POPULATION GENETICS OF BRYOPHYTES IN RELATION TO THEIR REPRODUCTIVE BIOLOGY

Abstract

  Several unusual features of bryophyte reproduction lead to predictions about levels of genetic variation and its partitioning among natural populations. Dominance of the life cycle by a free-living, haploid gametophyte suggests that levels of genetic variation should be low. Isozyme data indicate, however, that mosses display a range of variation comparable to that observed for diploid plants. The range for liverworts is less. Because of the tight coupling of ploidy level and sexuality in most bryophytes, it is difficult to determine if levels of genetic variation are higher in unisexual than in bisexual taxa. Bisexual allopolyploids in Plagiomnium and Rhizomnium typically show fixed heterozygosity, which inflates their gene diversity statistics relative to their unisexual, haploid congeners. Nevertheless, gene diversities are also higher for bisexual, haploid species of Plagiothecium than for their unisexual, haploid relatives. There are also few data available to test the prediction of higher rates of self-fertilization and greater differentiation among populations of bisexual than of unisexual mosses and liverworts. Restricted gene flow due to short sperm dispersal distances should lead to strong differentiation among populations, but this may be counteracted by long-distance dispersal of spores. Typically degrees of genetic differentiation among conspecific populations of bryophytes are similiar to those observed in seed plants, except that intercontinentally disjunct populations of bryophytes often are only weakly differentiated. Asexual reproduction is common and widespread in bryophytes and appears to lead to large clonal patches in some taxa. The recent discovery of several allopolyploid bryophytes suggests that interspecific hybridization is more common than has been thought. This conclusion fits better with the limited possibilities of bryophytes with respect to reproductive isolation, given that fertilization is external and effected by water in all taxa.