Evolution of nested genes with special reference to cuticle proteins in Drosophila melanogaster.

One of the pupal cuticle protein (PCP) genes has been found within an intron of a Drosophila housekeeping gene (the Gart locus) that encodes three enzymes involved in the purine pathway. This intronic gene has been described as a gene within a gene, and the gene is now called a "nested" gene. Because the intronic PCP gene has sequence similarity with the larval cuticle protein (LCP) gene, it may have been derived from one of the LCP genes or their ancestral gene. We have studied possible phylogenetic relationships among these five genes by comparing nucleotide sequences of four LCP genes with that of the PCP gene. The results obtained suggest that the PCP gene may have originated from an ancestral gene before duplication of the LCP genes occurred. Using the number of synonymous (silent) substitutions, we then estimated the divergence time between the PCP gene and the LCP genes to be about 70 million years (Myr). The divergence time estimated is much larger than that for the sibling species of D. melanogaster (about 2.5 Myr), indicating that the "nested" gene structure can be seen not only in Drosophila melanogaster, but also in other distantly related Drosophila species.