EVIDENCE FOR RECIPROCAL ORIGINS IN POLYPODIUM HESPERIUM (POLYPODIACEAE): A FERN MODEL SYSTEM FOR INVESTIGATING HOW MULTIPLE ORIGINS SHAPE ALLOPOLYPLOID GENOMES

Premise of the study: Many polyploid species are composed of distinct lineages originating from multiple, independent polyploidization events. In the case of allopolyploids, reciprocal crosses between the same progenitor species can yield lineages with different uniparentally inherited plastid genomes. While likely common, there are few well-documented examples of such reciprocal origins. Here we examine a case of reciprocal allopolyploid origins in the fern Polypodium hesperium and present it as a natural model system for investigating the evolutionary potential of duplicated genomes. Methods: Using a combination of uniparentally inherited plastid and biparentally inherited nuclear sequence data, we investigated the distributions and relative ages of reciprocally formed lineages in Polypodium hesperium, an allotetraploid fern that is broadly distributed in western North America. Key results: The reciprocally derived plastid haplotypes of Polypodium hesperium are allopatric, with populations north and south of 42 degrees N latitude having different plastid genomes. Incorporating biogeographic information and previously estimated ages for the diversification of its diploid progenitors, we estimate middle to late Pleistocene origins of P. hesperium. Conclusions: Several features of Polypodium hesperium make it a particularly promising system for investigating the evolutionary consequences of allopolyploidy. These include reciprocally derived lineages with disjunct geographic distributions, recent time of origin, and extant diploid progenitors.