Isozyme Evidence of Reticulate Evolution in Mosses: Plagiomnium medium is an Allopolyploid of P. ellipticum x P. insigne

Abstract

Electrophoretic data from 44 natural populations show that Plagiomnium medium is an allopolyploid, whose genome can be reconstructed by merely combining present-day variation of its extant haploid progenitors, P. ellipticum and P. insigne. Because the progenitor species are well- differentiated genetically, P. medium shows fixed heterozygosity in about 1/3 of the 23 loci assayed. High levels of polymorphism in P. ellipticum provide the opportunity to detect multiple origins of P. medium, which evidence shows must have originated from at least four separate hybridization events. We detected only one allele unique to P. medium and only one possible instance of gene silencing. This suggests a relatively recent origin of the allopolyploid. It may also suggest that natural selection does not favor silencing back to a haploid condition. Apparently, cross-fertilization is frequent in P. medium and generates a large number of genetic recombinants across loci. Thus, P. medium enjoys a favorable genetic situation in which there is fixed, intralocus heterozygosity combined with the potential for extensive interlocus recombination. We speculate that P. medium may have been launched as a species during the Pleistocene, when the ranges of the circumboreal, continental P. ellipticum and the coastal, Pacific Northwest endemic P. insigne may have overlapped broadly. Their present-day ranges include few sympatric populations and would not permit hy- bridization along a broad front. Contrary to conventional views, it appears that reticulate evolution occurs in bryophytes, just as it does in other land plants.