The overlooked dwarf males in mosses—Unique among green land plants

Abstract

Dwarf males occur in several animal groups but are unique to mosses among the green land plants. Mosses, with more than 50% unisexual species and common fertilisation distances in the range of a few decimetres, have evolved various means to cope with potential problems to successfully achieve fertilisation and to ensure sporophyte production and sexually produced diaspores. We explore the abundance of different kinds of extreme female-biased sexual size dimorphism in homocostate pleurocarpous mosses. Based on direct observations and literature information, we investigated the large-scale geographical distribution of this phenomenon, and whether male dwarfism was associated with family position. Almost 60% of 1737 species with information about sexuality are dioecious, and males are known in 769 dioecious species. Dwarf males occur in 178 of these (23%), and are considered obligate in 113 species (63% of 178). We thoroughly examined a subset of 162 species. Among these 72 species (44%) form dwarf males, but only 18 species (25% of 72) produce obligate dwarf males. We conclude that especially facultative male dwarfism is much overlooked among dioecious mosses. The distribution of dioecious species and dwarf males among the latter was related to family membership, but showed little correlation with geographic area. Male dwarfism thus appears useful as a taxonomic character to define bryophyte families. We estimate that between roughly one quarter and almost half of dioecious pleurocarpous moss species exhibit male dwarfism, and the majority of these (75%) form dwarf males facultatively. Male dwarfism in mosses is suggested to have evolved as a result of (1) competing selective pressures of the cytoplasmic and nuclear genomes, (2) competition between the sexes, (3) selection for reduced fertilisation distances, or a combination of these. The associated niche shift of the males may also provide these with a suitably humid and nutrient-rich habitat.