Pre- and Post-zygotic Isolation Between Sibling Species of Drosophila

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Charles Rodell


Speciation, the process by which new species arise, is not well understood. Even less is known about the underlying genetics of this process. This study examines the level of reproductive isolation between two closely related species, Drosophila melanogaster and D. simulans. All combinations of intra- and inter-specific single-pair matings were set up in a total of 288 crosses. In all cases, intra- specific crosses (controls) were more successful than inter-specific crosses (P<0.001). For all matings, data were obtained on the success of speram transfer (pre-zygotic reproductive isolation), production of viable offspring (post-zygotic), and, when possible, motility of sperm in hybrid males (post-zygotic). Competing hypotheses exist concerning the level of discrimination among females of derived versus ancestral species during inter-specific hybridization. The data obtained in this study support the hypothesis that the ancestral species (D. melanogaster) is the most discriminating (P<0.05). Furthermore, for the conditions tested, these data indicate that Haldane's rule is operating as a post-zygotic mechanism when the female of the inter-specific cross is D. melanogaster, but not when it is D. simulans.

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