The patchy distribution of the photosynthetic gene cluster in Erythrobacteraceae

Abstract

The photosynthetic gene cluster which encodes the ability to engage in aerobic anoxygenic photosynthesis is found in multiple clades of Erythrobacteraceae, but whether this patchy distribution was caused by horizontal operon transfer, secondary loss, or a mixed regime of transfer and loss has not previously been answered. We use whole genome sequencing and comparative genomic and phylogenetic methods to address these questions. Two new genomes for Erythrobacteraceae spp. isolates, Altererythrobacter sp. DSM 24483 and Erythrobacter sp. DSM 25594, were sequenced, assembled, and annotated. Despite their disparate sampling locations, these genomes are found to be highly similar, differing chiefly in the presence of four genomic islands unique to DSM 24483. A species phylogeny for 91 members of Erythrobacteraceae, including the two new genomes, was generated from whole genome data, expanding the most recently published phylogeny for this family by 17 species. Results of the phylogenetic analysis indicate that all species which both possess the photosynthetic gene cluster and express bacteriochlorophyll a in culture form one clade, corresponding to the Erythrobacter genus. Finally, two hypotheses about the evolution of phototrophy in Erythrobacteraceae are tested. Two methods of ancestral state reconstruction and gene order data are leveraged to determine whether secondary loss, horizontal operon transfer, or a mixed regime of transfer and loss explains the patchy distribution of the photosynthetic gene cluster in Erythrobacteraceae. Horizontal operon transfer is found to be the best explanation for the evolution of the patchy distribution of the photosynthetic gene cluster in Erythrobacteraceae.