Variation and Evolution in Proteins of the Drosophila Male Reproductive Tract

Abstract

<p>Improved two-dimensional gel electrophoresis (2DE) and silver staining were applied to Drosophila male reproductive tract proteins. Genic variation was scored for about 300 polypeptides in 20 isofemale lines each of Drosophila melanogaster and D. simulans. Approximately 10% of loci were polymorphic within each species, with average heterozygosity in the range: 2-3%. These estimates are significantly lower (2-5 fold) than analogous ones from one-dimensional gel electrophoresis (1DB) of soluble enzymes in the same Drosophila populations. This confirms earlier reports of low variability in 2DE proteins from Drosophila, Homo and Mus. It is argued that the technical improvements applied, as well as other considerations, indicate that the variability differences are not artifacts of electrophoretic technique. 2DE was also used to compare male reproductive tract proteins between D. melanogaster and D. simulans, D. simulans and D. mauritiana, and D. simulans and D. sechellia. About 10% of loci were apparently fixed for different alleles between D. melanogaster and D. simulans. About 25% of polypeptides within D. melanogaster or D. simulans lacked a detectably homologous spot in D. simulans or D. melanogaster, respectively. Many of these unpaired spots may represent large (>10-fold) changes in polypeptide expression. Male reproductive tract proteins may be evolving faster than other proteins in these four species, as judged by 2DE comparisons of imaginal disc proteins and 2DE proteins of whole-body extracts. Finally, polypeptides localized to glandular tissues of the male reproductive tract in D. simulans were on average more highly polymorphic than polypeptides expressed in both testis and glandular tissues or only in testes. Glandular polypeptides of D. simulans were also more highly diverged from those of the other three species, compared to polypeptides expressed in testes. These increases occurred along with decreases for polypeptides co-expressed in testes and glands. These patterns were weaker in D. melanogaster. The results are discussed with reference to theories of balanced genetic structure in populations, and shifts in such balance during species formation.</p>