Postpollination mechanisms influencing mating patterns and fecundity: An example from Eichhornia paniculata

Plant mating systems are influenced by the amount and genetic composition of pollen grains deposited on stigmas and by the ability of recipients to discriminate among pollen from different sources. We describe an experimental procedure that uses limiting and excess pollinations with mixtures of genetically marked pollen to partition the siring success of donors into three components: prefertilization gamete attrition (failure of male gametophytes before fertilization), pollen competitive ability (differences in pollen tube growth rate), and postfertilization gamete attrition (embryo abortion). Regression models for the relationships of pollen load size with each pollen's siring success and total recipient fecundity indicate that, for mixtures of self and outcross pollen, differences in gamete attrition, pollen competitive ability, and postfertilization success will have distinct and predictable effects on mating patterns and fecundity. Mating systems that rely on differences in pollen competitive ability result in outcrossing frequencies that increase with pollen load size, with seed production remaining high over a broad range of load sizes. In contrast, for mating systems governed by differences in gamete attrition, the frequency of outcrossed progeny will not vary greatly with pollen load size, but reduced fecundity will be expected over a wider range of pollen load sizes. These predictions were confirmed by analyzing the response of siring success and fecundity in response to pollen load size in the tristylous Eichhornia paniculata (Pontederiaceae). Experimental manipulations of the size and composition of pollen loads allow prediction of the frequency of outcrossed progeny produced under varying pollen environments.