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Drosophila mojavensis comprises three geographic forms occurring in the United States, the Baja California peninsula, and mainland Mexico. Peninsular and mainland forms were selected for increased sexual isolation from each other, while controls were maintained with maximum outbreeding. Response to selection was highly asymmetrical in that isolation was very high between selected peninsular males and mainland females, but nonexistent between selected mainland males and peninsular females. The heightened isolation is primarily due to some change in the peninsular males.

We present results deriving from the inception of an investigation (involving 4,000 scored matings) into the development of discriminatory behavior in Drosophila paulistorum. We examined two complementary situations for their effects on mating behavior: physical isolation (seclusion) beginning at each of six developmental stages and exposure to heterosemispecifics during all preadult stages (mixed culturing). Preadult seclusion at certain stages decreased sexual isolation between three of four semispecific pairs. Seclusion effect, or lack thereof, varied with semispecies and with the stage at which seclusion was imposed: the earlier the stage at which seclusion was initiated, the more drastic the effect. In contrast, mixed culturing increased sexual isolation between the one pair of semispecies that was tested.

Abstract. -Drosophila mojavensis comprises three geographic forms occurring in the United States, the Baja California peninsula, and mainland Mexico. Peninsular and mainland forms were selected for increased sexual isolation from each other, while controls were maintained with maximum outbreeding. Response to selection was highly asymmetrical in that isolation was very high between selected peninsular males and mainland females, but nonexistent between selected mainland males and peninsular females. The heightened isolation is primarily due to some change in the peninsular males.

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We have been utilizing members of the Drosophila paulistorum complex of semispecies to study the development of adult discriminatory behavior. In this study, three rearing methods were used to examine the effects of previous exposure to heterosemispecifics on reproductive isolation among the various semispecies. Experimental flies were exposed to heterosemispecifics by one of three methods: exposure to airborne heterosemispecific stimuli from egg through sexual maturity, physical contact with heterosemispecifics only during the postimaginal period, and mixed culturing (total physical contact with heterosemispecific individuals during each of five preimaginal stages). Effects of the first treatment varied with semispecies. The second treatment had no significant effect on mating behavior. The third treatment of mixed culturing reinforced sexual isolation.

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