Species of Bryopsidales form ecologically important components of seaweed communities worldwide. These siphonous macroalgae are composed of a single giant tubular cell containing millions of nuclei and chloroplasts, and harbor diverse bacterial communities. Little is known about the diversity of chloroplast genomes (cpDNAs) in this group, and about the possible consequences of intracellular bacteria on genome composition of the host. In a paper we recently published in BMC Genomics, we report on the complete cpDNAs of Bryopsis plumosa and Tydemania expeditionis, as well as a re-annotated cpDNA of B. hypnoides (which was shown to contain a higher number of genes than originally published).
The cpDNAs of B. plumosa and T. expeditionis are amongst the smallest and most gene dense chloroplast genomes in the core Chlorophyta, and both lack a large inverted repeat.
Both chloroplast genomes contain DNA regions with genes of bacterial origin, including rhs-family genes, and several genes involved in mobile functions, such as transposases, integrases, DNA polymerases, and phage/plasmid DNA primases. Another unexpected finding was the presence of two genes in T. expeditionis showing close similarity to bacterial DNA methyltransferases, a family of genes that have up till now not been found in any plastid genome.
The presence of bacterial genes, including genes typically found in mobile elements, suggest that these have been acquired through horizontal gene transfer, which may have been facilitated by the occurrence of obligate intracellular bacteria in these siphonous algae. Our data adds to the scarce knowledge of horizontal transfer of bacterial DNA to plastid genomes.
In addition, chloroplast genomic data were used to evaluate phylogenetic hypotheses in the Chlorophyta. These analyses, however, were largely inconclusive with respect to monophyly of the Ulvophyceae, and the relationship of the Bryopsidales within the Chlorophyta.