Browsing by Author "Britton, DM"
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Item Open Access Phylogenetic relationships and evolution of extant horsetails, Equisetum, based on chloroplast DNA sequence data (rbcL and trnL-F)(International Journal of Plant Sciences, 2003-01-01) Des Marais, DL; Smith, AR; Britton, DM; Pryer, KMEquisetum is a small and morphologically distinct genus with a rich fossil record. Two subgenera have been recognized based principally on stomatal position and stem branching: subg. Equisetum (eight species; superficial stomates; stems branched) and subg. Hippochaete (seven species; sunken stomates; stems generally unbranched). Prior attempts at understanding Equisetum systematics, phylogeny, and character evolution have been hampered by the high degree of morphological plasticity in the genus as well as by frequent hybridization among members within each subgenus. We present the first explicit phylogenetic study of Equisetum, including all 15 species and two samples of one widespread hybrid, Equisetum x ferrissii, based on a combined analysis of two chloroplast markers, rbcL and trnL-F. Our robustly supported phylogeny identifies two monophyletic clades corresponding to the two subgenera recognized by earlier workers. The phylogenetic placement of Equisetum bogotense, however, is ambiguous. In maximum likelihood analyses, it allies with subg. Hippochaete as the most basal member, while maximum parsimony places it as sister to the rest of the genus. A consensus phylogeny from the two analyses is presented as a basal trichotomy (E. bogotense, subg. Hippochaete, subg. Equisetum), and morphological character evolution is discussed. We detected rate heterogeneity in the rbcL locus between the two subgenera that can be attributed to an increased rate of nucleotide substitution (transversions) in subg. Hippochaete. We calculated molecular-based age estimates using the penalized likelihood approach, which accounts for rate heterogeneity and does not assume a molecular clock. The Equisetum crown group appears to have diversified in the early Cenozoic, whereas the Equisetaceae total group is estimated to have a Paleozoic origin. These molecular-based age estimates are in remarkable agreement with current interpretations of the fossil record.Item Open Access Spore studies in the genus Gymnocarpium(Canadian Journal of Botany, 1983-02-01) Pryer, Kathleen M; Britton, DMScanning electron microscopy was used to examine the variation in perispore characters within and between six currently recognized taxa of the genus Gymnocarpium Newm. in North America. Representative scanning electron micrographs are presented and depict the perispore features observed. Spores of those taxa studied here do not each possess a distinctive morphology providing practical species discrimination, rather they all exhibit similar patterns of variation. Spore size was also examined and shown to be correlated to ploidy level, permitting positive identification of the diploid taxon G. dryopteris ssp. disjunctum from the tetraploid G. dryopteris ssp. dryopteris.Item Open Access Studies on the Gymnocarpium robertianum complex in North America(Rhodora, 1981) Sarvela, J; Britton, DM; Pryer, K; Sarvela, J