The systematics of coccoid green algae: 18S rRNA gene sequence data versus morphology

The systematics of coccoid green algae is going through a dramatic phase of change. Classical approaches using morphological characters for the circumscription of taxa do not adequately reflect the phylogenetic relationships. Molecular data provide new insights into the phylogeny of coccoid Chlorophyta. Whereas the content of the orders Chlorococcales and Chlorellales needs to be reduced, the order Sphaeropleales comprises more families than in the traditional circumscription. According to molecular biological investigations the Neochloridaceae, Selenastraceae and Scenedesmaceae, formerly classified within the Chlorococcales and later within the Chlorellales, belong to the Sphaeropleales. The Hydrodictyaceae, formerly a member of the Protosiphonales, axe also to be classified in the Sphaeropleales. Some former members of the Ulotrichales, which propagate by binary fission, cluster together with autosporine coccoid taxa within the Chlorellales, Trebouxiophyceae. In this study, by presenting six new 18S rRNA gene sequences of chlorophytes we scroll through a phylogenetic tree that contains the main evolutionary lineages of the Chlorophyta, and we take up different topics of current interest for green algal systematists. (1) Chlorococcum robustum, formerly a member of the Chlorococcales s. str. is shown to be closely related to Chlamydopodium vacuolata; both belong to the Chlamydomonadales. (2) Pectodictyon pyramidale, formerly grouped with the Radiococcaceae, is closely related to Coelastrum reticulatum and both, like other coelastracean taxa, belong to the Scenedesmaceae, Sphaeropleales. (3) Pseudodietyosphaerium jurisii is a member of the Mychonastes clade containing picoplanktonic taxa. This clade is sister group to the Oedogoniales at the base of the Chlorophyceae. A new order will need to be established eventually to accommodate this clade. Whereas the position of (4) Lagerheimia genevensis within the Oocystaceae was confirmed, the former crucigenoid species (5) Cracigeniella rectangularis is to be transferred to this family. The latter two taxa demonstrate that the morphological diversity in the Oocystaceae is wider than expected. This observation is discussed in comparison to the family Chlorellaceae. (6) Catena viridis a former member of the Ulotrichales belongs to the Chlorellales, Trebouxiophyceae.