Chlorogonium complexum sp. nov. (Volvocales, Chlorophyceae), and morphological evolution of Chlorogonium

We isolated strains of a new species of Chlorogonium (Cg.), Cg. complexum Nakada sp. nov. The genus Chlorogonium was recently emended to have several, irregularly distributed contractile vacuoles, two-to-many pyrenoids associated with fragmented starch plates and tubular thylakoid lamellae penetrating their matrices, and stigma composed of a single layer of globules. Chlorogonium complexum has these diagnostic characters of the emended genus. It is distinguished from other species of Chlorogonium, Cg. euchlorum, Cg. elongatum and Cg. capillatum, on the combination of diagnostic characters, such as the number and stability of pyrenoids and the mode of encystment. Based on a dataset combining 18S rRNA, RuBisCO large subunit (rbcL) and P700 chl a-apoprotein A2 (psaB) gene sequences, the phylogenetic position of Chlorogonium was determined. Chlorogonium complexum represented an independent lineage, sister to Cg. euchlorum and Cg. capillatum. The evolution of diagnostic characters among Chlorogonium species is discussed based on the molecular phylogenetic analyses, and a key to species of Chlorogonium and fusiform chlamydomonads with irregularly distributed contractile vacuoles is presented.