MUTATION PRESSURE, NATURAL-SELECTION, AND THE EVOLUTION OF BASE COMPOSITION IN DROSOPHILA

Genome sequencing in a number of taxa has revealed variation in nucleo tide composition both among regions of the genome and among functional classes of sites in DNA. Mutational biases, biased gene conversion, a nd natural selection have been proposed as causes of this variation. H ere, we review patterns of base composition in Drosophila DNA. Nucleot ide composition in Drosophila melanogaster varys regionally, and base composition is correlated between introns and exons. Drosophila specie s also show striking patterns of non-random codon usage. Patterns of s ynonymous codon usage and the biochemistry of translation suggest that natural selection may act at 'silent' sites. A relationship between r ecombination rates and codon usage and comparisons of the evolutionary dynamics of silent mutations within and between species support natur al selection discriminating among synonymous codons. The causes of reg ional base composition variation are less clear. Progress in functiona l studies of non-coding DNA, further investigations of genome patterns , and statistical tests based on evolutionary theory will lead to a gr eater understanding of the contributions of mutational processes and n atural selection in patterning genome-wide nucleotide composition.