Messina Frank J
Department of Biology, Utah State University, Logan, Utah 84322-5305, USA.
Evolution. 2004 Dec;58(12):2788-97. doi: 10.1111/j.0014-3820.2004.tb01630.x.
Interfertile populations of the seed beetle Callosobruchus maculatus differ genetically in several behavioral, morphological, and life-history traits, including traits that affect the intensity of larval competition within seeds. Previous studies have suggested that this variation depends on differences in host size. I performed a selection experiment in which replicate beetle lines were either maintained on a small, ancestral host (mung bean) or switched to a larger, novel host (cowpea). After 40 generations, I estimated survival, development time, and adult mass on each host, both in the presence and absence of larval competition. The shift to cowpea substantially reduced body size; irrespective of rearing host, adults from the cowpea lines were more than 10% lighter than those from the mung bean lines. Switching to cowpea also improved survival and reduced development time on this host, but without decreasing performance on the ancestral host. The most striking effect of the shift to a larger host was a reduction in larval competitiveness. When two even-aged larvae co-existed within a seed, the probability that both survived to adult emergence was > or = 65% if larvae were from the cowpea lines but < or = 12% if they were from the mung bean lines. The adverse effects of competition on development time and adult mass were also less severe in the cowpea lines than in the mung bean lines. By rapidly evolving smaller size and reduced competitiveness, the cowpea lines converged toward populations chronically associated with cowpea. These results suggest that evolutionary trajectories can be predictable, and that host-specific selection can play a major role in the diversification of insect life histories. Because host shifts by small, endophagous insects are comparable to the colonization of new habitats, adaptive responses may often include traits (such as larval competitiveness) that are not directly related to host use.
绿豆象（Callosobruchus maculatus）的可育种群在行为、形态和生活史特征等多个方面存在遗传差异，这些特征包括影响种子内幼虫竞争强度的性状。先前的研究表明，这种变异取决于宿主大小的差异。我进行了一项选择实验，将重复的甲虫品系分别饲养在小型的、祖传宿主（绿豆）上，或者转换到更大的、新宿主（豇豆）上。40代之后，我估计了在有幼虫竞争和无幼虫竞争的情况下，每种宿主上的存活率、发育时间和成虫质量。转换到豇豆上会显著减小体型；无论饲养宿主如何，来自豇豆品系的成虫比来自绿豆品系的成虫轻10%以上。转换到豇豆上还提高了在这种宿主上的存活率并缩短了发育时间，但在祖传宿主上的表现并未下降。转换到更大宿主上最显著的影响是幼虫竞争力的降低。当两个同龄幼虫共存于一粒种子中时，如果幼虫来自豇豆品系，两者都存活到成虫羽化的概率≥65%，但如果来自绿豆品系，则≤12%。竞争对发育时间和成虫质量的不利影响在豇豆品系中也比在绿豆品系中不那么严重。通过快速进化出更小的体型和更低的竞争力，豇豆品系向长期与豇豆相关的种群趋同。这些结果表明，进化轨迹是可预测的，并且宿主特异性选择在昆虫生活史的多样化中可能起主要作用。由于小型内食性昆虫的宿主转换类似于新栖息地的定殖，适应性反应可能常常包括与宿主利用无直接关系的性状（如幼虫竞争力）。
