Parsons Karen E
Department of Zoology, University of Western Australia, Nedlands, Western Australia, Australia, 6907.
Evolution. 1997 Jun;51(3):784-796. doi: 10.1111/j.1558-5646.1997.tb03661.x.
Similar phenotypes do not always imply similar genotypes. In species distributed over a broad latitudinal range, geographical variation in morphological and life-history traits may reflect very different relations between genotypic and environmental effects on these traits. Patterns of selection among latitudinally separated sites may minimize phenotypic differences in life-history traits but promote phenotypic differences in form. Thus, for example, latitudinal variation in temperature often leads to genetically based metabolic differences that minimize differences in growth rate among populations at different latitudes (countergradient variation). However, variation in habitat experienced by the same populations may promote genetically based differences in shell form (cogradient variation). Few attempts have been made to assess simultaneously such mosaic effects of natural selection on the genetic basis of variation in both morphological and life-history traits among geographically separated populations. I quantified the extent to which widely separated populations of the rocky shore marine gastropod Bembicium vittatum exhibited genetic differences in shell shape, shell pattern, and growth rate. Bembicium vittatum occurs naturally at only three widely separated locations on the Western Australian coast. Individuals were transplanted from all three locations to a latitudinally intermediate site, where they were released in different pairwise combinations and allowed to reproduce. F offspring from crosses between same- or different-source parents were identified using allozyme markers. When grown in a common environment, offspring from same-source parents exhibited similar differences in shell shape and pattern, but dramatic differences in growth rates, compared to native populations. Genetic variation therefore exists for all three traits. Growth rates in the common environment were positively correlated with latitude of the source population, confirming the existence of countergradient variation for growth associated with metabolic compensation. In addition, for both shell shape and growth rate, hybrids exhibited phenotypes roughly midway between the same-source parents, suggesting that genetic differences have a large additive component. In contrast, when one parent had pigmented spots, the offspring also had spots, suggesting a strong dominance component to the genetic basis of shell pattern. Genetic differences therefore yield different morphological phenotypes but similar life-history phenotypes, among latitudinally distant populations, and confirm a pattern of mosaic evolution in B. vittatum.
相似的表型并不总是意味着相似的基因型。在分布于广泛纬度范围的物种中，形态和生活史特征的地理变异可能反映了基因型和环境对这些特征的影响之间非常不同的关系。在纬度上相隔较远的地点之间的选择模式可能会使生活史特征的表型差异最小化，但会促进形态上的表型差异。因此，例如，温度的纬度变化通常会导致基于遗传的代谢差异，从而使不同纬度种群之间的生长速率差异最小化（反梯度变异）。然而，同一种群所经历的栖息地变化可能会促进基于遗传的壳形差异（共梯度变异）。很少有人尝试同时评估自然选择对地理上隔离的种群中形态和生活史特征变异的遗传基础的这种镶嵌效应。我量化了岩岸海洋腹足纲动物斑纹滨螺广泛隔离的种群在壳形、壳纹和生长速率方面表现出遗传差异的程度。斑纹滨螺仅自然分布于西澳大利亚海岸三个广泛隔离的地点。个体从所有三个地点被移植到一个纬度居中的地点，在那里它们以不同的两两组合被释放并允许繁殖。使用等位酶标记识别来自同来源或不同来源亲本杂交的F后代。在共同环境中生长时，与本地种群相比，同来源亲本的后代在壳形和壳纹上表现出相似的差异，但在生长速率上表现出显著差异。因此，所有这三个特征都存在遗传变异。在共同环境中的生长速率与来源种群的纬度呈正相关，证实了与代谢补偿相关的生长存在反梯度变异。此外，对于壳形和生长速率，杂种表现出的表型大致介于同来源亲本之间，这表明遗传差异有很大的加性成分。相比之下，当一个亲本有色素斑点时，后代也有斑点，这表明壳纹的遗传基础有很强的显性成分。因此，遗传差异在纬度上相距较远的种群中产生了不同的形态表型，但生活史表型相似，并证实了斑纹滨螺的镶嵌进化模式。
