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Using highly polymorphic microsatellite markers, we assessed clonal structure and paternity in a population of the bryophyte species Polytrichum formosum. Identical multilocus genotypes of individual shoots were almost never observed in spatially separated cushions, but were found to be highly clustered within moss cushions. Therefore, asexual reproduction through dispersal of gametophyte fragments is not very important in P. formosum. However, asexual reproduction on a very localized scale through vegetative growth of genets (branching of gametophytes via clonal growth of rhizomes) is very extensive. The patchy spatial distribution of genets and the absence of intermingling among genets suggest that this species follows a 'phalanx' clonal growth strategy. Vegetative proliferation of genets will increase their size, and, consequently, will have considerable fitness consequences for individuals in terms of increased genet longevity and reproductive output. Although paternity analysis of sporophytes confirmed male genet size, i.e. gamete production, to be an important determinant of male reproductive fitness, it also showed that the spatial distance to female genets is the predominant factor that governs male reproductive success. Moreover, we showed that male gamete dispersal distances in P. formosum are much further than generally assumed, and are in the order of metres rather than centimetres. Combining the findings, we conclude that the high genotypic diversity observed for this facultatively clonal species is most likely explained by a preponderance of sexual reproduction over clonal reproduction.

Based on genetic differentiation, the haploid dioecious bryophyte taxa Polytrichum commune and P. uliginosum have been inferred to be completely reproductively isolated. However, analysing diploid sporophytes from a sympatric population for three diagnostic microsatellite markers, we show here that reproductive isolation between these taxa is far more complex and highly asymmetric. Isolation between ...-P. commune × ...-P. uliginosum seems to be complete and prezygotic (or early postzygotic) as no hybrid sporophytes were observed on P. commune females. In the other direction (...-P. uliginosum × ...-P. commune) isolation was clearly postzygotic as high frequencies of hybrid sporophytes were found on P. uliginosum females. However, during maturation these sporophytes showed irregular development, indicating that hybrid sporophytes are unlikely to produce ripe spores. Mechanisms possibly underlying this asymmetric reproductive isolation pattern are discussed. Notwithstanding hybrid offspring being unlikely, the high frequency of hybrid sporophytes observed suggests that viable spores may be formed occasionally through such rare processes as chromosome nondisjunction, possibly giving rise to allodiploids. Allodiploids have been reported in Polytrichum (and other bryophyte genera). Studies such as the one presented here will therefore help to elucidate the evolutionary importance of interspecific hybridization and allodiploidization in bryophyte speciation. [ABSTRACT FROM AUTHOR]

The distribution of genetic diversity in Mycelis muralis , or wall lettuce, was investigated at a European scale using 12 microsatellite markers to infer historical and contemporary forces from genetic patterns. Mycelis muralis has the potential for long-distance seed dispersal by wind, is mainly self-pollinated, and has patchily distributed populations, some of which may show metapopulation dynamics. A total of 359 individuals were sampled from 17 populations located in three regions, designated southern Europe (Spain and France), the Netherlands, and Sweden. At this within-region scale, contemporary evolutionary forces (selfing and metapopulation dynamics) are responsible for high differentiation between populations (0.34