Polak Michal, Markow Therese A
Department of Zoology, Arizona State University, Tempe, Arizona, 85287-1501.
Center for Insect Science, Tucson, Arizona, 85721.
Evolution. 1995 Aug;49(4):660-669. doi: 10.1111/j.1558-5646.1995.tb02302.x.
We conducted a field study and a laboratory experiment to test whether ectoparasitic mites, Macrocheles subbadius, generate parasite-mediated sexual selection in the Sonoran desert endemic fruit fly, Drosophila nigrospiracula. Male flies gather on the outer surfaces of necrotic saguaro cacti where they engage in male-male competitive interactions and vigorous female-directed courtship. At these sites, operational sex ratios were significantly skewed toward males. The degree to which mites were aggregated among flies varied across the 25 fly populations sampled. The degree of mite aggregation across fly populations was strongly positively related to the mean number of mites per fly (intensity of infestation). Both the intensity and prevalence of infestation (fraction of flies infested) increased with the age of the cactus necrosis. Infested flies of both sexes were significantly less likely to be found in copula than uninfested flies, and mean intensity of infestation was significantly more pronounced in noncopulating than in copulating flies. The effect of attached mites on copulatory success exhibited dose-dependency, and this effect was more stringent in males: males or females with more than two and four mites, respectively, were never found in copula. The magnitude of parasite-mediated sexual selection was estimated for 12 fly populations by calculating selection differentials for each sex separately. The relation between intensity of infestation and magnitude of parasite-mediated sexual selection was stronger in males but significant for both sexes. We also assayed copulatory success of field-caught males in the laboratory, both during infestation and after experimental removal of mites. Males infested with two mites copulated less frequently than uninfested individuals, and in mating trials after mites had been removed, previously infested males copulated as many times as flies with no history of infestation. These findings, and the lack of difference in the number of mite-induced scars on copulating and single individuals in nature, strongly suggest that the reduced copulatory success of infested flies is attributable to an effect of mites per se, rather than to a character correlated with parasitism or previous parasite infestation.
我们进行了一项实地研究和一项实验室实验，以测试外寄生螨类——亚氏巨螯螨（Macrocheles subbadius）是否会在索诺兰沙漠特有的果蝇——黑腹果蝇（Drosophila nigrospiracula）中产生寄生虫介导的性选择。雄蝇聚集在坏死的树形仙人掌的外表面，在那里它们进行雄雄竞争互动以及积极的向雌求偶行为。在这些地点，实际的性比显著偏向雄性。在所采样的25个果蝇种群中，螨在果蝇间的聚集程度各不相同。果蝇种群中螨的聚集程度与每只果蝇的平均螨数（感染强度）呈强烈正相关。感染强度和感染率（被感染果蝇的比例）都随着仙人掌坏死时间的增长而增加。与未感染的果蝇相比，感染螨的雌雄果蝇交尾的可能性显著更低，并且在未交尾的果蝇中，平均感染强度比正在交尾的果蝇中更为明显。附着的螨对交尾成功率的影响呈现剂量依赖性，并且这种影响在雄性中更为严格：分别有超过两只和四只螨的雄性和雌性从未被观察到处于交尾状态。通过分别计算每种性别的选择差，我们估计了12个果蝇种群中寄生虫介导的性选择强度。感染强度与寄生虫介导的性选择强度之间的关系在雄性中更强，但在两性中均显著。我们还在实验室中测定了野外捕获的雄性果蝇在感染期间以及实验性去除螨之后的交尾成功率。感染两只螨的雄性果蝇交尾频率低于未感染的个体，并且在螨被去除后的交配试验中，先前感染过的雄性果蝇交尾次数与没有感染史的果蝇一样多。这些发现，以及在自然状态下交尾果蝇和单个果蝇上螨诱导的伤疤数量没有差异，强烈表明感染果蝇交尾成功率降低是由于螨本身的影响，而不是与寄生相关的特征或先前的寄生虫感染。
