We have discovered a newly evolved gene, denoted Sdic, in region 19DE of the X chromosome of Drosophila. The new gene is found only in D. melanogaster. It codes for a sperm-specific axonemal dynein intermediate chain. Sdic was originally created from pieces of the genes Cdic, which encodes a cytoplasmic dynein intermediate chain, and AnnX, which encodes annexin X. This novel gene acquired a testes-specific promoter element. Considerable refashioning of the amino acid sequence occurred at the 5' end of the gene, and the gene became tandemly duplicated about tenfold. The Sdic gene is unusual in that the new regulatory region derives from sequences that were originally amino acid coding sequences, and the new 5' coding sequence derives from sequences that were originally noncoding. Several features of Sdic present unique opportunities for the study of the acquisition of new genetic functions and for examining the effects of a selective sweep on the polymorphism of linked genetic markers.
Aim 1 is to sequence the entire 70 kb region containing the Sdic repeats in D. melanogaster, as well as relevant regions of Cdic and AnnX in species closely related to D. melanogaster. The hypothesis is that many Sdic copies are undergoing mutational degeration, since preliminary cDNA analysis suggests that many of the copies are not expressed.
In Aim 2 additional cDNA and Northern analysis will be used to quantify the level of expression of each of the copies in the Sdic array.
Aim 3 deals with the patterns of genetic polymorphism in the Sdic region of the X chromosome. The theory of genetic hitchhiking predicts that positive selection for Sdic should have left a """"""""signature"""""""" on the frequency spectrum of genetic polymorphisms in the region, provided the selection was recent enough and strong enough. The innovation in this approach is that, instead of looking first for reduced polymorphisms and tying to identify the target, we have already identified an apparent target of selection and propose to study its effects on polymorphism.
Aim 4 is to ascertain whether Sdic is essential for male fertility by isolating point mutations and/or deletions that eliminate Sdic function, and also to determine whether expression of the Sdic::GFP fusion protein quantitatively reduces male fertility in males, as might be expected if the extended GFP carboxyl end interferes with axonemal dynein assembly.
Aim 5 recognizes Sdic as a candidate gene with a possible role in postmating reproductive isolation between D. melanogaster and D. simulans, which will be tested by interspecific germline transformation.
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