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Genetics, Vol. 168, 1421-1432, November 2004, Copyright © 2004
doi:10.1534/genetics.104.027631
Rapid Evolution Through Gene Duplication and Subfunctionalization of the Testes-Specific 
4 Proteasome Subunits in Drosophila
Dara G. Torgerson1 and Rama S. Singh
Department of Biology, McMaster University, Hamilton, Ontario L8S 4K1, Canada
1 Corresponding author: Department of Biology, McMaster University, 1280 Main St., West Hamilton, ON L8S 4K1, Canada.
E-mail: torgerdg{at}mcmaster.ca
Gene duplication is an important mechanism for acquiring new genes and creating genetic novelty in organisms. Evidence suggests that duplicated genes are retained at a much higher rate than originally thought and that functional divergence of gene copies is a major factor promoting their retention in the genome. We find that two Drosophila testes-specific 
4 proteasome subunit genes (4-t1 and 4-t2) have a higher polymorphism within species and are significantly more diverged between species than the somatic 4 gene. Our data suggest that following gene duplication, the 4-t1 gene experienced relaxed selective constraints, whereas the 4-t2 gene experienced positive selection acting on several codons. We report significant heterogeneity in evolutionary rates among all three paralogs at homologous codons, indicating that functional divergence has coincided with genic divergence. Reproductive subfunctionalization may allow for a more rapid evolution of reproductive traits and a greater specialization of testes function. Our data add to the increasing evidence that duplicated genes experience lower selective constraints and in some cases positive selection following duplication. Newly duplicated genes that are freer from selective constraints may provide a mechanism for developing new interactions and a pathway for the evolution of new genes.
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