Evolutionary Dynamics of Sex Ratio and Gender Dimorphism in Silene latifolia: II. Sex Ratio and Flowering Status in a Potentially Male-Biased Population

Abstract

We examined sex ratio in flowering and nonflowering plants from a single population of the dioecious species Silene latifolia in (1) a greenhouse experiment, (2) the field population, and (3) a field garden. To sex nonflowering plants we extracted DNA and amplified it in PCR reactions with a RAPD primer complementary to a DNA fragment on the male chromosome of S. latifolia. In the greenhouse, germination was low (26%–58%), and across all plants, males significantly outnumbered females (633 males to 544 females). Among those plants that flowered, significantly more were male (395 males to 292 females), while among nonflowering plants marginally more were female (238 males to 252 females). The male bias among flowering plants was not caused by higher female mortality or a lower proportion of females flowering. Males were more likely to flower and at high density were marginally more likely to flower earlier. In the field population the sex ratio was significantly female biased (75 males to 112 females). In the first generation of the field garden experiment, germination was very low (11%) and there was a significant male bias (153 males to 115 females). In the second generation, germination increased to >90%, and there was a significant female bias (256 males to 352 females). We conclude that the male bias in the greenhouse was probably a consequence of sex-differential germination behavior and that the sex ratio in the natural population is likely to be female biased both at the seed and adult stage. We discuss how the tendency for males to flower earlier might favor the evolution of such a female-biased sex ratio.