A comparative study of cultured and field plants provides evidence for the shy male hypothesis in tropical genotypes of Bryum argenteum Hedw.

Abstract

ABSTRACT Introduction The female-biased sex ratio is recurrently observed in bryophyte populations, although the sex ratio of spores is usually balanced. The results of several studies have suggested explanations for this observation, one of which is the shy male hypothesis. The shy male hypothesis predicts that the sex ratio of populations is balanced in the field, but due to a low or lack of sex expression of male plants, the populations present a female bias when the sex ratio is based on observations of sex-expressing plants. Methods We tested the shy male hypothesis using samples of the moss Bryum argenteum collected from Brazilian seasonally dry tropical forests and the Brazilian Atlantic Forest. Field sex ratios were determined by classifying shoots as male, female, or non–sex-expressing. Non–sex-expressing shoots were further analysed for sex expression after culture regeneration. Results and discussion Our results showed that populations of Bryum argenteum in the field had a low rate of sex expression, and that most of the shoots expressed were female. However, when shoots without sex expression were cultured, a greater number of male shoots were observed compared with in the field plants. Because more males appeared in the shoots cultured from non–sex-expressing field plants, our data for tropical B. argenteum tend to support the shy male hypothesis. Some genotypes from the wet forest were synoicous (i.e. had male and female gametangia in the same gametoecia), suggesting polyploidy.