Systematics of Green Coccoid Algae: the Genus Bracteacoccus
Green algae (Chlorophyta) are a morphologically heterogeneous group that is undergoing considerable revisions at present. Especially in coccoid genera, there have been striking cases of polyphyly, when species originally placed in one genus were shown to belong to up to three different classes. The coccoid chlorophycean genus Bracteacoccus Tereg was until recently considered monophyletic, but with the advent of new molecular data, it no longer appears as such.
The goal of my project is to monograph the genus Bracteacoccus. I collect 18S ribosomal DNA sequences (nuclear gene) as well as rbcL sequences (chloroplast, protein-coding gene). Phylogeny obtained from the sequence data can be subsequently used as a starting point for further research: well supported clades can be examined for defining traits. Like other coccoid genera, Bracteacoccus has very simple morphology and therefore few characters to be scored. Transmission electron microscopy may provide one or several taxonomically useful characters, but perhaps other type of traits will need to be found.
Some of the main questions involved in monographing Bracteacoccus are:
1) What is Bracteacoccus? I.e. how is the genus delimited? Is the original description informative enough? Provided that there is no live culture material associated with the description, how do we go about evaluating the molecular phylogeny that lacks the type species?
2) Are the currently described species of Bracteacoccus valid? Several of the type cultures cluster together in one clade, exhibiting little or no variation in either gene. Will more variable markers help solving this question?
3) What is the closest relative to Bracteacoccus? Among the "hottest" candidates, we already know it is not Dictyochloris. Is it Dictyococcus? If not, what does that mean in terms of informativeness of morphological traits used to distinguish them and group them together in older literature? Other genera have recently emerged as putative closest relatives: Radiococcus and Planktosphaeria. Will Bracteacoccus end up being a nomenclatural "black hole" or will it end up being split into several genera?
4) There are more than a hundred recent isolates of Bracteacoccus available from the Biotic Crust Project. Where do these and other additional strains appear on the phylogeny? Are they species new to science?