Traditional generic concepts versus 18S rRNA gene phylogeny in the green algal family Selenastraceae (Chlorophyceae, Chlorophyta)

Abstract

Coccoid green algae of the Selenastraceae were investigated by means of light microscopy, TEM, and 18S rRNA analyses to evaluate the generic concept in this family. Phylogenetic trees inferred from the 18S rRNA gene sequences showed that the studied species of autosporic Selenastraceae formed a well-resolved monophyletic clade within the DO group of Chlorophyceae. Several morphological characteristics that are traditionally used as generic features were investigated, especially the arrangement of autospores in the mother cells, colony formation, and pyrenoid structure. The parallel arrangement of autospores was confirmed for the genera Ankistrodesmus, Podohedriella, and Quadrigula. In mother cells of Monoraphidium and Kirchneriella the autospores were arranged serially. Colony formation was either stable (Quadrigula) or variable (Ankistrodesmus, Podohedriella) within genera. All strains studied possessed naked or starch-covered pyrenoids within the chloroplast. The pyrenoid matrix was homogenous or penetrated by thylakoids. In contrast to considerations of traditional systematics, the present study showed that the presence and structure of pyrenoids are unsuitable for differentiation of genera in Selenastraceae. Furthermore, the molecular analyses showed that any morphological criterion considered so far is not significant for the systematics of the Selenastraceae on the generic level. Species assigned to different genera such as Ankistrodesmus and Monoraphidium were not monophyletic and therefore not distinguishable as separate genera. Species of Monoraphidium appeared in four different lineages of the Selenastraceae. Our phylogenetic analyses support earlier discussions to abandon the common practice of conceiving "small" genera (i.e. genera that are differentiated from other genera by only a few diacritic characteristics and that contain only a small number of species) and to reestablish "large" genera of Selenastraceae such as Ankistrodesmus.