Seasonal changes in components of male and female reproductive success in Raphanus sativus L. (Brassicaceae).

Abstract

To document seasonal changes in the reproductive behavior of the perfect-flowered, self-incompatible mustard, Raphanus sativus L., we monitored individual survival, flower and fruit production among 58 individuals in a California population over six census dates (cohorts). Population size declined dramatically and mean individual levels of fruit set changed significantly between cohorts. The frequency distribution of flower and fruit production became increasingly skewed over the first four cohorts. The phenotypic maleness of individuals, a standardized measure of phenotypic gender, oscillated during the reproductive season, peaking in the third and fourth cohorts. We calculated a simple estimate of expected male reproductive success of each plant (the number of fruits sired on conspecifics); this estimate was a function of an individual's flower production and the fruit production of its potential mates in our sampled population. Mean expected male success did not differ significantly among cohorts; expected male success per flower did, however, change significantly among cohorts. Among individuals within each cohort, maternal fruit production and expected male success were both positively correlated with flower production throughout the season. Spearman rank correlation coefficients indicate that the strength of these associations, however, changed during the season. Linear regressions of transformed variables indicated that the shape of several fitness functions also changed over time. In addition, the amount of variation in maternal or expected paternal success explained by flower production declined over the first four cohorts. If typical of wild populations, these temporal changes in these functions suggest that measurements of the intensity of phenotypic selection on flower production will depend on when and how fitness is measured in natural populations.