A major theoretical consequence of selection at a locus is the genetic hitchhiking of linked sites (selective sweep). The extent of hitchhiking around a gene is related to the strength of selection and the rate of recombination, with its impact diminishing with distance from the selected site. At the Rop-1 locus of the sheep blowfly, Lucilia cuprina, polymorphisms at two different sites within the LcαE7 gene encode forms of the protein that confer organophosphorus insecticide resistance. To assess the impact of selection at these two sites on variation around LcαE7, we sequenced regions within six other genes along chromosome IV across isogenic (IV) strains of L. cuprina. High levels of linkage disequilibrium, characterized by low haplotype number (K) and diversity (H), and significant R2 values were observed for two genes, LcαE1 and LcαE10, both members of the same α-esterase gene cluster as LcαE7. A significant R2 value was also observed for a gene predicted to be the next closest to LcαE7, AL03, but not for any of the other genes, LcRpL13a, Lcdsx, or LcAce. Skews in the site frequency spectra toward high-frequency variants were significant for LcαE1 (Fay and Wu's H = -2.91), LcαE10 (H = -1.85), and Lcdsx (H = -2.00). Since the selective sweeps, two forms of likely returning variation were observed, including variation in microsatellites in an intron of LcαE10 and a recombination event between LcαE7 and LcαE10. These data suggest that two incomplete soft sweeps have occurred at LcαE7 that have significantly affected variation across, and beyond, the α-esterase gene cluster of L. cuprina. The speed and impact of these selective sweeps on surrounding genomic variation and the ability of L. cuprina to respond to future environmental challenges are discussed.
- genetic hitchhiking
- linkage disequilibrium
- microsatellite evolution
- organophosphorus insecticide resistance
- selective sweep