title: Mitochondrial genomes of the Baltic clam Macoma balthica (Bivalvia: Tellinidae): setting the stage for studying mito-nuclear incompatibilities.
abstract: BACKGROUND
  Allopatric divergence across lineages can lead to post-zygotic reproductive isolation upon secondary contact and disrupt coevolution between mitochondrial and nuclear genomes, promoting emergence of genetic incompatibilities. A previous F ST scan on the transcriptome of the Baltic clam Macoma balthica highlighted several genes potentially involved in mito-nuclear incompatibilities (MNIs). As proteins involved in the mitochondrial oxidative phosphorylation (OXPHO) chain are prone to MNIs and can contribute to the maintenance of genetic barriers, the mitochondrial genomes of six Ma. balthica individuals spanning two secondary contact zones were sequenced using the Illumina MiSeq plateform.
  
  
  RESULTS
  The mitogenome has an approximate length of 16,806 bp and encodes 13 protein-coding genes, 2 rRNAs and 22 tRNAs, all located on the same strand. atp8, a gene long reported as rare in bivalves, was detected. It encodes 42 amino acids and is putatively expressed and functional. A large unassigned region was identified between rrnS and tRNA (Met) and could likely correspond to the Control Region. Replacement and synonymous mutations were mapped on the inferred secondary structure of all protein-coding genes of the OXPHO chain. The atp6 and atp8 genes were characterized by background levels of replacement mutations, relative to synonymous mutations. However, most nad genes (notably nad2 and nad5) were characterized by an elevated proportion of replacement mutations.
  
  
  CONCLUSIONS
  Six nearly complete mitochondrial genomes were successfully assembled and annotated, providing the necessary roadmap to study MNIs at OXPHO loci. Few replacement mutations were mapped on mitochondrial-encoded ATP synthase subunits, which is in contrast with previous data on nuclear-encoded subunits. Conversely, the high population divergence and the prevalence of non-synonymous mutations at nad genes are congruent with previous observations from the nuclear transcriptome. This further suggest that MNIs between subunits of Complex I of the OXPHO chain, coding for NADH dehydrogenase, may play a role in maintaining barriers to gene flow in Ma. balthica.
authors.affiliation: Littoral, Environnement et Sociétés, UMR 7266 CNRS, Université de La Rochelle, 2 rue Olympe de Gouges, La Rochelle, 17000, France. saunier.alice@gmail.com.
authors.forename: Alice
authors.initials: A
authors.lastname: Saunier
authors.affiliation: Littoral, Environnement et Sociétés, UMR 7266 CNRS, Université de La Rochelle, 2 rue Olympe de Gouges, La Rochelle, 17000, France. pascale.garcia@univ-lr.fr.
authors.forename: Pascale
authors.initials: P
authors.lastname: Garcia
authors.affiliation: Littoral, Environnement et Sociétés, UMR 7266 CNRS, Université de La Rochelle, 2 rue Olympe de Gouges, La Rochelle, 17000, France. vanessa.becquet-buren@univ-lr.fr.
authors.forename: Vanessa
authors.initials: V
authors.lastname: Becquet
authors.affiliation: GeT-PlaGe, Genotoul, INRA Auzeville, Castanet-Tolosan, 31326, France. nmarsaud@insa-toulouse.fr.
authors.forename: Nathalie
authors.initials: N
authors.lastname: Marsaud
authors.affiliation: GeT-PlaGe, Genotoul, INRA Auzeville, Castanet-Tolosan, 31326, France. frederic.escudie@toulouse.inra.fr.
authors.forename: Frédéric
authors.initials: F
authors.lastname: Escudié
authors.affiliation: Littoral, Environnement et Sociétés, UMR 7266 CNRS, Université de La Rochelle, 2 rue Olympe de Gouges, La Rochelle, 17000, France. pante.eric@gmail.com.
authors.forename: Eric
authors.initials: E
authors.lastname: Pante
mesh_terms: D000818:Animals; D049872:Bivalvia / Q000166:cytology* / Q000235:genetics*; D002467:Cell Nucleus; D021901:DNA, Intergenic; D019143:Evolution, Molecular; D054629:Genome, Mitochondrial; D008565:Membrane Proteins / Q000235:genetics; D008928:Mitochondria; D058977:Molecular Sequence Annotation; D012335:RNA, Ribosomal / Q000235:genetics; D012343:RNA, Transfer / Q000235:genetics
chemical_list: D021901:DNA, Intergenic; D008565:Membrane Proteins; D012335:RNA, Ribosomal; D012343:RNA, Transfer