Mopper Susan, Beck Michael, Simberloff Daniel, Stiling Peter
Department of Biology, University of Southwestern Louisiana, Lafayette, Louisiana, 70504-2451.
Department of Biological Science, Florida State University, Tallahassee, Florida, 32306.
Evolution. 1995 Oct;49(5):810-815. doi: 10.1111/j.1558-5646.1995.tb02317.x.
The deme-formation hypothesis states that selection can produce adaptive genetic variation within and among phytophagous insect populations. We conducted three field experiments and tested this prediction by transferring eggs and measuring performance of a mobile leafmining insect, Stilbosis quadricustatella. In Experiment 1, we compared the rate of mine initiation of leafminers transferred to natal and novel sites. In Experiment 2, we compared mine-initiation rate of leafminers transferred to natal and novel host-plant species. In Experiment 3, we compared the mine-initiation rate, mine-completion rate, and sources of mortality of miners transferred to neighboring natal and novel Quercus geminata trees. In the first, second, and third experiments, leafminer larvae initiated significantly more mines at the natal site, on the natal plant species, and on the natal Q. geminata tree, evidence for adaptive differentiation. Furthermore, plant-mediated mortality was significantly lower among miners transferred to natal Q. geminata trees. This result supports a key assumption of the deme-formation hypothesis: insects adapt to the defensive phenotypes of individual trees. However, natural-enemy mortality was significantly higher among miners transferred to natal trees, essentially reversing the plant effect. Therefore, rates of successful mine completion were similar on natal (19%) and novel (17%) trees. This experiment suggests that host plants and natural enemies may represent opposing forces of selection. Leafminers adapted to individual trees may realize a selective advantage only when natural-enemy densities are low.
种群形成假说认为，选择能够在植食性昆虫种群内部以及不同种群之间产生适应性遗传变异。我们进行了三项田间实验，并通过转移卵以及测量一种能移动的潜叶昆虫——四斑潜叶蛾（Stilbosis quadricustatella）的表现来检验这一预测。在实验1中，我们比较了转移到出生地和新地点的潜叶蛾开始潜道的速率。在实验2中，我们比较了转移到出生地和新寄主植物物种上的潜叶蛾开始潜道的速率。在实验3中，我们比较了转移到相邻的出生地和新的双子叶栎树上的潜叶蛾开始潜道的速率、完成潜道的速率以及死亡原因。在第一项、第二项和第三项实验中，潜叶蛾幼虫在出生地、出生地的植物物种以及出生地的双子叶栎树上开始潜道的数量显著更多，这是适应性分化的证据。此外，转移到出生地的双子叶栎树上的潜叶蛾中，由植物介导的死亡率显著更低。这一结果支持了种群形成假说的一个关键假设：昆虫适应单个树木的防御表型。然而，转移到出生地树木上的潜叶蛾中，天敌造成的死亡率显著更高，基本上抵消了植物的影响。因此，在出生地树木（19%）和新树木（17%）上成功完成潜道的速率相似。该实验表明，寄主植物和天敌可能代表了相反的选择力量。只有当天敌密度较低时，适应单个树木的潜叶蛾才可能实现选择优势。
