• cell wall structure;
  • compensatory base change;
  • Desmodesmus;
  • ITS2 rDNA;
  • new species;
  • secondary structure;
  • species delimitation

Desmodesmus species taxonomy is one of the most long-standing issues in green microalgal systematics due to problems associated with phenotypic plasticity. Whereas more recent species descriptions and identifications are mainly based on cell wall structures and the use of cultures, comparisons with molecular phylogenies are largely lacking. In this study, the phylogenetic relationships between 22 clones identified as Desmodesmus costato-granulatus (Skuja) E. H. Hegew. were assessed using ITS2 rDNA sequence data in combination with cell wall morphology. The unrooted ITS2 phylogeny showed that the clones cluster into five groups, which also differ in their cell wall structures. Therefore, the taxon is split into five species: D. costato-granulatus, D. elegans, D. fennicus, D. regularis, and D. ultrasquamatus. Compared with other Desmodesmus species, intraspecific sequence variation is extensive and may contain additional (pseudo)cryptic diversity. Compensatory base changes were near-absent within the species and varied from one to 11 between species. Relationships among the species were unresolved. Despite this, they clustered together with the two other Desmodesmus species having a combination of small and large warts in a well-supported lineage. Remarkably, ITS2 sequence variation in this lineage is as high as between all other included Desmodesmus species, even though the morphology of its members is rather uniform.