Rapid Evolution of Testes-specific Proteasome Subunits in the Drosophila melanogaster Complex. D.G. Torgerson , R.S. Singh. Department of Biology, McMaster University, Hamilton, Ontario, Canada.

   Proteasomes are large multi-catalytic proteins involved in the ubiquitin protein degradation pathway. During spermatogenesis, proteasomes are thought to recognize and degrade discarded proteins, and to regulate the fine structural tuning of the sperm tail. We have chosen a family of Drosophila proteasome subunit genes to study the molecular evolution of two testes-specific isoforms (a4-t1 and a4-t2) compared to their somatic counterpart (a4). Both testes-specific genes show higher polymorphism and higher divergence than the somatically expressed a4 gene. The testes-specific a4-t1 gene is evolving rapidly in the D. melanogaster complex, and shows a significantly higher level of polymorphism and divergence than both the somatic a4 gene and the testes-specific a4-t2 gene. There are no significant differences in the synonymous substitution rates between testes-specific and somatic isoforms, suggesting that differences in the nonsynonymous substitution rate are due to differences in selective pressures and not mutation rate. The higher divergence of testes-specific proteasome subunits compared to their somatic counterpart adds to the increasing support for the theory that genes involved in sex and reproduction related processes evolve rapidly.