A New Type of Isolating Mechanism in Drosophila

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Abstract
An insemination reaction is described as occurring in the [female] [female] of 27 out of 34 spp. of Drosophila. Almost immediately after copulation the vagina begins to enlarge and takes on an edematous-like condition, increasing to 3 or 4 times the normal size. In homogamic matings the vagina returns to its normal size in a few hrs. but in interspecific crosses it may remain swollen for several days and undergo changes which prevent it from carrying on its normal function. In D. buzzatti [female] X D. arizonensis [male], 88% of the [female] [female] were inseminated but no hybrids were ever obtained and among eggs discharged frotn the ovary none (of 105 checked) contained sperm. The reaction occurs whether or not spermatozoa are delivered with the semen and seems to represent an inherited foreign-protein reaction of the hypersensitive mucous membrane of the vagina. Three possible functions are suggested: prepn. of the reproductive tract for fertilization; prevention of the [male] from copulating more than once with the same [female] ; and reduction or prevention of interspecific exchange of genes.[long dash]G. W. Lasker.