title: Comparison of whole mitochondrial genome sequences from two clades of the invasive ascidian, Didemnum vexillum.
abstract: The mitochondria are the main source of cellular energy production and have an important role in development, fertility, and thermal limitations. Adaptive mitochondrial DNA mutations have the potential to be of great importance in determining aspects of the life history of an organism. Phylogenetic analyses of the globally invasive marine ascidian Didemnum vexillum using the mitochondrial cytochrome c oxidase 1 (COX1) coding region, revealed two distinct clades. Representatives of one clade (denoted by 'B') are geographically restricted to D. vexillum's native region (north-west Pacific Ocean, including Japan), whereas members of the other clade (denoted by 'A') have been introduced and become invasive in temperate coastal areas around the world. Persistence of clade B's restricted distribution may reflect it being inherently less invasive than clade A. To investigate this we sought to determine if the two clades differ significantly in other mitochondrial genes of functional significance, specifically, alterations in amino acids encoded in mitochondrial enzyme subunits. Differences in functional mitochondrial genes could indicate an increased ability for clade A colonies to tolerate a wider range of environmental temperature. Full mitochondrial genomic sequences from D. vexillum clades A and B were obtained and they predict significant sequence differences in genes encoding for enzymes involved in oxidative phosphorylation. Diversity levels were relatively high and showed divergence across almost all genes, with p-distance values between the two clades indicating recent divergence. Both clades showed an excess of rare variants, which is consistent with balancing selection or a recent population expansion. Results presented here will inform future research focusing on examining the functional properties of the corresponding mitochondrial respiration enzymes, of A and B clade enzymes. By comparing closely related taxa that have differing distributions it is possible to identify genes and phenotypes suited to particular environments. The examination of mitochondrial genotypes, and associated enzyme functioning, across populations may aid in our understanding of thermal tolerance and environmental adaptation.
authors.affiliation: Shimoda Marine Research Center, University of Tsukuba, 5-10-1, Shimoda City, Shizuoka 415-0025, Japan; Cawthron Institute, Private Bag 2, Nelson 7042, New Zealand. Electronic address: kirsty.smith@cawthron.org.nz.
authors.forename: Kirsty F
authors.initials: KF
authors.lastname: Smith
authors.affiliation: Biological Station, Fisheries and Oceans Canada, 3190 Hammond Bay Road, Nanaimo, British Columbia V9T 6N7, Canada.
authors.forename: Cathryn L
authors.initials: CL
authors.lastname: Abbott
authors.affiliation: Shimoda Marine Research Center, University of Tsukuba, 5-10-1, Shimoda City, Shizuoka 415-0025, Japan.
authors.forename: Yasunori
authors.initials: Y
authors.lastname: Saito
authors.affiliation: Cawthron Institute, Private Bag 2, Nelson 7042, New Zealand; Maurice Wilkins Centre for Molecular Biodiscovery, University of Auckland, Auckland 1142, New Zealand; Institute of Marine Science, University of Auckland, Auckland 1142, New Zealand.
authors.forename: Andrew E
authors.initials: AE
authors.lastname: Fidler
mesh_terms: D000220:Adaptation, Biological / Q000235:genetics*; D063147:Animal Distribution; D000818:Animals; D001483:Base Sequence; D019295:Computational Biology; D017931:DNA Primers / Q000235:genetics; D014644:Genetic Variation; D005828:Genetics, Population; D054629:Genome, Mitochondrial / Q000235:genetics*; D058977:Molecular Sequence Annotation; D008969:Molecular Sequence Data; D010641:Phenotype; D010802:Phylogeny; D020641:Polymorphism, Single Nucleotide / Q000235:genetics; D017422:Sequence Analysis, DNA; D013045:Species Specificity; D014561:Urochordata / Q000235:genetics*
chemical_list: D017931:DNA Primers
keywords: Divergence; Functional genes; Invasive species; Mitochondrial protein-coding genes; Mitogenome; Speciation