Hybrid Male Sterility in Crosses Between Field and Laboratory Strains of Anopheles quadrimaculatus (Say) (Diptera: Culicidae)

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Abstract
Hybridization experiments involving matings between nine geographic strains and a laboratory strain (ORL) of Anopheles quadrimaculatus (Say) revealed the existence of two genetically distinct forms of this species. Matings between some individuals (designated type A) of each of the geographic strains produced normal, fertile progeny. However, a second type (designated type B), which was genetically incompatible to the ORL strain, was found in three natural populations. Crosses between type B males and ORL females resulted in high mortality of male progeny in the pupal stage, and all surviving adult male progeny were sterile. The reciprocal cross produced a normal 1:1 sex ratio, and all of the hybrid male progeny were sterile. Female progeny of type B × ORL matings (in both directions) were semisterile. Hybrid females, when backcrossed to either type A or type B males, produced egg batches with lower percent hatch than in the F1 crosses. Male sterility persisted in the backcross progeny. It is concluded, on the basis of mating compatibility to the ORL strain, that type A and type B individuals are genetically incompatible and that populations of each type are reproductively isolated. Samples of adult males collected from two of the three sites were found to be fertile, suggesting that hybridization in nature occurs rarely, if at all. These results and the results of concurrent studies on allozymes and chromosomes led to the conclusion that type A and type B represent sympatrically occurring sibling species, tentatively designated A. quadrimaculatus species A and A. quadrimaculatus species B.

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