Geographic Variation in the Occurrence and Extent of Sexual Dimorphism in a Dioecious Shrub, Simmondsia chinensis

Abstract

Simmondsia chinensis (Simmondsiaceae), a dioecious desert shrub, varies throughout its geographic distribution in both the occurrence and the extent of sexual dimorphism present in secondary sex characteristics. Three desert populations showed significant morphological differences between sexes, while five populations in more mesic sites were sexually isomorphic. Females had larger leaves than males in two of the eight populations surveyed, and longer internodes in three populations. Females tended to branch more frequently, although this was statistically significant in only one population. At one desert site, male shrubs were taller and larger than females, while at a coastal site, sexes did not differ in gross morphology. The extent of sexual dimorphism appears to be associated with the potential evapo-transpiration of the environment, as the degree of dimorphism in leaf size was inversely correlated with the amount of rainfall during the growing season, and that of internode length was associated with large annual temperature fluctuations. It is hypothesized that selection for larger seed size, advantageous in more xeric environments, contributed to selection for larger vegetative features in females through allometric responses. The extent of dimorphism in internode length increased with mean female internode length, indicating that the dimorphism may have arisen through modifications in female morphology. Also, females tend to show greater variability in measured characters ; this differential variance could contribute to the evolution of sexual dimorphism in this species.