Intron evolution of chicken, mouse, and human mitochondrial carrier genes

In vertebrates the coding sequences of the mitochondrial carrier (MC) genes are highly conserved; in these genes, relatively few intronic sequences are well conserved in chicken, mouse and human, and many more in mouse and human. In the chicken/mouse/human MC intronic alignments encompassing about 13,700 nucleotides, 77.8% of the homotopic nucleotides are identical in all three species. The non-identical nucleotides form three distinct subpopulations of nucleotides: chicken-specific, mouse-specific and human-specific. The nucleotide compositions of the chicken-specific and human-specific subpopulations do not differ significantly from each other and from the identical-nucleotide subpopulation. In contrast, the mouse-specific subpopulation is remarkably different from the chicken-specific and human-specific ones, being richer in C and G and poorer in A and T. A similar analysis was carried out for mouse/human MC intronic alignments encompassing about 56,800 nucleotides, and 74.9% of the homotopic nucleotides were found to be identical in the two species. Also in this case the human-specific subpopulation appears to be markedly similar to the mouse/human identical-nucleotide subpopulation, whereas the mouse-specific subpopulation is richer in C and G and poorer in A and T. These results can be interpreted as indicating that the human and chicken intronic sequences of the MC genes investigated largely retain the characteristics of the bird/mammal ancestor sequence, while the mouse intronic sequences diverge from those of the rodent/primate ancestor much more than the human sequences do. Remarkably, the bird/mammal and rodent/primate ancestors probably shared a significant amount of homotopic nucleotides of the investigated intronic sequences.