Feder Jeffrey L, Stolz Uwe, Lewis Kristin M, Perry William, Roethele Joseph B, Rogers Allan
Department of Biological Sciences, University of Notre Dame, Notre Dame, Indiana, 46556.
Evolution. 1997 Dec;51(6):1862-1876. doi: 10.1111/j.1558-5646.1997.tb05109.x.
Host plant-associated fitness trade-offs are central to models of sympatric speciation proposed for certain phytophagous insects. But empirical evidence for such trade-offs is scant, which has called into question the likelihood of nonallopatric speciation. Here, we report on the second in a series of studies testing for host-related selection on pupal life-history characteristics of apple- (Malus pumila L.) and hawthorn- (Crataegus mollis L. spp.) infesting races of the Tephritid fruit fly, Rhagoletis pomonella (Walsh). In particular, we examine the effects of winter length on the genetics of these flies. We have previously found that the earlier fruiting phenology of apple trees exposes apple-fly pupae to longer periods of warm weather preceding winter than hawthorn-fly pupae. Because R. pomonella has a facultative diapause, we hypothesized that this selects for pupae with more recalcitrant pupal diapauses (or slower metabolic/development rates) in the apple-fly race. A study in which we experimentally manipulated the length of the prewintering period for hawthorn-origin pupae supported this prediction. If the period preceding winter is important for apple- and hawthorn-fly pupae, then so too should be the length (duration) of winter; the rationale for this prediction is that "fast developing" pupae that break diapause too early will deplete their energy reserves and disproportionately die during long winters. To test this possibility, we chilled apple- and hawthorn-origin pupae collected from a field site near Grant, Michigan, in a refrigerator at 4°C for time periods ranging from one week to two years. Our a priori expectation was that longer periods of cold storage would select against allozyme markers that were associated with faster rates of development in our earlier study. Since these electromorphs are typically found at higher frequencies in hawthorn flies, extending the overwintering period should favor "apple-fly alleles" in both races. The results from this "overwinter" experiment supported the diapause hypothesis. The anticipated genetic response was observed in both apple and hawthorn races, as allele frequencies became significantly more "apple-fly-like" in eclosing adults surviving longer chilling periods. This indicates that it is the combination of environmental conditions before and during winter that selects on the host races. Many tests for trade-offs fail to adequately consider the interplay between insect development, host plant phenology, and local climatic conditions. Our findings suggest that such oversight may help to explain the paucity of reported fitness trade-offs.
寄主植物相关的适合度权衡是为某些植食性昆虫提出的同域物种形成模型的核心。但这种权衡的实证证据很少，这对非异域物种形成的可能性提出了质疑。在这里，我们报告了一系列研究中的第二项，该研究测试了对苹果实蝇（Rhagoletis pomonella (Walsh)）侵染苹果（Malus pumila L.）和山楂（Crataegus mollis L. spp.）的品系蛹期生活史特征的寄主相关选择。特别是，我们研究了冬季长度对这些果蝇遗传学的影响。我们之前发现，苹果树较早的结果物候使苹果实蝇蛹在冬季前比山楂实蝇蛹暴露于温暖天气的时间更长。由于苹果实蝇具有兼性滞育，我们假设这会选择苹果实蝇品系中滞育更顽固（或代谢/发育速率更慢）的蛹。一项我们对山楂源蛹的越冬前期长度进行实验性操纵的研究支持了这一预测。如果冬季前的时期对苹果和山楂实蝇蛹很重要，那么冬季的长度（持续时间）也应该很重要；这一预测的依据是，过早打破滞育的“快速发育”蛹会耗尽它们的能量储备，并在漫长的冬季中不成比例地死亡。为了测试这种可能性，我们将从密歇根州格兰特附近的一个田间地点采集的苹果源和山楂源蛹在4°C的冰箱中冷藏1周至2年不等的时间。我们的先验预期是，更长时间的冷藏会淘汰与我们早期研究中更快发育速率相关的等位酶标记。由于这些电泳变体通常在山楂实蝇中以更高的频率出现，延长越冬期应该会在两个品系中都有利于“苹果实蝇等位基因”。这个“越冬”实验的结果支持了滞育假说。在苹果和山楂品系中都观察到了预期的遗传反应，因为在经历更长冷藏期后羽化的成虫中，等位基因频率变得明显更像“苹果实蝇的”。这表明是冬季前后环境条件的组合对寄主品系进行了选择。许多对权衡的测试未能充分考虑昆虫发育、寄主植物物候和当地气候条件之间的相互作用。我们的研究结果表明，这种疏忽可能有助于解释所报道的适合度权衡为何很少。
