Multiple nuclear insertions of mitochondrial cytochrome b sequences in callitrichine primates

We report the presence of four nuclear paralogs of a 380-bp segment of cyto chrome b in callitrichine primates (marmosets and tamarins). The mitochondr ial cytochrome b sequence and each nuclear paralog were obtained from sever al species, allowing multiple comparisons of rates and patterns of substitu tion both between mitochondrial and nuclear sequences and among nuclear seq uences. The mitochondrial DNA had high overall rates of molecular evolution and a strong bias toward substitutions at third codon positions. Rates of molecular evolution among the nuclear sequences were low and constant, and there were small differences in substitution patterns among the nuclear cla des which were probably attributable to the small number of sites involved. A novel method of phylogenetic reconstruction based on the large differenc e in rates of evolution at different codon positions among mitochondrial an d nuclear clades was used to determine whether different nuclear paralogs r epresent independent transposition events or duplications following a singl e insertion. This method is generally applicable in cases where differences in pattern of molecular evolution are known, and it showed that at least t hree of the four nuclear clades represent independent transposition events. The insertion events giving rise to two of the nuclear clades predate the divergence of the callitrichines, whereas those leading to the other two nu clear clades may have occurred in the common ancestor of marmosets.