Armbruster Peter, Bradshaw William E, Holzapfel Christina M
Department of Biology, University of Oregon, Eugene, Oregon, 97403-1210.
Evolution. 1997 Apr;51(2):451-458. doi: 10.1111/j.1558-5646.1997.tb02432.x.
We examined the genetic basis for evolutionary divergence among geographic populations of the pitcher-plant mosquito, Wyeomyia smithii, using protein electrophoresis and line-cross analysis. Line-cross experiments were performed under both low density, near-optimal conditions, and at high, limiting larval densities sufficient to reduce fitness (r ) in parental populations by approximately 50%. We found high levels of electrophoretic divergence between ancestral and derived populations, but low levels of divergence between two ancestral populations and between two derived populations. Assessed under near-optimal conditions, the genetic divergence of fitness (r ) between ancestral and derived populations, but not between two derived populations or between two ancestral populations, has involved both allelic (dominance) and genic (epistatic) interactions. The role of dominance and epistasis in the divergence of r among populations affects its component traits in a pattern that is unique to each cross. Patterns of genetic differentiation among populations of W. smithii provide evidence for a topographically complex "adaptive landscape" as envisioned by Wright in his "shifting balance" theory of evolution. Although we cannot definitively rule out the role of deterministic evolution in the divergence of populations on this landscape, ecological inference and genetic data are more consistent with a stochastic than a deterministic process. At high, limiting larval density, hybrid vigor is enhanced and the influence of epistasis disappears. Thus, under stressful conditions, the advantages to fitness due to hybrid heterozygosity can outweigh the deleterious effects of fragmented gene complexes. These results have important implications for the management of inbred populations. Outbreeding depression assessed in experimental crosses under benign lab, zoo, or farm conditions may not accurately reveal the increased advantages of heterozygosity in suboptimal or marginal conditions likely to be found in nature.
我们运用蛋白质电泳和品系杂交分析，研究了猪笼草蚊（Wyeomyia smithii）地理种群间进化分歧的遗传基础。品系杂交实验分别在低密度、接近最优条件下，以及高密度、足以使亲代种群适合度（r）降低约50%的极限幼虫密度条件下进行。我们发现，祖先种群和衍生种群之间存在高水平的电泳分歧，但两个祖先种群之间以及两个衍生种群之间的分歧水平较低。在接近最优条件下评估发现，祖先种群和衍生种群之间适合度（r）的遗传分歧涉及等位基因（显性）和基因（上位性）相互作用，但两个衍生种群之间或两个祖先种群之间不存在这种情况。显性和上位性在种群间r分歧中的作用，以一种每个杂交组合特有的模式影响其组成性状。W. smithii种群间的遗传分化模式，为赖特在其“动态平衡”进化理论中设想的地形复杂的“适应性景观”提供了证据。尽管我们不能明确排除确定性进化在这一景观中种群分歧中的作用，但生态推断和遗传数据更符合随机过程而非确定性过程。在高密度、极限幼虫密度条件下，杂种优势增强，上位性影响消失。因此，在压力条件下，杂种杂合性对适合度的优势可能超过基因复合体碎片化的有害影响。这些结果对近交种群的管理具有重要意义。在良性实验室、动物园或农场条件下的实验杂交中评估的远交衰退，可能无法准确揭示在自然环境中可能出现的次优或边缘条件下杂合性增加的优势。
