Following species divergence, differences arise in the expression levels of various genes as a result of natural selection. Differences in gene expression levels also occur within a species, but do so between the sexes, with most of these differences being attributable to sexual dimorphism and sex-specific behavior and reproduction. In the June 13 issue of Science, Jose Ranz and colleagues at Harvard University use microarray technology coupled with statistical analysis to examine the evolutionary pattern of gene expression differences at the whole genome level in Drosophila melanogaster and D. simulans, which diverged about 2.5 million years ago (Science, 300:1742-1745, June 13, 2003).

Ranz et al. analyzed both transcription products and genomic DNA in multiple hybridizations against microarrays containing 4776 Drosophila coding sequences. They observed that 2493 genes examined had not changed in relative expression levels since species divergence, and of these, 57.5% showed sex-biased expression. The...

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