The reproductive biology of Polytrichum formosum: clonal structure and paternity revealed by microsatellites

Using highly polymorphic microsatellite markers, we assessed clonal structu re and paternity in a population of the bryophyte species Polytrichum formo sum. Identical multilocus genotypes of individual shoots were almost never observed in spatially separated cushions, but were found to be highly clust ered within moss cushions. Therefore, asexual reproduction through dispersa l of gametophyte fragments is not very important in P. formosum. However, a sexual reproduction on a very localized scale through vegetative growth of genets (branching of gametophytes via clonal growth of rhizomes) is very ex tensive. The patchy spatial distribution of genets and the absence of inter mingling among genets suggest that this species follows a 'phalanx' clonal growth strategy. Vegetative proliferation of genets will increase their siz e, and, consequently, will have considerable fitness consequences for indiv iduals in terms of increased genet longevity and reproductive output. Altho ugh paternity analysis of sporophytes confirmed male genet size, i.e. gamet e production, to be an important determinant of male reproductive fitness, it also showed that the spatial distance to female genets is the predominan t factor that governs male reproductive success. Moreover, we showed that m ale gamete dispersal distances in P. formosum are much further than general ly assumed, and are in the order of metres rather than centimetres. Combini ng the findings, we conclude that the high genotypic diversity observed for this facultatively clonal species is most likely explained by a prepondera nce of sexual reproduction over clonal reproduction.