Phylogeny of Chloromonas (Chlorophyceae): A study of 18S ribosomal RNA gene sequences

The unicellular, biflagellate genus Chloromonas differs from its ally, Chlamydomonas, primarily by the absence of pyrenoids in the vegetative stage of the former. As with most green flagellate genera, little is known about phylogenetic affinities within and among Chloromonas species. Phylogenetic analyses of nuclear-encoded small-subunit ribosomal RNA gene sequences demonstrate that a sampling of five Chloromonas taxa, obtained from major culture collections, do not forma monophyletic group. However, only three of these isolates, Chloromonas clathrata, Chloromonas serbinowi, and Chloromonas rosae, are diagnosable morphologically as Chloromonas species by the absence of a pyrenoid in the vegetative state. The three diagnosable Chloromonas taxa form an alliance with two pyrenoid-bearing chlamydomonads, Chlamydomonas augustae and Chlamydomonas macrostellata. With the exception of Chloromonas serbinowi, which represents the basal lineage within the clade, each of the diagnosable Chloromonas taxa and their pyrenoid-bearing Chlamydomonas allies were isolated originally from mountain soils, snow, or cold peat. These observations suggest that habitat, independent of pyrenoid status, may be most closely linked to the natural history of this clade of chlamydomonad flagellates.