A taxonomic revision of Chloromonas reticulata (Volvocales, Chlorophyceae), the type species of the genus Chloromonas, based on multigene phylogeny and comparative light and electron microscopy

Abstract

Matsuzaki R., Hara Y. and NOZAKI H. 2012. A taxonomic revision of Chloromonas reticulata (Volvocales, Chlorophyceae), the type species of the genus Chloromonas, based on multigene phylogeny and comparative light and electron microscopy. Phycologia 51: 74–85. DOI: 10.2216/11-18.1

Chloromonas reticulata (Goroschankin) Gobi, the type species for Chloromonas, was taxonomically revised using nine strains previously assigned to the species, based on light and transmission electron microscopy and multigene phylogenetic analyses. Based on vegetative cell and papilla shape, eyespot form and position, and the ultrastructure of the eyespot and pyrenoid, the nine strains were classified into four species: C. reticulata; C. rosae (H. & O. Ettl) H. Ettl; C. chlorococcoides (H. Ettl & K. Schwarz) Matsuzaki, Y. Hara & Nozaki comb. nov.; and C. typhlos (Gerloff) Matsuzaki, Y. Hara & Nozaki comb. nov. The taxonomic revision was supported by the robust monophyly of the species resolved in phylogenetic analyses using a combined dataset that included 18S ribosomal (r) RNA, ATP synthase β-subunit, P700 chlorophyll a-apoprotein A1 and A2 genes, and nuclear rDNA internal transcribed spacer 2 (ITS2) sequences. The phylogenetic analyses demonstrated that the pyrenoid-lacking species C. reticulata and the pyrenoid-containing species C. chlorococcoides, which has a dense pyrenoid matrix associated with starch grains, formed a robust monophyletic group to which the pyrenoid-lacking species C. rosae and the pyrenoid-containing species C. typhlos were basal. Comparison of the C. reticulata and C. chlorococcoides nuclear rDNA ITS2 secondary structures revealed two compensatory base changes in the conserved helix III region, suggesting a separation between these sister species.

Key Words::

• Chloromonadinia
• Chloromonas
• Chloromonas chlorococcoides comb. nov.
• Chloromonas lineage
• Chloromonas typhlos comb. nov.
• Molecular phylogeny
• Morphology
• Taxonomy
• Ultrastructure
• Volvocales

ACKNOWLEDGEMENTS

We are grateful to Dr Thomas Pröschold (Culture Collection of Algae and Protozoa, Scottish Association for Marine Science) for his kind and scholar comments and advise on our earlier version of nuclear rDNA ITS2 secondary models. We also thank Dr Takashi Nakada (Institute for Advanced Biosciences, Keio University) for his kind support and valuable suggestions. This work was supported by Grants-in-Aid for Scientific Research (Nos. 20247032 and 22112505 to HN) from the Ministry of Education, Culture, Sports, Science and Technology, Japan.