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

The green algal genusOncosaccus, endemic to China, has previously been classified in Chlamydomondales (Chlorophyceae). In the present study, two green algal specimens from west China were identified asOncosaccus tetrasporoidesdue 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 thatOncosaccusdid not have close affinities to the Chlamydomondales, but instead belonged to the order Chaetopeltidales in the family Chaetopeltidaceae, and had a close relationship withChaetopeltisandPseudulvella. The ultrastructure ofOncosaccus tetrasporoidesresembledChaetopeltisandPseudulvella, 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.

Automated summary

This study confirmed that the green algal genus Oncosaccus, endemic to China, actually belongs to the order Chaetopeltidales instead of Chlamydomondales, based on molecular data and ultrastructure analysis. The morphological features of Oncosaccus tetrasporoides were found to be similar to Chaetopeltis and Pseudulvella, supporting its placement within the Chaetopeltidales.