PATTERNS OF GENDER ALLOCATION IN A MONOECIOUS CUCURBIT, APODANTHERA UNDULATA, AND THEIR REPRODUCTIVE CONSEQUENCES.

Abstract

I investigated the patterns of male and female flower production in two populations of a monoecious, self-compatible, prostrate vine, Apodanthera undulata Gray (Cucurbitaceae). Small, and probably young, plants produce no flowers. Larger and older plants produce only male flowers, while a somewhat greater threshold size is necessary for female flower production. Beyond these threshold effects, femaleness, a proportional measure of allocation to female function, did not increase with plant size. Thus, allocation to both male and female functions increased with size. In contrast, femaleness decreased with increasing flower production; plants with many flowers opened relatively more male flowers than plants with fewer flowers. This trend, which seemed stronger in the low density population, can have important effects on pollinator behavior and thus on pollen dispersal and receipt. Femaleness was positively correlated between years for all plants and for all cosexes, plants that produced both male and female flowers. Thus, plants that opened only male flowers one year were likely to open only male flowers the next year. Similarly, cosexes were likely to be cosexes again in the following year, with similar femaleness values. Approximately 10% of all flowering plants changed gender group. These patterns suggested that all plants were male until they reached a certain size and that plants had an intrinsic femaleness value due to either genotype, microsite or environmental effects. Finally, some of the reproductive consequences of these gender allocation patterns were investigated. The factors determining success, such as predation and rainfall, were unpredictable between populations and years. Unless success was especially low (i.e., few fruits matured in the population), success through male and female functions were positively correlated, as could be predicted for a monoecious species. The gains for male success were greater with increasing allocation to that function in the high density as opposed to the low density population. This suggested that plants in the high density population should allocate more to male function, as was observed. Though preliminary, these data support the predictions of sex allocation theory for cosexual species.