Recherche

Rothfels, Carl J.; Li, Fay-Wei; Sigel, Erin M.; Huiet, Layne; Larsson, Anders; Burge, Dylan O.; Ruhsam, Markus; Deyholos, Michael; Soltis, Douglas E.; Stewart Jr., C. Neal; Shaw, Shane W.; Pokorny, Lisa; Chen, Tao; dePamphilis, Claude; DeGironimo, Lisa; Chen, Li; Wei, Xiaofeng; Sun, Xiao; Korall, Petra; Stevenson, Dennis W.; Graham, Sean W.; Wong, Gane Ka-Shu; Pryer, Kathleen M.; Stewart, C. Neal; Wong, Gane K-S.; de Pamphilis, Claude — 2016-06-24 Premise of the study: Understanding fern (monilophyte) phylogeny and its evolutionary timescale is critical for broad investigations of the evolution of land plants, and for providing the point of comparison necessary for studying the evolution of the fern sister group, seed plants. Molecular phylogenetic investigations have revolutionized our understanding of fern phylogeny, however, to date, these studies have relied almost exclusively on plastid data. Methods: Here we take a curated phylogenomics approach to infer the first broad fern phylogeny from multiple nuclear loci, by combining broad taxon sampling (73 ferns and 12 outgroup species) with focused character sampling (25 loci comprising 35877 bp), along with rigorous alignment, orthology inference and model selection. Key results: Our phylogeny corroborates some earlier inferences and provides novel insights; in particular, we find strong support for Equisetales as sister to the rest of ferns, Marattiales as sister to leptosporangiate ferns, and Dennstaedtiaceae as sister to the eupolypods. Our divergence-time analyses reveal that divergences among the extant fern orders all occurred prior to ∼200 MYA. Finally, our species-tree inferences are congruent with analyses of concatenated data, but generally with lower support. Those cases where species-tree support values are higher than expected involve relationships that have been supported by smaller plastid datasets, suggesting that deep coalescence may be reducing support from the concatenated nuclear data. Conclusions: Our study demonstrates the utility of a curated phylogenomics approach to inferring fern phylogeny, and highlights the need to consider underlying data characteristics, along with data quantity, in phylogenetic studies.

Percy, Diana M.; Argus, George W.; Cronk, Quentin C.; Fazekas, Aron J.; Kesanakurti, Prasad R.; Burgess, Kevin S.; Husband, Brian C.; Newmaster, Steven G.; Barrett, Spencer C. H.; Graham, Sean W.; Barrett, Spencer C.H. — 2014-06-17 Willows (Salix: Salicaceae) form a major ecological component of Holarctic floras, and consequently are an obvious target for a DNA-based identification system. We surveyed two to seven plastid genome regions (~3.8 kb; ~3% of the genome) from 71 Salix species across all five subgenera, to assess their performance as DNA barcode markers. Although Salix has a relatively high level of interspecific hybridization, this may not sufficiently explain the near complete failure of barcoding that we observed: only one species had a unique barcode. We recovered 39 unique haplotypes, from more than 500 specimens, that could be partitioned into six major haplotype groups. A unique variant of group I (haplotype 1*) was shared by 53 species in three of five Salix subgenera. This unusual pattern of haplotype sharing across infrageneric taxa is suggestive of either a massive non-random coalescence failure (incomplete lineage sorting), or of repeated plastid capture events, possibly including a historical selective sweep of haplotype 1* across taxonomic sections. The former is unlikely as molecular dating indicates that haplotype 1* originated recently, and is nested in the oldest major haplotype group in the genus. Further, we detected significant non-neutrality in the frequency spectrum of mutations in group I, but not outside group I, and demonstrated a striking absence of geographic structure to the haplotype distributions in this group. The most likely explanation for the patterns we observed involves recent repeated plastid capture events, aided by widespread hybridization and long-range seed dispersal, but primarily propelled by one or more trans-species selective sweeps.