Assortative Mating in the Jewel Wasp 1 Figueredo and Sage

An experiment was performed to test the influence of two factors on assortative mating in the Jewel wasp (Nasonia vitripennis): (1) inherited eye-color genotype, and (2) conditioned foraging phenotype. To determine the female choice of sires, genetically marked wasps were used of two recessive mutant eye-color genotypes: white or red. Each female (of independently crossed eye-color and juvenile host) was given a choice of two males, one of each eye-color, counterbalanced for juvenile host: blowfly (Sarcophaga bullata) or housefly (Musca domestica) pupae. Blowflyreared females produced more total offspring, indicating higher fertility. Red-eyed females produced more offspring sired by red-eyed males, indicating assortative mating by eye color. Blowfly-reared females, however, did not produce more offspring sired by blowfly-reared males, indicating no assortative mating by juvenile host. ASSORTATIVE MATING IN THE JEWEL WASP 1 FIGUEREDO AND SAGE 52 and Haynes 1997). In addition, active assortative mating may also be based on traits that are not necessarily genetic in origin. For example, behaviorally-mediated assortative mating based on adult size has been shown in water striders (Heteroptera: Gerridae) (Rowe and Arnqvist 1996) and in meloid beetles (Lytta magister) (Snead and Alcock 1985). Furthermore, active assortative mating may be based upon acquired characteristics that are host-specific and based upon feeding experience. For example, in the case of the cactophilic fruit fly (Drosophila mojavensis), assortative mating is based on the distinct cuticular hydrocarbon compositions acquired as adults by larvae raised on different hosts, producing behaviorally-mediated reproductive isolation (Etges 1992, Brazner and Etges 1993, Etges and Ahrens 2001). The development of host-specific cuticular hydrocarbons has also been documented in the case of the bethylid wasp (Cephalonomia tarsalis Ashmead) (Howard 1998). Thus, assortatively mating individuals are sometimes able to discriminate potential sexual partners based on host-specific chemosensory cues. In the case of parasitoid feeding traditions, we need to pay close attention to the proximate behavioral mechanisms presumably involved. Because of the observed plasticity in foraging behavior, the putative mechanism or behavioral predisposition for assortative mating are not likely to be under direct genetic control because there might be no simple correspondence between an individual’s genotype and its host-feeding phenotype. For the same reason, philopatric patterns of dispersal (Shields 1982), the tendency to remain in the natal area through adulthood, might be similarly ineffective mechanisms for assortative mating. The major candidate for a proximate behavioral mechanism therefore appears to be phenotype matching, as has been documented in the monarch butterfly (Norton-Griffiths 1968). There are several theoretical considerations to be taken into account when proposing assortative mating hypotheses for the Jewel wasp. There are ecological factors which might mitigate against assortative mating for host specificity. For example, the population densities of different species of hosts may vary significantly and behaviorally-inflexible daughters might then suffer, and the foraging opportunity costs of not finding a preferred host might exceed the benefits of increased offspring fecundity associated with ovipositing on the preferred host. For the Jewel wasp, it has been found that adult foraging experience may modify the behavioral effects of juvenile feeding experience to track the relative local abundance of alternative host species (Figueredo 1987, 1989). Thus, it was concluded that sympatric speciation based on host-specific assortative mating was an unlikely result of such a lifelong learning mechanism. On the other hand, the influence of juvenile feeding experience on adult mate choice, per se, was not determined. Because adult experience might modify foraging behavior without altering any sexual preferences imprinted earlier, reproductive isolation by differential juvenile feeding experience — affecting mate choice directly, rather than indirectly through diet — was still technically possible. It remained possible that Jewel wasps might mate assortatively based on juvenile feeding experience. Alternatively, because blowfly-reared females were found to be significantly more fecund (Figueredo 1987, 1989), one might instead hypothesize a possible species-typical disposition for mate choice to favor males that were blowfly-reared to maintain that host predisposition. Finally, there is also the possibility of assortative mating for other phenotypic characteristics reflecting genetic differences that are unrelated to feeding experience. This paper reports the results of an experiment which was designed to test the effects of juvenile feeding experience on adult mate choice in the Jewel wasp. The principal hypothesis was that adult female Jewel wasps, when given a choice of males as sires for their offspring, would assortatively mate with males reared on the same juvenile host as themselves. This hypothesis was tested in the context of an alternative hypothesis that adult female Jewel wasps would assortatively mate, given the same choice conditions, on the basis of an unrelated and purely genetically determined phenotypic trait, such as eye color. The latter finding would instead support assortative mating based on genetic similarity (e.g., Rushton 1989, Rushton and Bons 2005).