Evolution of GC content in the histone gene repeating units from Drosophila lutescens, D. takahashii and D. pseudoobscura.

A subset of histone genes (H1, H2A, H2B and H4), which are encoded along with H3 within repeating units, were analyzed in Drosophila lutescens, D. takahashii and D. pseudoobscura to investigate the evolutionary mechanisms influencing this multigene family and its GC content. Nucleotide divergence among species was more marked in the less functional regions. A strong inverse relationship was observed between the extent of evolutionary divergence and GC content within the repeating units; this finding indicated that the functional constraint on a region must be associated with both divergence and GC content. The GC content at 3(rd) codon positions in the histone genes from D. lutescens and D. takahashii was higher than that from D. melanogaster, while that from D. pseudoobscura was similar. These evolutionary patterns were similar to those of H3 gene regions. Based on these findings, we propose that the evolutionary mechanisms governing nucleotide content at 3(rd) codon positions tend to eliminate A and T nucleotides more frequently than G and C nucleotides. These changes might be the consequence of negative selection and would result in GC-rich 3(rd) codon positions. In addition, interspecific differences in GC content, which exhibited the same pattern for all histone genes, could be explained by different selection efficiencies that result from changes in population size.