Highly conserved complete organellar genomes of Bangia atropurpurea (Bangiales, Rhodophyta) revealed by refined annotation

Bangia atropurpurea is the only freshwater species in the ancient order Bangiales, which dates back to approximately 1,047 Mya. It was speculated that B. atropurpurea originated from a marine ancestor. However, the timing and mechanism by which this species transitioned from the marine to the freshwater environment remain unclear. Genomic data could shed light on these problems. We provided the complete organellar genomes of a sample of B. atropurpurea from China in the present study. The gene content, genomic organization, tRNA secondary structures and codon usage of the organellar genomes were analyzed. The gene annotations were manually refined. Comparative genomics revealed that the chloroplast genomes in Bangiaceae are highly conserved, whereas the mitochondrial genomes are relatively less conserved. The factors and probable mechanisms that affect the genome were discussed. Genes unique to freshwater or filamentous species could be interesting candidates for the next step of adaptive evolution research. The phylogeny based on genomic data provides solid evidence to split Pyropia into four genera.