Even without spatial or temporal (time-related) separation, chemical incompatibility between the stigma or style and pollen of the same plant can also promote outcrossing. Molecular data suggest that self-compatibility is the ancestral condition in flowering plants and that self-incompatibility has evolved independently many times. In plants such as tobacco (Nicotiana) that have gametophytic self-incompatibility (GSI), pollen tubes germinate but fail to grow through the style if they are chemically incompatible. In GSI, the incompatibility reaction is determined by the combination of the self-incompatibility (SI) genes of the maternal plant and the SI genes of the pollen grain. GSI is found in several species, including tobacco and some grasses.

In sporophytic self-incompatibility (SSI), the incompatibility reaction is controlled by the combination of maternal plant SI genes in the stigma and the SI genes of the plant that produced the pollen, rather than those of the pollen grain itself. Incompatible reactions cause the pollen tube to stop growing on or near the stigma. Multi-allelic SSI systems have many incompatibility types, and proteins that cause the incompatibility reaction are produced by the anthers and are present in the outer layer of the pollen grain. Broccoli and many other members of the mustard family (Brassicaceae) have multi-allelic SSI.

In contrast, many plants with SSI have only two or three incompatibility types, but are heterostylous (e.g., shamrock [Oxalis], and water hyacinth [Eichhornia], a noxious, invasive aquatic weed). see also Flowers; Pollination Biology; Reproduction, Asexual; Reproduction, Fertilization and; Reproduction, Sexual; Seed Dispersal.

Ann K. Sakai

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