KRIENITZ, LOTHAR1*, MATTHIAS WOLF1, EBERHARD HEGEWALD2, and DOMINIK HEPPERLE1,3. 1Leibniz-Institute of Freshwater Ecology and Inland Fisheries, D-16775 Stechlin-Neuglobsow, Germany; 2Institute of Chemistry and Dynamics of the Geosphere, Research Centre Juelich, D-52425 Juelich, Germany; 3Albrecht-von-Haller-Institute of Plant Sciences, University of Goettingen, D-37037 Goettingen, Germany. - Systematics of coccoid green algae: morphology versus 18S rRNA gene phylogeny.
Classical approaches using morphological characters for the
circumscription of taxa in coccoid green algae do not adequately
reflect the phylogenetic relationships. Phylogenetic trees based on
18S rRNA gene sequence analysis provide new insights into the
systematics of these algae. We give a review and discuss the content
of orders of coccoid green algae and evaluate the usefulness of some
traditionally used diacritic features. 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
in the Chlorococcales and later into the Chlorellales belong to the
Sphaeropleales. The Hydrodictyaceae, formerly a member of the
Protosiphonales, are also to be grouped in the Sphaeropleales. Some
members of the Ulotrichales, which propagate by binary fission (e.g.
Nannochloris and Catena) cluster together with autosporine coccoid
taxa within the Trebouxiophyceae. With the example of members of the
Selenastraceae, Oocystaceae and Chlorellacea it is shown that the
morphology of cells as well as the formation of coenobia, spines,
mucilage and incrustations cannot be used to separate genera.
Key words: 18S rDNA, Chlorophyceae, coccoid green algae, molecular phylogeny, morphology, Trebouxiophyceae