Taxonomic complexity analysis of the Chlorella clade (Chlorellaceae)

Abstract

In this study, 253 of 18S rDNA sequences, 155 of ITS2 sequences, 63 of rbcL sequences, and 23 of tufA sequences were collected on GenBank to evaluate and analyse the taxonomic complexity of the Chlorella clade. C. vulgaris and C. sorokiniana had the most collected sequences of DNA barcoding. Analysis of phylogenetic tree results showed that the strains of these two species were divided into 2 to 3 small groups with genetic differences. In addition, different species belonging to the same clade in phylogenetic trees were observed. For example, Actinastrum hantzschii, C. chlorelloides, C. singularis, C. sorokiniana, C. thermophila, C. volutis, C. vulgaris, Micractinium pusillum, and M. reisseri belonged to the same group of the 18S rDNA phylogenetic tree. This showed the complexity in the taxonomy of the Chlorella clade came not only from the cryptic species, but also from the genetic relationships of the remaining species. Therefore, identifying and determining the relationship of Chlorella clade species have been still challenging, because of the complexity of the database on GenBank as well as the incomplete taxonomic system. Clarification of the genetic relationship is needed to improve the taxonomy and identification of new isolates of the Chlorella clade.