Rapid unidirectional change of hybrid dysgenesis potential in Drosophila

Two main types of Drosophila melanogaster strains have been previously described in relation to hybrid gonadal sterility: P (paternally derived) and M (maternally derived). When M strain females are mated with P strain males, cytoplasm-chromosome interactions result in variable frequencies of sterility in their hybrid progeny. A third neutral strain type, Q, generally has no significant potential for sterility in any hybrid strain combination. Mixed populations of these three types of strains were set up in various combinations and initial frequencies. They were subsequently maintained without artificial selection and monitored for their gonadal sterility and potential. All 24 mixed cultures initiated with P and M parents independently evolved rapidly towards a state of at least moderate P activity, which was maintained in succeeding generations. The M cytotype was not maintained after 10-20 generations except at very low frequencies. Changes in sterility potential were less clear cut in those mixed populations that originally included neutral strain flies, but there was a tendency for the frequencies of sterility of the majority of PQ and QM mixed cultures to change in the same direction as for the P and M combinations. The results are discussed in terms of conventional mechanisms of segregation and selection and in terms of an alternative hypothesis of chromosome contamination in which sterility factors may be transposed from P to M chromosomes, resulting in extremely rapid unidirectional population changes. Possible implications of these results for general models of the evolution of hybrid dysgenesis are explored.