Genome duplication and gene-family evolution: the case of three OXPHOS gene families

DNA duplication is one of the main forces acting on the evolution of organisms because it creates the raw genetic material that natural selection can subsequently modify. Duplicated regions are mainly due to "errors" in different phases of meiosis, but DNA transposable elements and reverse transcription also contribute to amplify and move the genomic material to different genomic locations. As a result, redundancy affects genomes to variable degrees: from the single gene to the whole genome (WGD). Gene families are clusters of genes created by duplication and their size reflects the number of duplicated genes, called paralogs, in each species. The aim of this review is to describe the state of the art in the identification and analysis of gene families in eukaryotes, with specific attention to those generated by ancient large scale events in vertebrates (WGD or large segmental duplications). As a case study, we report our work on the evolution of gene families encoding subunits of the five OXPHOS (oxidative phosphorylation) complexes, fundamental and highly conserved in all respiring cells. Although OXPHOS gene families are smaller than the general trend in nuclear gene families, some exceptions are observed, such as three gene families with at least two paralogs in vertebrates. These gene families encode cytochrome c (Cyt c, the electron shuttle protein between complex III and IV), Lipid Binding Protein (LBP, the channel protein of complex V which transfers protons through the inner mitochondrial membrane) and the MLRQ subunit (MLRQ, a supernumerary subunit of the large complex I, with unknown function). We provide a two-step approach, based on structural genomic data, to demonstrate that these gene families should have arisen through WGD (or large segmental duplication) events at the origin of vertebrates and, only afterwards, underwent species-specific events of further gene duplications and loss. In summary, this review reflects the need to apply genome comparative approaches, deriving from both "classical" molecular phylogenetic analysis and "new" genome map analysis, to successfully define the complex evolutionary relations between gene family members which, in turn, are essential to obtain any other comparative phylogenetic or functional results.