Sex ratio, spatial segregation, and fertilization rates of the epiphyllous moss Crossomitrium patrisiae (Brid.) Müll.Hal. in the Brazilian Atlantic rainforest

In order to investigate which mechanisms the epiphyllous moss Crossomitrium patrisiae employs to overcome the typical constraints of dioicy in light of its spatially isolated habitat, we tested the predictions that high fertilization rates are achieved due to (1) high rates of sexual expression, (2) balanced sex ratios, and (3) high co-occurrence of both sexes in the same colony (leaf). Seven hundred and ninety-seven ramets were studied for total length, presence, and number of gametoecia and number of fertilized perichaetia in the female according to spatial segregation at leaf- and shrub-scales. High rates of sexual expression (76%) were confirmed. The other two predictions, however, were not upheld as C. patrisae exhibited a strong male bias (0·43 ♀:1 ♂ at ramet level, n = 604). Despite considerable spatial segregation (co-occurrence of sexes was seen in 36·7% of shrubs and 12·8% of colonies), this species revealed one of the highest fertilization rates ever reported for a dioicous bryophyte at three spatial scales: nearly 90% of the mixed colonies bore sporophytes, 40% of all female ramets, and 74% of female ramets occurring in mixed colonies bore sporophytes. Thus, the relative rareness of females may be a phenotypic response due to heavy investment in sporophyte maturation as demonstrated by the high fertilization success of females. High rates of sexual expression and the occurrence of fertilization in nearly all cases of coincidence of sexes in common colonies, with low levels of abortion, contribute to the sexual reproductive success and, subsequently, the long-term survival of this species. © British Bryological Society 2013