Molecular phylogenetic relationship between two symbiotic photo-oxygenic prokaryotes, Prochloron sp. and Synechocystis trididemni

Abstract

Synechocystis trididemni is a photo-oxygenic prokaryotic symbiont in some didemnid ascidians. Although the cells are pink and contain phycoerythrin, they morphologically resemble those of another symbiont of other didemnids, Prochloron didemni, which has no phycobilin pigments but has both chlorophyll a and chlorophyll b. The cytological similarities suggest that they might have evolved from a common ancestor. We examined this hypothesis by comparing nucleotide sequences in gene fragments of the 16S ribosomal RNA (16S rRNA) gene and in the large subunit of Rubisco gene (rbcL). ‘Blast’ search of GenBank showed that the most similar sequences to the 16S rRNA gene and rbcL of S. trididemni were those of Prochloron sp. The percent nucleotide and amino acid identities of the 16S rRNA genes, rbcL and the deduced Rubisco amino acid sequences between Prochloron sp. and S. trididemni were 94%, 90% and 99%, respectively. In trees based on the 16S rRNA gene and rbcL sequences, S. trididemni formed a clade with Prochloron. These data thus support the hypothesis that Prochloron sp. and S. trididemni arose from a common cyanobacterial ancestor and that the ancestor of Prochloron sp. somehow acquired the ability to synthesize chlorophyll b, as well as chlorophyll a, and subsequently lost the ability to synthesize phycobiliproteins.