SEX ALLOCATION IN THE MONOECIOUS HERB BEGONIA-SEMIOVATA

Sex-allocation models predict that the evolution of self-fertilization should result in a reduced allocation to male function and pollinator attraction in plants. The evolution of sex allocation may be constrai ned by both functional and genetic factors, however. We studied sex al location and genetic variation for floral sex ratio and other reproduc tive traits in a Costa Rica population of the monoecious, highly selfi ng annual Begonia semiovata. Data on biomass of floral structures, flo wer sex ratios, and fruit set in the source population were used to ca lculate the average proportion of reproductive allocation invested in male function. Genetic variation and genetic correlations for floral s ex ratio and for floral traits related to male and female function wer e estimated from the greenhouse-grown progeny of field-collected mater nal families. The proportion of reproductive biomass invested in male function was low (0.34 at flowering, and 0.07 for total reproductive a llocation). Significant among-family variation was detected in the siz e (mass) of individual male and female flowers, in the proportion of m ale Rowers produced, and in the proportion of total flower mass invest ed in male flowers. Significant among-family variation was also found in flower number per inflorescence, petal length of male and female fl owers, and petal number of female flowers. Except for female petal len gth, we found no difference in the mean value of these characters betw een selfed and outcrossed progeny, indicating that, with the possible exception of female petal length, the among-family variation detected was not the result of variation among families in the level of inbreed ing. Significant positive phenotypic and broad-sense genetic correlati ons were detected between the mass of individual male and female flowe rs, between male and female petal length, and between number of male a nd number of female flowers per inflorescence. The ratio of stamen-to- pistil mass (0.33) was low compared to published data for autogamous s pecies with hermaphroditic flowers, suggesting that highly efficient s elfing mechanisms may evolve in monoecious species. Our results indica te that the study population harbors substantial genetic variation for reproductive characters. The positive genetic correlation between inv estment in male and female flowers may reflect selection for maximum p ollination efficiency, because in this self-pollinating species.