DELETERIOUS MUTATIONS AT THE MITOCHONDRIAL ND3 GENE IN SOUTH-AMERICANMARSH RATS (HOLOCHILUS)

Statistical analyses of DNA sequences have revealed patterns of nonneu tral evolution in mitochondrial DNA of mice, humans, and Drosophila. H ere we report patterns of mitochondrial sequence evolution in South Am erican marsh rats (genus Holochilus). We sequenced the complete mitoch ondrial ND3 gene in 82 Holochilus brasiliensis and 21 H. vulpinus to t est the neutral prediction that the ratio of nonsynonymous to synonymo us nucleotide changes is the same within and between species. Within H . brasiliensis we observed a greater number of amino acid polymorphism s than expected based on interspecific comparisons. This contingency t able analysis suggests that many amino acid polymorphisms are mildly d eleterious. Several tests of the frequency distribution also revealed departures from a neutral, equilibrium model, and these departures wer e observed for both nonsynonymous and synonymous sites. In general, an excess of rare sites was observed, consistent with either a recent se lective sweep or with populations not at mutation-drift equilibrium.