Widespread atypical mitochondrial DNA structure in isopods (Crustacea, Peracarida) related to a constitutive heteroplasmy in terrestrial species

Metazoan mitochondrial DNA (mtDNA) is generally composed of circular monomeric molecules. However, a few exceptions do exist and among them two terrestrial isopods Armadillidium vulgare and Porcellionides pruinosus have an atypical mtDNA composed of linear monomers associated with circular head-to-head dimers: a very unusual structure for animal mtDNA genome. To assess the distribution of this atypical mtDNA among isopods, we performed RFLP and Southern blot analyses on mtDNA of 16 terrestrial (Oniscidea family) and two aquatic isopod species: the marine Sphaeroma serratum (suborder Flabellifera, sister group of Oniscidea) and the freshwater Asellus aquaticus (Asellota, early derived taxon of isopod). The atypical mtDNA structure was observed in 15 terrestrial isopod species and A. aquaticus, suggesting a wide distribution of atypical mtDNA among isopods. However, a typical metazoan mtDNA structure was detected in the marine isopod S. serratum and the Oniscidea Ligia oceanica. Our results suggest two possible scenarios: an early origin of the atypical mtDNA in isopods followed by reversion to the typical ancestral mtDNA structure for several species, or a convergent appearance of the atypical mtDNA structure in two isopod suborders. We compare this distribution of the atypical mtDNA structure with the presence of a heteroplasmy also observed in the mtDNA of several terrestrial isopod species. We discuss if this transmitted heteroplasmy is vectored by the atypical mtDNA and its impact on the maintenance of the atypical mtDNA in isopods.