Sex expression and sex ratios in dwarf male-producing pleurocarpous mosses – have we missed something?

Abstract

Background: We recently demonstrated that dwarf male plants are much more common among mosses than hitherto thought. Dwarf plants, producing functional male sexual organs occur in 10–20% of the moss species worldwide. Aims: We investigated how an inadvertent omission of dwarf males affects estimates of male sex expression rates and sex ratios. Methods: We recorded frequencies of specimens with female-expressing plants, male-expressing normal-sized and/or dwarf plants, and sporophytes in herbarium material of five model species from Macaronesia. We analysed how frequency estimates of male occurrence and sex ratios were affected by excluding dwarf males. Results: Occurrence of dwarf males was positively associated with sporophyte formation. When dwarf males were excluded, male mate availability was missed in 51–61% of specimens (in 74–76% of sporophyte-containing specimens). Specimen-level sex ratio was balanced in material with sporophytes in the three species exhibiting male dwarfism, but strongly female biased if dwarf males were omitted. Conclusions: We provide the first quantitative evidence that sex-expressing dwarf males are essential for sexual reproduction and sporophyte production in mosses that form such males. Considering its abundance, male dwarfism therefore deserves more attention among plant biologists. Innovative approaches are required to further study the minute male plants.