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Selection is thought to be partially responsible for patterns of molecular evolution at synonymous sites within numerous Drosophila species. Recently, "per-site" and likelihood methods have been developed to detect loci for which positive selection is a major component of synonymous site evolution. An underlying assumption of these methods, however, is a homogeneous mutation process. To address this potential shortcoming, we perform a complementary analysis making gene-by-gene comparisons of paired synonymous site and intron substitution rates toward and away from the nucleotides G and C because preferred codons are G or C ending in Drosophila. This comparison may reduce both the false-positive rate (due to broadscale heterogeneity in mutation) and false-negative rate (due to lack of power comparing small numbers of sites) of the per-site and likelihood methods. We detect loci with patterns of evolution suggestive of synonymous site selection pressures predominately favoring unpreferred and preferred codons along the Drosophila melanogaster and Drosophila sechellia lineages, respectively. Intron selection pressures do not appear sufficient to explain all these results as the magnitude of the difference in synonymous and intron evolution is dependent on recombination environment and chromosomal location in a direction supporting the hypothesis of selectively driven synonymous fixations. This comparison identifies 101 loci with an apparent switch in codon preference between D. melanogaster and D. sechellia, a pattern previously only observed at the Notch locus.

That natural selection affects molecular evolution at synonymous sites in protein-coding sequences is well established and is thought to predominantly reflect selection for translational efficiency/accuracy mediated through codon bias. However, a recently developed maximum likelihood framework, when applied to 18 coding sequences in 3 species of Drosophila, confirmed an earlier report that the Notch gene in Drosophila melanogaster was evolving under selection in favor of those codons defined as unpreferred in this species. This finding opened the possibility that synonymous sites may be subject to a variety of selective pressures beyond weak selection for increased frequencies of the codons currently defined as "preferred" in D. melanogaster. To further explore patterns of synonymous site evolution in Drosophila in a lineage-specific manner, we expanded the application of the maximum likelihood framework to 8,452 protein coding sequences with well-defined orthology in D. melanogaster, Drosophila sechellia, and Drosophila yakuba. Our analyses reveal intragenomic and interspecific variation in mutational patterns as well as in patterns and intensity of selection on synonymous sites. In D. melanogaster, our results provide little statistical evidence for recent selection on synonymous sites, and Notch remains an outlier. In contrast, in D. sechellia our findings provide evidence in support of selection predominantly in favor of preferred codons. However, there is a small subset of genes in this species that appear to be evolving under selection in favor of unpreferred codons, which indicates that selection on synonymous sites is not limited to the preferential fixation of mutations that enhance the speed or accuracy of translation in this species.

We present a likelihood method for estimating codon usage bias parameters along the lineages of a phylogeny. The method is an extension of the classical codon-based models used for estimating d N /d S ratios along the lineages of a phylogeny. However, we add one extra parameter for each lineage: the selection coefficient for optimal codon usage (S), allowing joint maximum likelihood estimation of S and the d N /d S ratio. We apply the method to previously published data from Drosophila melanogaster, Drosophila simulans, and Drosophila yakuba and show, in accordance with previous results, that the D. melanogaster lineage has experienced a reduction in the selection for optimal codon usage. However, the D. melanogaster lineage has also experienced a change in the biological mutation rates relative to D. simulans, in particular, a relative reduction in the mutation rate from A to G and an increase in the mutation rate from C to T. However, neither a reduction in the strength of selection nor a change in the mutational pattern can alone explain all of the data observed in the D. melanogaster lineage. For example, we also confirm previous results showing that the Notch locus has experienced positive selection for previously classified unpreferred mutations.

To identify genomic regions affected by the rapid fixation of beneficial mutations (selective sweeps), we performed a scan of microsatellite variability across the Notch locus region of Drosophila melanogaster. Nine microsatellites spanning 60 kb of the X chromosome were surveyed for variation in one African and three non-African populations of this species. The microsatellites identified an ∼14-kb window for which we observed relatively low levels of variability and/or a skew in the frequency spectrum toward rare alleles, patterns predicted at regions linked to a selective sweep. DNA sequence polymorphism data were subsequently collected within this 14-kb region for three of the D. melanogaster populations. The sequence data strongly support the initial microsatellite findings; in the non-African populations there is evidence of a recent selective sweep downstream of the Notch locus near or within the open reading frames CG18508 and Fcp3C. In addition, we observe a significant McDonald-Kreitman test result suggesting too many amino acid fixations species wide, presumably due to positive selection, at the unannotated open reading frame CG18508. Thus, we observe within this small genomic region evidence for both recent (skew toward rare alleles in non-African populations) and recurring (amino acid evolution at CG18508) episodes of positive selection.

In 2002 Kim and Stephan proposed a promising composite-likelihood method for localizing and estimating the fitness advantage of a recently fixed beneficial mutation. Here, we demonstrate that their composite-likelihood-ratio (CLR) test comparing selective and neutral hypotheses is not robust to undetected population structure or a recent bottleneck, with some parameter combinations resulting in a false positive rate of nearly 90%. We also propose a goodness-of-fit test for discriminating rejections due to directional selection (true positive) from those due to population and demographic forces (false positives) and demonstrate that the new method has high sensitivity to differentiate the two classes of rejections.