The evolution of mitogenomic architecture in Bilateria

We used a dataset comprising over ten thousand bilaterian mitogenomes to address multiple open questions regarding the evolution of mitogenomic architecture. Double-stranded architecture is the most likely ancestral state for all major radiations (phylum and above), aside from Kinorhyncha, Syndermata and Brachiopoda. We found over twenty transitions from double-stranded to single-stranded (all genes on a single strand) mitogenomic architectures, as well as the first evidence of single-stranded to double-stranded transitions: one in Annelida and two or more in Mollusca. Gene order rearrangement rates (GORR) were strongly positively correlated with sequence evolution rates (r=0.69, R2≈35%). Both variables were increased in single-stranded mitogenomes, parasitic species, and those with low locomotory capacity. The latter two categories also exhibited increased proportions of single-stranded mitogenomes. Contrary to this, mitogenome size exhibited a negative correlation with both GORR and sequence evolutionary rates. Purifying selection plays a major role in the mitogenomic architectural evolution, but other variables need to be invoked to fully explain the observed patterns.