Oncosaccus: a rare green alga endemic to China belongs to Chaetopeltidales (Chlorophyceae, Chlorophyta)

The green algal genus Oncosaccus, endemic to China, has previously been classified in Chlamydomondales (Chlorophyceae). In the present study, two green algal specimens from west China were identified as Oncosaccus tetrasporoides due to their unique colonial form with numerous cells embedded in a thick gelatinous envelope without perforations and pseudocilia. Molecular data from two nuclear loci (18S rDNA and ITS) were used to test the hypothesised membership of this taxon. The molecular data clearly showed that Oncosaccus did not have close affinities to the Chlamydomondales, but instead belonged to the order Chaetopeltidales in the family Chaetopeltidaceae, and had a close relationship with Chaetopeltis and Pseudulvella. The ultrastructure of Oncosaccus tetrasporoides resembled Chaetopeltis and Pseudulvella, and adjacent cells lacked plasmodesmata, and the pyrenoid matrix was traversed by a cytoplasmic channel instead of the thylakoid membrane were present. This morphology supported the molecular data and confirmed this group’s placement in the Chaetopeltidales. The present study expanded our understanding of the morphological features of green algae, particularly those of the poorly studied Chaetopeltidales. Nonetheless, a phylogenetic re-evaluation based on large samples in conjunction with molecular analyses is needed.