Phylogeny of Mentzelia section Trachyphytum: origins and evolutionary ecology of polyploidy

Joshua Michael Brokaw

doi:10.7273/000005668

This study explores the systematics and evolutionary ecology of Mentzelia section Trachyphytum. Trachyphytum is a monophyletic group of annual flowering plants with no fewer than 24 species. Nearly two thirds of the species in this group are polyploids, suggesting that polyploidy has had a fundamental role in diversification. We examine phylogenetic relationships within Trachyphytum in order to understand the evolutionary origins of diploid and polyploid species. We examine habitats of extant species and consider how ecological change has contributed to the current patterns of diversity. We used phylogenetic reconstructions based on DNA sequences from the plastid regions, trnH-psbA, trnS-trnG, trnS-trnfM, ndhF-rpl32 and rpl32-trnL, and the isocitrate dehydrogenase nuclear gene to infer sister group relationships among diploids and identify the progenitors of polyploids in Trachyphytum. Phylogenetic analyses of the diploid species recovered two subsections consistent with morphology. However, species level relationships revealed evidence of homoplasy in morphological evolution. Therefore, modifications in the subsectional taxonomy have been suggested. Investigations of polyploid origins suggest that most polyploid species have recent allopolyploid origins. Only one species was inferred to have progenitors from separate subsections. In a few cases, autopolyploid origins could not be rejected. Further, several species were inferred to have early origins and/or extinct progenitors. We used habitat characteristics for extant species to infer patterns of ecological change during cladogenesis. We compared phylogenetic and ecological distances between progenitors of allopolyloids to further investigate patterns of polyploid speciation. Diploid lineages in Trachyphytum exhibited early niche divergence and subsequent convergence leading to coexistence of species from different subclades. Polyploid origins were generally associated with shifts in habitat, with habitats exhibiting both intermediate and transgressive characteristics with respect to progenitors. Although the mean of phylogenetic distances between progenitors of allopolyploids was not significantly different from expectations given random progenitors from Trachyphytum, the distribution of phylogenetic distances was narrower than expected. The variability of allopolyploid niches was found to increase with increasing ecological distance between progenitors. Finally, most polyploids did not exhibit significant increases in ecological variability with respect to progenitors, but one allooctoploid species displayed the greatest ecological amplitude in Trachyphytum.

Phylogeny of Mentzelia section Trachyphytum: origins and evolutionary ecology of polyploidy

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