Taxonomy of Desmodesmus serratus (Chlorophyceae, Chlorophyta) and related taxa on the basis of morphological and DNA sequence data

The green alga Desmodesmus serratus is a common small Desmodesmus species. The highly variable morphology among strains and natural populations suggested that this taxon was in need of revision using a combination of molecular and morphological characters. Thirty-six isolates that were assigned to D. serratus on the basis of morphology or that were known to be closely related to D. serratus on the basis of the results of DNA sequence analysis were examined. These isolates were evaluated by light microscopy, electron microscopy, and phylogenetic analysis of DNA sequences from the nuclear ribosomal internal transcribed spacer (ITS) regions, nuclear 5.8S ribosomal DNA, and the plastid rbcL gene. Results of the analyses of ITS and rbcL sequence data were congruent. On the basis of the results of these analyses, we propose the new species Desmodesmus itascaensis, Desmodesmus perdix, Desmodesmus serratoides, Desmodesmus pseudoserratus, and Desmodesmus santosii. Two of these species, D. perdix and D. pseudoserratus, possess morphologies identical to that of D. serratus. However, these species were all clearly separated on the basis of the results of DNA sequence analysis. The new species as well as D. serratus form a monophyletic lineage that is allied with the small, spineless species, Desmodesmus costato-granulatus, Desmodesmus lunatus, Desmodesmus elegans, Desmodesmus ultrasquamatus, and Desmodesmus regularis.