The genetic basis of susceptibility to infection withDirofilaria immitisinAedes aegypti

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Abstract
The genetics of susceptibility to Dirofilaria immitis in Aedes aegypti were studied by means of a series of crosses and backcrosses between a susceptible and refractory stock. Progeny tests were employed to determine the genotype of backcross offspring of doubtful status. The results confirmed that susceptibility to D. immitis infection is under the control of a single major, sex-linked gene, susceptibility being recessive to refractoriness. The susceptibility gene, designated fi, was found to have a high penetrance and expressivity. In a preliminary experiment, when susceptibility was scored at nine to 14 days after infection, the recombination fraction between the locus for Ji and that for sex (m) was calculated as 0·155 ± 0·036. In more detailed experiments in which susceptibility was scored four to five days after infection, and with fi and m in coupling, the recombination fraction was 0·111 ± 0·025. In addition to the differences in the development of D. immitis within the Malpighian tubules of susceptible and refractory mosquitoes, there was evidence that a larger number of parasites entered the tubules of susceptible than those of refractory mosquitoes. This distinction was maintained even in the progeny of a second backcross.