The complete mitochondrial genome of the Antarctic sea spider Ammothea carolinensis (Chelicerata; Pycnogonida)

Mitochondria are responsible for the oxidative phosphorylation process. Accordingly, putatively adaptive changes in their genomic features have been variously associated with major eco-physiological shifts in animal evolution, including increased metabolic rates and heat adaptation. Antarctic pycnogonids offer an interesting system to test whether the selective pressure for heat production and increased aerobic metabolism may be driving genomic changes like: (a) unusual compositional biases at the nucleotide and amino acid level, possibly related to cold adaptation; (b) an accelerated rate of mutations/genomic rearrangements, possibly related to the mutagenic effects of oxygen intermediates. The complete mitochondrial genome (mtDNA) of the Antarctic sea spider Ammothea carolinensis Leach, 1814 (Arthropoda: Pycnogonida), the type species for the genus Ammothea, has been determined and is here compared to known genomes from Antarctic and temperate species. We describe a marked heterogeneity in base composition skewness parameters as well as a strong signature of purifying selection toward an increase in thymines at second codon positions, possibly associated with an increased stability of hydrophobic inter-membrane domains. We further observe a fairly high rate of genomic changes, including a possible hot spot of recombination at the level of tRNA-Q. Nevertheless, these features do not seem to be restricted to the two Antarctic pycnogonids analyzed, as to suggest a causal relationship between cold adaptation and genomic changes, and are better interpreted as basal features shared by the entire group. The relevance of the newly determined sequence for the phylogeny of pycnogonids, including its base composition and genomic rearrangements, is further discussed.