Mitogenomes reveal alternative initiation codons and lineage-specific gene order conservation in echinoderms

The mitochondrial genetic code is much more varied than the standard genetic code. The invertebrate mitochondrial code, for instance, comprises six initiation codons, including five alternative start codons. However, only two initiation codons are known in the echinoderm and flatworm mitochondrial code, the canonical ATG and alternative GTG. Here, we analysed 23 Asteroidea mitogenomes, including 10 newly-sequenced species, and unambiguously identified at least two other start codons, ATT and ATC, both of which also initiate translation of mitochondrial genes in other invertebrates. These findings underscore the diversity of the genetic code and expand upon the suite of initiation codons among echinoderms to avoid erroneous annotations. Our analyses have also uncovered the remarkable conservation of gene order among asteroids, echinoids and holothuroids, with only an interchange between two gene positions in asteroids over approximately 500 million years of echinoderm evolution.