| Comparatively few studies have combined cytological, morphological, genetic, and molecular data to elucidate processes of allopolyploid evolution. Approaches using nuclear and plastid molecular markers indicate that polyploid derivatives can originate independently and allopatrically from progenitors. Allopolyploidy is an important evolutionary process in plants. Multiple origins within allopolyploid complexes are more common than once thought and genetic diversity, which results from these independent origins involving different progenitors, is recognized as a source of variation on which selection can act. In the genus Chelone, morphological, cytological, and molecular evidence is used to test evolutionary and taxonomic hypotheses of multiple origins within an allopolyploid complex. Species and varietal delimitation, as proposed by Pennell in 1935, are also examined. Morphological analyses indicate that the three diploid species of Chelone (C. cuthbertii, C. glabra, and C. lyonii) are taxonomically distinct whereas populations of the polyploid complex (C. obliqua) recombine characters of the diploids. Populations of C. obliqua in the southern Blue Ridge are tetraploids, whereas those in the Interior Highland and Plain and the Atlantic Coastal Plain are hexaploids. Fixed heterozygosity inferred from isozyme data supports hypotheses of allopolyploid origins involving divergent populations of progenitors that are most like extant C. glabra and C. lyonii. Molecular markers indicate multiple origins of allopolyploid derivatives likely occurred in the southern Blue Ridge, Central Lowland, Ozark Plateau, Interior Low Plateau, and Atlantic Coastal Plain. Isozyme, nuclear ribosomal DNA (nrDNA), and chloroplast DNA (cpDNA) data are concordant with the hypothesis that the extant tetraploids are not progenitors of hexaploids. Marker alleles are not shared between the two, and both cpDNA and nrDNA markers indicate progenitors of allohexaploid derivatives may be extinct because of their unique cpDNA haplotype and nrDNA type. Both isozyme and cpDNA data suggest that Chelone has been effected by Pleistocene and Holocene climatic changes. Patterns of genetic similarity and chloroplast DNA haplotypes may reflect recolonization of glacial bog habitats and meltwater lakes in formerly glaciated regions from southern refugial areas. Contraction of species ranges as a result of the southern migration of species ranges in the Pleistocene may have brought diploid populations of Chelone in closer proximity, thereby increasing chances of interspecific hybridization and subsequent allopolyploidy. |
