The true sex ratio in European Pseudocalliergon trifarium (Bryophyta: Amblystegiaceae) revealed by a novel molecular approach

Abstract

Dioecious plants, including many bryophytes, rarely exhibit discernible sexual dimorphism before sexual maturity. Because many species and populations of dioecious bryophytes do not express their sex, it remains mostly unresolved whether expressing individuals reflect the ratios of genetically male and female plants. The present study assesses the population sex ratio of the wetland moss Pseudocalliergon trifarium in central and northern Europe. For the first time in a bryophyte, we estimate the sex ratio in a population by assessing directly both expressing and non-expressing plants. Expressed gender ratio was assessed from herbarium specimens. Single shoots from non-expressing specimens were sexed using a recently developed molecular sex marker. On the basis of the female and male frequencies in these two data sets and the overall proportion of expressing specimens, we estimate the European population sex ratio to be 1.93 : 1 (female/male). Expressed, non-expressed, and population sex ratios are not significantly different from each other, suggesting that gender differences in rates of sex expression cannot account for the female bias. Earlier studies of P. trifarium failed to reveal gender-specific growth rates or pre-zygotic reproductive costs. Gender differences at the spore to protonemal stage, in mortality, or niche preferences could potentially explain the uneven sex ratio. © 2010 The Linnean Society of London, Biological Journal of the Linnean Society, 2010, 100, 132–140.