Monograph of the Genus Bracteacoccus (Sphaeropleales, Chlorophyceae), with Systematic Clarification of Taxonomically Related Genera

Abstract

Green algae are ubiquitous, abundant, and ecologically important worldwide, and therefore an accurate understanding of their biodiversity and evolutionary relationships is of great interest to biologists. This systematic study focused on a group of morphologically simple unicellular algae from the order Sphaeropleales (Chlorophyceae, Chlorophyta), specifically four genetically divergent genera that share a similar vegetative morphology: Bracteacoccus, Chromochloris, Dictyococcus, and Pseudomuriella . This study employed phylogenetic analyses of nuclear ribosomal loci (18S, 5.8S and ITS2) as well as two plastid protein-coding genes ( rbcL and tufA) and one mitochondrial protein-coding gene (cox1) to distinguish among the morphologically similar genera and species within them using the phylogenetic species concept. Contrary to the expectation formulated historically in the literature, the genus Dictyococcus was found to be distantly related to Bracteacoccus and this finding was supported by light microscope observations that revealed that Dictyococcus can be identified by its chloroplast morphology. On the other hand, Pseudomuriella and the hereby resurrected genus Chromochloris were found to be close relatives of Bracteacoccus and morphologically indistinguishable from it and one another using standard microscopic means, providing an example of cryptic genera. Species-level diversity within Bracteacoccus and Pseudomuriella was assessed, and by adding a number of newly isolated strains the current knowledge about the diversity and geographic distribution of these genera was greatly increased. Furthermore, the utility of four putative barcode markers was investigated for recognizing the newly revealed diversity within the previously monotypic genus Pseudomuriella. Based on the phylogenetic findings, new species were also described within Bracteacoccus and an exhaustive revision of the genus is presented in form of a monograph.