Switch in codon bias and increased rates of amino acid substitution in theDrosophila saltans species group

We investigated the nucleotide composition of five genes, Xdh, Adh, Sod, Pe r; and 28SrRNA, in nine species of Drosophila (subgenus Sophophora) and one of Scaptodrosophila. The six species of the Drosophila saltans group marke dly differ from the others in GC content and codon use bias. The GC content in the third codon position, and to a lesser extent in the first position and the introns, is higher in the D. melanogaster and D. obscura groups tha n in the D. saltans group tin Scaptodrosophila it is intermediate hut close r to the melanogaster and obscura species). Differences are greater for Xdh than for Adh, Sod, Per, and 28SrRNA, which are functionally more constrain ed. We infer that rapid evolution of GC content in the saltans lineage is l argely due to a shift in mutation pressure, which may have been associated with diminished natural selection due to smaller effective population numbe rs rather than reduced recombination rates. The rate of GC content evolutio n impacts the rate of protein evolution and may distort phylogenetic infere nces. Previous observations suggesting that GC content evolution is very li mited in Drosophila may have been distorted due to the restricted number of genes and species (mostly D. melanogaster) investigated.