Xylochloris irregularis gen. et sp. nov. (Trebouxiophyceae, Chlorophyta), a novel subaerial coccoid green alga

The phylogenetic diversity of subaerial coccoid green algae remains still poorly explored. We characterised in detail two unicellular green algae found on tropical trees in Singapore. Light microscopy revealed morphological identity of these two strains. Depending on the age of cultures, the cells were spherical to cylindrical, and ranged in size from 13.5 to 20.5 mm. Each cell contained a pyrenoid-bearing parietal chloroplast that was typically somewhat detached from the plasma membrane on its parietal side. The cells reproduced by 4–16 globular autospores. The 18S rRNA gene sequences of the two strains differed by only a single nucleotide, indicating probable conspecificity. Because the strains were morphologically most comparable to species of the genus Parietochloris, we determined the 18S rRNA gene sequences from authentic strains of three Parietochloris species (P. alveolaris, P. cohaerens and P. ovoidea) for comparison. Molecular phylogenetic analyses placed all five examined strains into the class Trebouxiophyceae. The two novel tropical strains were found to be an independent lineage without an obvious sister group. The type species of the genus Parietochloris, P. alveolaris formed a monophyletic lineage with Parietochloris pseudalveolaris. Finally, P. cohaerens and P. ovoidea fell into another independent clade that also contained Lobosphaera tirolensis, L. incisa and Myrmecia bisecta, indicating that the genus Parietochloris as previously defined is polyphyletic. Based on our morphological and molecular phylogenetic data, we describe the two novel tropical strains as representatives of a new trebouxiophycean genus and species, Xylochloris irregularis gen. et sp. nov.