Revisiting the taxonomic significance of aplanozygote morphologies of two cosmopolitan snow species of the genus Chloromonas (Volvocales, Chlorophyceae)

Abstract:

Chloromonas brevispina and C. nivalis (Volvocales, Chlorophyceae) are considered cosmopolitan, as the aplanozygotes assignable to the two species are distributed worldwide. Although recent phylogenetic analyses have indicated the possible polyphyly of aplanozygotes identified as C. nivalis, the analytical resolution was not robust, possibly because only single genes were analysed. Here, we obtained long sequences of multiple DNA regions from 50 identical [on the basis of light microscopy (LM)] aplanozygotes in a field-collected sample, and the phylogenetic positions of these aplanozygotes (identified as C. brevispina and C. nivalis on the basis of LM) were determined with high statistical support using a multigene phylogeny. Comparisons of the present and previously published sequence data indicated that aplanozygotes of C. nivalis originating from Japan and Austria represented at least four different lineages, all of which were robustly separated from the North American strain CU563D of C. nivalis. Molecular analyses including the comparison of highly evolving nuclear ribosomal DNA internal transcribed spacer 2 demonstrated that one Japanese lineage of C. nivalis aplanozygotes was conspecific with C. miwae strains. In addition, the present C. brevispina aplanozygote specimens from Japan were assigned to C. krienitzii sp. nov., which was herein delineated on the basis of LM and electron microscopy and the molecular phylogeny of newly established strains.

Key Words::

• Chloromonas brevispina
• Chloromonas krienitzii sp. nov.
• Chloromonas nivalis
• Molecular phylogeny
• Morphology
• Snow algae
• Ultrastructure

ACKNOWLEDGEMENTS

We are grateful to Dr Koji Yonekura (Tohoku University, Japan) for his kind help for collecting field samples of coloured snow at Mt Hakkoda. We thank to Drs Akihiko Nakano, Takashi Ueda and Tomohiro Uemura (University of Tokyo, Japan) and members of their laboratory for permitting us to use their cold room. This work was supported by Grants-in-Aid for JSPS Fellows (No. 24·2217 to R.M.), Challenging Exploratory Research (No. 24657045 to H.N.) and Scientific Research (A) (No. 24247042 to H.N.) from MEXT/JSPS KAKENHI.