Chapter 16 Patterns and mechanisms of polyploid speciation in bryophytes

Abstract

Past cytological and molecular studies of bryophytes have generated hypotheses about the importance of polyploidy, and challenged our views on allopolyploidy and reticulate evolution. Nevertheless, we still know little about mechanisms of polyploid formation, requirements for polyploid bryophyte establishment, and adaptive significance and constraints of genome duplication. Assessing the incidence of polyploidy from some threshold chromosome number is problematic and has little bearing on what the relative importance of polyploidy in speciation is. Instead, here number of unique ploidy levels and number of species within genera was used to obtain maximum and minimum estimates of the fraction of species originating by polyploidy. Estimates for mosses (6.4-18.6%) and liverworts (4.9-10.3%) show that polyploidy is an active and ongoing process in both groups. Molecular studies give overwhelming evidence that allopolyploidy prevails in bryophytes. The predominance of allopolyploidy also suggests that apospory is rare, and that the fraction of unreduced spores produced is relatively high in sporophytes of hybrid origin. Polyploid moss gametophytes are autonomous, which has large potential for vegetative reproduction. Moreover, bisexual gametophytes can produce sporophytes by intragametophytic selfing. This may entail that successful polyploid establishment chiefly depends on relative fitness, and not on the frequency of the cytotype in the population. Chromosome doubling may have beneficial genetic consequences by allowing masking of deleterious alleles in the dominant gametophyte generation. In bryophytes with high chromosome numbers it is unclear to what extent functional haploidy indicates silencing or sub-functionalization of genes, and whether the high numbers reflect increases by chromosome translocation or fission rather than polyploidy. Putative constraints on polyploid formation in bryophytes are briefly discussed.

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