Stylar Polymorphism in Epigaea repens, a Dioecious Species

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Abstract
Heterostyly may serve as an intermediate step in the evolution of dioecism in the angiosperms, but there are few clear-cut examples of this evolutionary pathway. Since E. repens, a dioecious species with stylar polymorphism, is a possible example, its reproductive biology was studied in 7 populations in North Carolina (USA). Staminate flowers have long styles, unlobed stigmas and anthers which produce abundant pollen. Pistillate flowers have significantly shorter styles, well-lobed stigmas and degenerate (if any) anthers. The few morphologically hermaphroditic individuals were functionally staminate. Sex ratios and the reduction in effective population size due to skewed sex ratios were estimated for each population. The populations were either near 50:50 or significantly pistillate biased. With the exception of stylar polymorphism, E. repens shows no evidence of past heterostyly. Stamen size and pollen tetrad size are not dimorphic. Although the differences in style length are significant between sexes, style lengths are not discrete; variation of style length is continuous and fairly variable. It was concluded that E. repens was never heterostylous; dioecism evolved from hermaphroditism.

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