The rapidly evolving and often extraordinarily complex appearance of male genital morphology of internally fertilizing animals has been recognized for centuries. Postcopulatory sexual selection is regarded as the likely evolutionary engine of this diversity, but direct support for this hypothesis is limited. We used two complementary approaches, evolution through artificial selection and microscale laser surgery, to experimentally manipulate genital morphology in an insect model system. We then assessed the competitive fertilization success of these phenotypically manipulated males and studied the fate of their ejaculate in females using high-resolution radioisotopic labeling of ejaculates. Males with longer genital spines were more successful in gaining fertilizations, providing experimental evidence that male genital morphology influences success in postcopulatory reproductive competition. Furthermore, a larger proportion of the ejaculate moved from the reproductive tract into the female body following mating with males with longer spines, suggesting that genital spines increase the rate at which seminal fluid passes into the female hemolymph. Our results show that genital morphology affects male competitive fertilization success and imply that sexual selection on genital morphology may be mediated in part through seminal fluid.

Numerous insects, including Drosophila nigrospiracula, that inhabit necrotic cacti of the Sonoran desert are host to Macrocheles subbadius, a cosmopolitan mesostigmatid mite. This paper shows that the mite pierces Drosophila integument and ingests haemolymph while attached to the abdomen of its host. Among field—caught flies, there was a strong negative relationship between mite load and wet mass of adult flies of both sexes. Thus, M. subbadius is an ectoparasite of D. nigrospiracula, contrary to the widespread belief that macrochelids form only phoretic associations with adult flies. The parasite harms its host in many ways. Under laboratory conditions, there was a significant negative effect of mite load on survivorship. In the field, there was a significant negative relationship between mite load and number of eggs carried by sexually mature females, suggesting that infested females resorb their oocytes to compensate for nutritional stress imposed by feeding mites. Likewise, in the laboratory, infestation lengthened somewhat the period prior to onset of oviposition, and decreased the number of eggs laid by females over their life. However, mite—infested females maintained on a yeast—supplemented diet overcame the potentially debilitating effects of mites, and were actually slightly, but not significantly, more fertile than uninfested females. Since the dietary conditions of flies influenced the degree to which fecundity was affected, the debilitating effect of mites is comparable to that of starvation. This conclusion derives support from the observation that laboratory females infested for 4 d, but whose mites were subsequently removed, produced fewer progeny than uninfested females. Although this effect was significant only among old females, it nevertheless indicates that the observed effects of mites on fecundity are not the result of mechanical constraints on oviposition behavior. Since intensities of infestation used in laboratory experiments were similar to those encountered in nature, observed ectoparasitic effects on fly fitness suggest that M. subbadius can reduce fly numbers in nature.

A trade-off between sex and somatic maintenance is fundamental to life-history theory. Tests of this trade-off usually emphasize deleterious consequences of increased reproduction on life span. Here we show the reverse effect, that reductions in the expected life span elevate sexual activity. Experimentally parasitized male Drosophila nigrospiracula lived shorter lives, but before dying, they courted females significantly more than unparasitized controls. This greater courtship resulted in increased mating speed, and potentially greater reproductive success than parasitized males would have achieved otherwise. The results show that an environmental reduction in life span increases reproductive effort, and support the hypothesis of a trade-off between these key life-history traits.

The leading hypothesis for the evolution of male genital complexity proposes that genital traits evolve in response to post-insemination sexual selection; that is, via cryptic female choice or sperm competition. Here, we describe a laser ablation technique for high-precision manipulation of microscale body parts of insects, and employ it to discern the adaptive function of a rapidly evolving and taxonomically important genital trait: the intromittent claw-like genital spines of male Drosophila bipectinata Duda. We demonstrate experimentally and unambiguously that the genital spines of this species function to mechanically couple the genitalia together. The excision of the spines by laser ablation sharply reduced the ability of males both to copulate and to compete against rival males for mates. When spineless males did succeed to copulate, their insemination success and fertilization rate were not statistically different from controls, at odds with the post-insemination sexual selection hypothesis of genital function and evolution. The results provide direct experimental support for the hypothesis that genital traits evolve in response to sexual selection occurring prior to insemination.

The occurrence of closely related strains of maternally transmitted endosymbionts in distantly related insect species indicates that these infections can colonize new host species by lateral transfer, although the mechanisms by which this occurs are unknown. We investigated whether ectoparasitic mites, which feed on insect haemolymph, can serve as interspecific vectors of Spiroplasma poulsonii, a male-killing endosymbiont of Drosophila. Using Spiroplasma-specific primers for PCR, we found that mites can pick up Spiroplasma from infected Drosophila nebulosa females and subsequently transfer the infection to Drosophila willistoni. Some of the progeny of the recipient D. willistoni were infected, indicating successful maternal transmission of the Spiroplasma within the new host species. However, the transmission rate of the infection from recipient flies to their offspring was low, perhaps due to low Spiroplasma density in the recipient flies.