Lill J T
Department of Biology, University of Missouri-St Louis, 63121, USA.
Evolution. 2001 Nov 11;55(11):2236-47. doi: 10.1111/j.0014-3820.2001.tb00738.x.
The timing of life-history events in insects can have important consequences for both survival and reproduction. For insect herbivores with complex life histories, selection is predicted to favor those combinations of traits that increase the size at metamorphosis while minimizing the risk of mortality from natural enemies. Studies quantifying selection on life-history traits in natural insect herbivore populations, however, have been rare. The purpose of this study was to measure phenotypic selection imposed by elements of the first and third trophic levels on variation in two life-history traits, the timing of egg hatch and pupal mass, in a population of oak-feeding caterpillars, Psilocorsis quercicella (Lepidoptera: Oecophoridae). Larvae were collected from the field throughout each of two generations per year for three years and reared to determine the effects of the date of egg hatch on both the risk of attack from parasitoids and the pupal mass of the survivors. The direction and strength of phenotypic selection attributed to aspects of the first and third trophic levels, as well as their combined effects, on the date of egg hatch was measured for each of the six generations. Heritabilities of and genetic correlations between pupal mass and the date of adult emergence from diapause (the life-history trait expected to have the largest influence on the timing of egg hatch, and thus larval development) were estimated from laboratory matings. In four of the six generations examined, significant directional selection attributed to the first trophic level was detected, always favoring early-hatching cohorts predicted to experience higher leaf quality than late-hatching cohorts. Directional phenotypic selection by the third trophic level was detected in only one of three years, and in that year the direction of selection was in opposite directions during the two successive generations. The combined effect of selection by both trophic levels indicated that the third trophic level acted to either reduce or enhance the more predictable pattern of selection attributed to the first trophic level. In addition, I found evidence of truncation selection acting to increase the mean and decrease the variance of pupal mass during the pupa-adult transition in the laboratory. Pupal mass and diapause duration were found to vary significantly among full-sibling families; upper bounds for heritability estimates were 0.57 and 0.30, respectively. Furthermore, these two traits were found to be positively genetically correlated (families with larger pupae had longer diapause durations), resulting in a fitness trade-off, because larger pupae enjoy higher survival through metamorphosis and female fecundity but emerge later, when average leaf quality for offspring is generally poorer.
昆虫生活史事件的时间安排对其生存和繁殖都可能产生重要影响。对于具有复杂生活史的食草昆虫而言，预计自然选择会青睐那些能在变态时增加体型，同时将天敌造成的死亡风险降至最低的性状组合。然而，量化自然昆虫食草动物种群生活史性状选择的研究却很少见。本研究的目的是测量第一和第三营养级因素对以橡树为食的毛虫（Psilocorsis quercicella，鳞翅目：巢蛾科）种群中两个生活史性状（卵孵化时间和蛹质量）变异所施加的表型选择。在连续三年中，每年从田间收集两代幼虫并进行饲养，以确定卵孵化日期对寄生蜂攻击风险和存活幼虫蛹质量的影响。针对六个世代中的每一代，测量了归因于第一和第三营养级方面的表型选择的方向和强度，以及它们对卵孵化日期的综合影响。通过实验室交配估计了蛹质量与滞育成虫羽化日期（预期对卵孵化时间影响最大，进而影响幼虫发育的生活史性状）之间的遗传力和遗传相关性。在六个被研究的世代中，有四个世代检测到归因于第一营养级的显著定向选择，总是有利于预测比晚孵化幼虫经历更高叶片质量的早孵化群体。第三营养级的定向表型选择仅在三年中的一年被检测到，并且在那一年，连续两代的选择方向相反。两个营养级选择的综合效应表明，第三营养级要么起到减少，要么增强归因于第一营养级的更可预测的选择模式的作用。此外，我发现有证据表明在实验室中，蛹到成虫转变期间存在截尾选择，其作用是增加蛹质量的平均值并降低其方差。发现全同胞家系之间的蛹质量和滞育持续时间存在显著差异；遗传力估计的上限分别为0.57和0.30。此外，发现这两个性状存在正遗传相关性（蛹较大的家系滞育持续时间较长），这导致了一种适合度权衡，因为较大的蛹在变态过程中具有更高的存活率和雌虫繁殖力，但羽化较晚，此时后代的平均叶片质量通常较差。
