A battle of the sexes? Sex ratio and performance at small scales in dioecious Mercurialis perennis

Abstract

Mercurialis perennis is a dioecious clonal woodland herb. Its male-biased shoot sex ratio may result from differential selection (with skewed genet sex ratio) and/or differential performance (fairly even genet sex ratio but skewed shoot sex ratio since male clones grow more vigorously or have a higher flowering propensity). To test both scenarios, a total of 1024 shoots were mapped and excavated in five 1 m2 plots in an old-growth forest in northeastern Germany. SSR genotyping revealed a less skewed intrinsic male:female shoot sex ratio (1.6) and a nearly even genet sex ratio. The sexes showed intermingling rather than sexual niche segregation. Most of the 36 genets found split into numerous independent units (merigenets). A slight sexual size dimorphism (SSD) was found: male clones possess on average more shoots, have longer stolons and a higher proportion of flowering shoots. Analyses of resource allocation patterns showed 1.5–2.8 times higher costs for sexual reproduction (dry mass, nitrogen) for females. Males invest surplus resources into leaves and stolons, accentuating the SSD; although for genets the difference may be smaller due to the higher flowering rate of males (53% vs. 34%). Photosynthetic performance, deduced from rapid light curves measured with a portable fluorometer, was similar for both sexes, especially in spring; the main challenge being the transformation from light to shade plant with tree foliation. Our data indicate that differential performance accounts for the male bias in shoot sex ratio; competition between sexes is unlikely given the strongly intermingled clones.