Showing papers by "Elizabeth A. Zimmer published in 2012"

Phylogeny, divergence times, and historical biogeography of New World Dryopteris (Dryopteridaceae).

Abstract:  Premise of the study: Dryopteris is a large, cosmopolitan fern genus ideal for addressing questions about diversifi cation, biogeography, hybridization, and polyploidy, which have historically been understudied in ferns. We constructed a highly resolved, well-supported phylogeny for New World Dryopteris and used it to investigate biogeographic patterns and divergence times.  Methods: We analyzed relationships among 97 species of Dryopteris , including taxa from all major biogeographic regions, with analyses based on 5699 aligned nucleotides from seven plastid loci. Phylogenetic analyses used maximum parsimony, maximum likelihood, and Bayesian inference. We conducted divergence time analyses using BEAST and biogeographic analyses using maximum parsimony, maximum likelihood, Bayesian, and S-DIVA approaches. We explored the monophyly of subgenera and sections in the most recent generic classifi cation and of geographic groups of taxa using Templeton tests.  Key results: The genus Dryopteris arose ca. 42 million years ago (Ma). Most of the Central and South American species form a well-supported clade which arose 32 Ma, but the remaining New World species are the result of multiple, independent dispersal and vicariance events involving Asia, Europe, and Africa over the last 15 Myr. We identifi ed six long-distance dispersal events and three vicariance events in the immediate ancestry of New World species; reconstructions for another four lineages were ambiguous.  Conclusions: New World Dryopteris are not monophyletic; vicariance has dominated the history of the North American species, while long-distance dispersal prevails in the Central and South American species, a pattern not previously seen in plants.

Unraveling reticulate evolution in North American Dryopteris (Dryopteridaceae)

Abstract: The thirteen species of Dryopteris in North America have long been suspected of having undergone a complicated history of reticulate evolution via allopolyploid hybridization. Various explanations for the origins of the allopolyploid taxa have been suggested, and though most lines of evidence have supported the so-called “semicristata” hypothesis, contention over the group’s history has continued in several recent, conflicting studies. Sequence data from nine plastid and two nuclear markers were collected from 73 accessions representing 35 species of Dryopteris. Sequences from each of the allopolyploids are most closely related to their progenitor species as predicted by the “semicristata” hypothesis. Allotetraploid D. campyloptera appears to be derived from a hybrid between diploid D. expansa and D. intermedia; D. celsa, from diploid D. ludoviciana x D. goldiana; and D. carthusiana and D. cristata, from diploid “D. semicristata” x D. intermedia and D. ludoviciana, respectively. Allohexaploid D. clintoniana appears to be derived from D. cristata x D.goldiana. The earliest estimated dates of formation of the allopolyploids, based on divergence time analyses, were within the last 6 Ma. We found no evidence for recurrent formation of any of the allopolyploids. The sexual allopolyploid taxa are derived from crosses between parents that show intermediate levels of genetic divergence relative to all pairs of potential progenitors. In addition, the four allotetraploids are transgressive with respect to geographic range relative to one or both of their parents (their ranges extend beyond those of the parents), suggesting that ecological advantages in novel habitats or regions may promote long-term regional coexistence of the hybrid taxa with their progenitors. This study provides the first thorough evaluation of the North American complex of woodferns using extensive sampling of taxa and genetic markers. Phylogenies produced from each of three datasets (one plastid and two nuclear) support the “semicristata” hypothesis, including the existence of a missing diploid progenitor, and allow us to reject all competing hypotheses. This study demonstrates the value of using multiple, biparentally inherited markers to evaluate reticulate complexes, assess the frequency of recurrent polyploidization, and determine the relative importance of introgression vs. hybridization in shaping the histories of such groups.

Phylogenetic placement of a recently described taxon of the genus Pleodendron (Canellaceae)

Abstract: In order to determine placement of the recently described plant species, Pleodendron costaricense (Canellaceae), five DNA regions were sequenced. For the new species, those included two from the nuclear rRNA coding region, ITS and 18S, and three from the chloroplast genome, the genes for rbcL and atpB and the spacer trnLF region. For the 18 taxa of Canellaceae and sister group Winteraceae, ITS and trnLF sequences were published (Karol et al. 2000), while the other three regions were sequenced for this study. The aligned sequences were combined and analyzed with parsimony, likelihood and Bayesian programs. The single tree produced in these analyses provided 100% support for placement of P. costaricense in a monophyletic group with Pleodendron macranthum and Cinnamodendron ekmanii. This result suggests that nomenclatural changes for those three species should be considered. Phytologia 94(3): 404-412 (December 1, 2012).