A taxonomic study of snow Chloromonas species (Volvocales, Chlorophyceae) based on light and electron microscopy and molecular analysis of cultured material

Abstract:

Although zygote morphology is important in traditional taxonomic systems of snow-inhabiting species of Chloromonas (Volvocales, Chlorophyceae), induction of sexual reproduction or hypnospore formation is difficult in culture. Thus, alternative taxonomic methodologies are required when sexual reproduction does not occur in cultures. Here, we performed a taxonomic study of snow-inhabiting Chloromonas species based on culture strains with elongate or ellipsoidal vegetative cells. Our comparative light and electron microscopy demonstrated that the strains were clearly distinguished into six species based on differences in vegetative cell shape and chloroplast morphology, the number of zoospores within the parental cell and the presence or absence of cell aggregates in old cultures. The six species are Chloromonas chenangoensis, C. fukushimae sp. nov., C. hohamii, C. pichinchae, C. tenuis sp. nov. and C. tughillensis. Although four of the six species formed a small clade based on phylogenetic analysis of multiple genes, their separation was supported by comparison of secondary structures of the internal transcribed spacer 2 of nuclear ribosomal DNA and genetic differences of nuclear- and chloroplast-encoded genes. The present study demonstrated that polyphasic analyses of culture strains of snow Chloromonas species could be used for the identification of natural species.

Key Words::

• Chloromonadinia
• Chloromonas fukushimae
• Chloromonas tenuis
• Molecular phylogeny
• Morphology
• Snow algae
• Taxonomy
• Ultrastructure

ACKNOWLEDGEMENTS

We are grateful to Mr Tomofumi Shitara (Yamagata University) and Dr Kouji Yonekura (Tohoku University) for their kind help for collecting field sample of coloured snow at Mt. Gassan and Mt. Hakkoda, respectively. We thank Dr Takahiro Segawa (National Institute of Polar Research) for his valuable suggestions for establishing snow algal cultures. We also would like to thank Drs Akihiko Nakano, Takashi Ueda and Tomohiro Uemura (University of Tokyo) 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.), Scientific Research on Innovative Areas (No. 24112707 to H.N.) and Scientific Research (A) (No. 24247042 to H.N.) from MEXT/JSPS KAKENHI.

SUPPLEMENTARY DATA

Supplementary data associated with this article can be found online at http://dx.doi.org/10.2216/14-3.1.s1.