A revision of the species Desmodesmus perforatus and D. tropicus (Scenedesmaceae, Chlorophyceae, Chlorophyta)

Abstract

S.L. Jeon and E. Hegewald. 2006. A revision of the species Desmodesmus perforatus and D. tropicus (Scenedesmaceae, Chlorophyceae, Chlorophyta). Phycologia 45: 567–584. DOI: 10.2216/05-63.1

We have made a synopsis of the morphology, ultrastructure, taxonomy, distribution and genetics of two widely distributed but little known species because this is important for the general understanding of species delineation in green algae. The taxa Desmodesmus perforatus and D. tropicus have in common central gaps between coenobial cells, but otherwise according to the type descriptions and type illustrations, they are morphologically well defined and easy to distinguish. However, in screening the literature, the situation becomes confusing and a clear delineation seems to be impossible. To solve the question of whether the two species are in fact different, or extremes of a single morphologically variable taxon, we isolated strains from different localities worldwide as well as strains of the related taxon D. maximus. The strains were studied under both optical and electron microscopes (EM) and the ITS-2r DNA of available strains was compared. With the results of the studied strains, and a comparison with natural populations, we could delimit the taxa D. perforatus var. perforatus, D. perforatus var. iberaënsis and D. tropicus and revise the literature data. Under the optical microscope, populations are clearly distinguishable, although single specimens may not be. According to ITS-2 rDNA all strains of D. perforatus (except Hegewald 1998–18) and D. tropicus differ by 10 bases and D. perforatus var. perforatus and D. perforatus var. iberaënsis by 2 bases. Regarding the compensatory base changes (CBC), there are zero exchanges between D. perforatus and D. tropicus (except strain Hegewald 1998–18) but D. tropicus and D. perforatus var. iberaënsis have one CBC. A tepwise xchange of bases was shown for CBCs. Other accepted taxa are D. perforatus f. bicaudatus (Compère) E. Hegewald, D. tropicus var. longiclathratus (Tell) S.L. Jeon & E. Hegewald and D. perforatus var. mirabilis (Massjuk) E. Hegewald. A sister group to D. perforatus and D. tropicus is D. maximus. This taxon is similar in EM cell wall structure to D. perforatus and D. tropicus, but has no holes between the cells.

Key Words::

• Compensatory base change (CBC)
• Cultures
• Desmodesmus maximus
• D. perforatus
• D. perforatus var. iberaënsis (Tell) E. Hegewald & S.L. Jeon comb. nov.
• D. perforatus var. mirabilis (Massjuk) E. Hegewald comb. nov.
• D. perforatus f. bicaudatus
• D. tropicus
• D. tropicus var. longiclathratus (Tell) S.L. Jeon & E. Hegewald comb. nov.
• Distribution
• ITS-2
• Morphology
• Phylogeny
• Taxonomy

ACKNOWLEDGMENTS

For the excellent TEM studies we thank Prof. E. Schnepf and Mrs. G. Deichgräber, Heidelberg, Germany, and also Mrs. J. Carter-Sigglow, MA, for proofreading the English. For strains we thank the curators of ACOI, Portugal, CCAP, UK, and Y. Tsukii, Japan.