to self-fertilization are the plant reproductive incompatibility systems known as self-incompatibility (SI). SI, which occurs in more than half of the approximately 250,000 species of flowering plants, is defined as the inability of plants having functional female and male gametes to self-fertilize and set seed in the absence of pollinators. SI acts as a prefertilization barrier that allows cells of the pistil to discriminate between “self” (incompatible) pollen (that is, pollen derived from the same flower, the same plant, or genetically related plants) and “cross” (compatible) pollen (that is, pollen derived from genetically unrelated plants of the same species). As a consequence of this discrimination, the germination of “self” pollen and elongation of “self” pollen tubes into the stigma, style, or ovary are inhibited. As a result, the sperm cells that are carried by the pollen tube never reach the ovule and fail to effect fertilization.

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