NONRANDOM MATING IN CLARKIA-GRACILIS (ONAGRACEAE) - A CASE OF CRYPTIC SELF-INCOMPATIBILITY

In plants capable of both self-fertilization and outcrossing, the selfing rate depends on the proportion of self pollen in pollen loads and on the relative postpollination success of self pollen in siring offspring. While the composition of pollen loads is subject to unpredictable variation, paternity success of self vs. outcross pollen following pollen deposition may be controlled by maternal plants. This study examined postpollination paternity success in Clarkia gracilis ssp. sonomensis, in which deposition of self pollen is common. Pure loads of self and outcross pollen produced similar numbers of mature seeds, but equal mixtures of self and outcross pollen yielded more than three times as many outcrossed offspring as selfed offspring. The finding that the paternity success of self pollen depends on whether it is in competition with outcross pollen helps to explain an earlier finding that the selfing rate in experimental populations was highest when pollinator activity was lowest. Cryptic self-incompatibility allows paternity by self pollen when outcross pollen is unavailable.