ECOLOGICAL AND GENETIC-FACTORS CONTRIBUTING TO THE LOW-FREQUENCY OF MALE-STERILITY IN CHAMAECRISTA-FASCICULATA (FABACEAE)

Bumble bee pollinated Chamaecrista fasciculata provides pollen as the sole reward to its pollinators. Male sterility, expressed as an absenc e or nearly complete absence of pollen production, occurs in low frequ ency in populations of C. fasciculata. Here we describe experiments, u sing C. fasciculata, to examine frequently cited determinants of the s pread and maintenance of male sterility: compensation and the genetic basis of male sterility. In addition, we examine the role the pollinat ion system plays in determining the reproductive success of the male s teriles. Seventeen populations in Maryland, Illinois, and Kansas were surveyed and found to range from 0 to 6% male sterility per population . An artificial population of male-sterile simulants and hermaphrodite s was created to examine how the local frequency of nonrewarding male steriles might affect male-sterile female reproductive success. Male s teriles performed equally poorly, with respect to seed production, whe ther surrounded by other male-sterile simulants or hermaphrodites. Com pensation was examined by comparison of male steriles and hermaphrodit es with respect to several reproductive and nonreproductive characters . Male steriles outperformed hermaphrodites in terms of nonreproductiv e biomass, but performed equally in terms of ovule number and produced many fewer flowers. The genetic basis of male sterility was examined by performing both intra- and interpopulational crosses of male steril es to hermaphrodites and indicate that male sterility is not purely cy toplasmic. The low frequency of male sterility in C. fasciculata popul ations may reflect reduced female reproductive success because of poll inator avoidance, lack of reproductive compensation, and a mode of inh eritance that is not purely cytoplasmic.