Mitochondrial genomic phylogeny of gadid fish: implications for biogeographic origins and taxonomy

Coulson, Mark W. (2003) Mitochondrial genomic phylogeny of gadid fish: implications for biogeographic origins and taxonomy. Masters thesis, Memorial University of Newfoundland.

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Abstract

The phylogeny of ten species of gadine fishes was assessed with complete mitochondrial DNA (mtDNA) genome sequences from the following species: Microgadus proximus, Pollachius virens, Melanogrammus aeglefinus, Merlangius mer/angus, Boreogadus saida, Arctogadus glacialis, Theragra chalcogramma and the three species of Gadus (G. morhua, G. macrocephalus and G. ogac ). As most prior mitogenomic studies have addressed resolving basal or very divergent relationships, this study uses mitochondrial genomes among a closely related group of taxa to address taxonomic relationships with a focus on the biogeography of recently diverged species. Maximum parsimony, neighbour-joining and maximum likelihood all produced the same relationships when using mtDNA genomic sequences representing 14036 base pairs and consistently supported at least 8 of 10 nodes. Of these nodes, at least 6 were supported with 95% or greater bootstrap support. -- Among the individual mtDNA protein-coding genes, subunits of the ND complex included both the most successful (e.g. ND1 and ND5) and some of the least successful (e.g. ND6) genes for resolving phylogenetic relationships among these taxa. Amino acid sequences supported at least 6 of the nodes in common with nucleotide data and even suggested a functional evolutionary difference for ND5 among Theragra, relative to Gadus. -- The phylogenetic analysis identified the following relationships: Melanogrammus and Merlangius as sister taxa, a clade composed of Boreogadus, Arctogadus, Theragra and Gadus with Theragra and G. morhua as sister taxa, and a close relationship between one of two G. macrocephalus individuals and the two G. ogac individuals. Microgadus proximus was used to root the tree, and Pollachius was resolved as the outgroup to the other above mentioned species. These results suggest that Theragra chalcogramma should now be included as Gadus chalcogrammus, and that G. ogac represents a northward and eastward extension of the Pacific G. macrocephalus and should be included as a subspecies, G. macrocephalus ogac. The close evolutionary relationship between Theragra and G. morhua may therefore explain their common ability to have sustained the two largest fisheries in the world. The data also supports separate invasions of the Pacific basin by species endemic to these waters, and suggests that there has been at least one occurrence of secondary Atlantic Ocean contact with the introduction of G. macrocephalus ogac. The evidence also supported Melanogrammus aeglefinus and Merlangius mer/angus as sister taxa which was surprising considering these species represent very different ecological niches and prior morphological work has failed to yield such a relationship.

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