POSTPOLLINATION MECHANISMS INFLUENCING MATING PATTERNS AND FECUNDITY - AN EXAMPLE FROM EICHHORNIA-PANICULATA

Plant mating systems are influenced by the amount and genetic composit ion of pollen grains deposited on stigmas and by the ability of recipi ents to discriminate among pollen from different sources. We describe an experimental procedure that uses limiting and excess pollinations w ith mixtures of genetically marked pollen to partition the siring succ ess of donors into three components: prefertilization gamete attrition (failure of male gametophytes before fertilization), pollen competiti ve ability (differences in pollen tube growth rate), and postfertiliza tion gamete attrition (embryo abortion). Regression models for the rel ationships of pollen load size with each pollen's siring success and t otal recipient fecundity indicate that, for mixtures of self and outcr oss pollen, differences in gamete attrition, pollen competitive abilit y, and postfertilization success will have distinct and predictable ef fects on mating patterns and fecundity. Mating systems that rely on di fferences in pollen competitive ability result in outcrossing frequenc ies that increase with pollen load size, with seed production remainin g high over a broad range of load sizes. In contrast, for mating syste ms governed by differences in gamete attrition, the frequency of outcr ossed progeny will not vary greatly with pollen load size, but reduced fecundity will be expected over a wider range of pollen load sizes. T hese predictions were confirmed by analyzing the response of siring su ccess 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 frequ ency of outcrossed progeny produced under varying pollen environments.