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

Coccoid green algae of the Selenastraceae were investigated by means of lig ht microscopy, TEM, and 18S rRNA analyses to evaluate the generic concept i n 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 featur es were investigated, especially the arrangement of autospores in the mothe r 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 auto spores were arranged serially. Colony formation was either stable (Quadrigu la) 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 contras t to considerations of traditional systematics, the present study showed th at the presence and structure of pyrenoids are unsuitable for differentiati on of genera in Selenastraceae. Furthermore, the molecular analyses showed that any morphological criterion considered so far is not significant for t he systematics of the Selenastraceae on the generic level. Species assigned to different genera such as Ankistrodesmus and Monoraphidium were not mono phyletic and therefore not distinguishable as separate genera. Species of M onoraphidium appeared in four different lineages of the Selenastraceae. Our phylogenetic analyses support earlier discussions to abandon the common pr actice of conceiving "small" genera (i.e. genera that are differentiated fr om other genera by only a few diacritic characteristics and that contain on ly a small number of species) and to reestablish "large" genera of Selenast raceae such as Ankistrodesmus.