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The amino acid sequence of the transformer (tra) gene exhibits an extremely rapid rate of evolution among Drosophila species, although the gene performs a critical step in sex determination. These changes in amino acid sequence are the result of either natural selection or neutral evolution. To differentiate between selective and neutral causes of this evolutionary change, analyses of both intraspecific and interspecific patterns of molecular evolution of tra gene sequences are presented. Sequences of 31 tra alleles were obtained from Drosophila americana. Many replacement and silent nucleotide variants are present among the alleles; however, the distribution of this sequence variation is consistent with neutral evolution. Sequence evolution was also examined among six species representative of the genus Drosophila. For most lineages and most regions of the gene, both silent and replacement substitutions have accumulated in a constant, clock-like manner. In exon 3 of D. virilis and D. americana we find evidence for an elevated rate of nonsynonymous substitution, but no statistical support for a greater rate of nonsynonymous relative to synonymous substitutions. Both levels of analysis of the tra sequence suggest that, although the gene is evolving at a rapid pace, these changes are neutral in function.

Type

Journal article

Journal

Genetics

Publication Date

04/2000

Volume

154

Pages

1711 - 1720

Addresses

Institute for Cell, Animal and Population Biology, University of Edinburgh, United Kingdom. bryantm@uta.edu

Keywords

Animals, Drosophila, Nuclear Proteins, Evolution, Molecular, Species Specificity, Base Sequence, Sequence Homology, Nucleic Acid, Molecular Sequence Data, Genetic Variation, Sex Determination Processes