The adaptive background of nannandry: dwarf male distribution and fertilization in the moss Homalothecium lutescens

Dwarf males (nannandry) occur in many unrelated, mostly aquatic, groups of organisms. Among land plants they are unique to bryophytes. In this study our aim was to explain variation in frequency of dwarf males and fertilization within populations in the moss Homalothecium lutescens . We compared parameters related to dwarf male presence and sporophyte production in 90 colonies from three localities. Dwarf male density was positively associated with colony moisture at two of the localities, suggesting increased spore germination and dwarf male survival with moist conditions. At one of these localities, dwarf male density was also positively associated with the presence of perichaetia (female sexual branches). Dwarf male density and fertilization frequency were positively associated in two of the localities. Furthermore, in one population, fertilization was also positively associated with canopy cover, which could be attributed to improved nutrient status as a result of throughfall, increased sperm‐dispersal efficiency because of larger water drops, or more favourable moisture conditions as a result of shading. Nannandry thus appears to strongly reduce the problem of short fertilization distances in bryophytes, but the presence of water is still critical because the dwarf males are dependent on a certain level of humidity for recruitment and/or development. © 2014 The Linnean Society of London, Biological Journal of the Linnean Society , 2014, 113 , 74–84.