Morphological diversity masks phylogenetic similarity of Ettlia and Haematococcus (Chlorophyceae)

Abstract:

Ettlia is a green algal genus comprised of several economically important species that have contributed to a taxonomic nightmare for phycologists – the recognition that coccoid green algae are polyphyletic on a grand scale. This taxonomic confusion is slowly being resolved with the aid of molecular evidence and, in some cases, new structural data. The most recent investigation using 18S ribosomal (r)DNA data corroborated an extraordinarily close alliance between Haematococcus pluvialis, a predominantly flagellate unicell, and the coccoid alga, Ettlia carotinosa. This putative evolutionary relationship between the green algal species H. pluvialis and E. carotinosa has, once again, placed coccoid algae at the center of systematic debate. The only apparent similarity between these two species is the ability to produce astaxanthin. Because the 18S rDNA results were not particularly robust, additional data from the internal transcribed spacer 2 rRNA and 26S rRNA genes were collected. Results from analyses of these data corroborated a close alliance between E. carotinosa and H. pluvialis. In addition, the 18S rDNA data set was expanded using new, published data from other species of Ettlia. Phylogenetic analyses of this updated matrix confirmed the existence of at least four distinct Ettlia lineages. These observations indicated that (1) additional taxonomic revision of the genus Ettlia is needed, (2) E. carotinosa need not be merged into the genus Haematococcus, (3) the case of H. pluvialis and E. carotinosa is an example of cryptic similarity, and (4) the extraordinarily close alliance between E. carotinosa and H. pluvialis offers an ideal system to study patterns of gene expression and morphogenesis in unicellular eukaryotes.

Key Words::

• Cryptic similarity
• Ettlia
• Haematococcus
• ITS2
• Phylogeny
• 18S rDNA
• 26S rDNA
• Sequence structure

ACKNOWLEDGEMENTS

We thank Ian Bellovich (Tulsa Undergraduate Research Challenge Junior Scholar, summer 2013) and Joe Kuiper (independent study student, spring 2014) for assistance with DNA extractions and PCR reactions. We thank Marsha Howard, Rashmi Mohanty and Jessie Myers for offering comments on early drafts of the manuscript. We thank Tobias Müller and Christian Koetschan for assistance using the statistical framework R. Financial support for this study was provided by the Deutsche Forschungsgemeinschaft (Mu-2831/1-1 to MW) and the US National Science Foundation (MCB-0132083 and DEB-0129030 to MAB). Additional support from the Office of Research at the University of Tulsa (CP and MB) and the Department of Biological Science (MB) is gratefully acknowledged.

SUPPLEMENTARY DATA

Supplementary data associated with this article can be found online at http://dx.doi.org/10.2216/15-015.1.s1.