A taxonomic and phylogenetic study of Nephroselmis Stein (Prasinophyceae, Chlorophyta)

Abstract

The Prasinophycean genus Nephroselmis Stein is a group of largely marine, photosynthetic,scaly biflagellates which has undergone several taxonomic changes since its erection in 1878. The taxonomy and phylogeny of the genus was investigated in the present study using separate and combined morphological and molecular maximum parsimony cladistic analyses. New data from light and electron microscopy were combined with data from the literature. Partial 18S gene molecular data from fresh material and from GenBank were included. Seven species of Nephroselmis were confirmed as discrete entities: N. anterostigmatica, N. astigmatica, N. olivacea, N. pyriformis, N. rotunda, N. spinosa and N. viridis. The existing generic and specific descriptions are emended. Samples of N. fissa, N. gaoae, N. marina and N. minuta could not be sourced from culture collections or opportunistic sampling, and as no detailed and reliable electron microscopic work is available for these species, they were not included and could not be confirmed by this study. The cladistic analyses undertaken in this study confirmed that the genus Nephroselmis is monophyletic. Reweighted morphological cladistic analysis yielded distinct clades within the genus, grouped predominantly by body scale number and scale complexity. Partial 18S molecular cladistic analysis agreed well with the morphological analysis, as did the combined analysis. N. pyriformis, which shows the greatest number of plesiomorphic features and possesses the simplest scale morphology, was placed in a basal position in all three analyses. The greatest scale complexity is found in N. anterostigmatica and N. astigmatica, which occured as sister species in all three analyses. Eyespot modifications are found in these two species only: N. astigmatica lacks an eyespot and the eyespot of N. anterostigmatica is atypically located on the anterior surface of the cell. N. olivacea and N. viridis show intermediate scale complexity and were placed as sister species in all three analyses. The relationship between N. rotunda and N. spinosa varied between the three analyses. The reweighted morphological cladistic analysis placed N. rotunda, which has three body scale layers, in its own clade, between N. pyriformis, which has two body scale layers, and the clade consisting of all other Nephroselmis species, which possess four or five body scale layers. The partial 18S molecular cladistic analysis grouped N. rotunda and N. spinosa in a clade sister to the N. olivacea / N. viridis clade. The combined analysis grouped N. rotunda and N. spinosa in a clade sister to the remaining Nephroselmis species. The only freshwater species, N. olivacea, occured in a clade with N. viridis (in a more derived position), rather than in a basal position or alone in a clade. This is consistent with N. olivacea having adapted to a freshwater environment as a result of, for example, a marine overwash event. Of the nine other genera included in the molecular analysis, Dolichomastix, Mamiella, Pseudoscourfieldia and Tetraselmis appeared as the sister clade of Nephroselmis.