Data from: Body size and allometric shape variation in the molly Poecilia vivipara along a gradient of salinity and predation

Background: Phenotypic diversity among populations may result from divergent natural selection acting directly on traits or via correlated responses to changes in other traits. One of the most frequent patterns of correlated response is the proportional change in the dimensions of anatomical traits associated with changes in growth or absolute size, known as allometry. Livebearing fishes subject to predation gradients have been shown to repeatedly evolve larger caudal peduncles and smaller cranial regions under high predation regimes. Poecilia vivipara is a livebearing fish commonly found in coastal lagoons in the north of the state of Rio de Janeiro, Brazil. Similar to what is observed in other predation gradients, lagoons inhabited by P. vivipara vary in the presence of piscivorous fishes; contrary to other poeciliid systems, populations of P. vivipara vary greatly in body size, which opens the possibility of strong allometric effects on shape variation. Here we investigated body shape diversification among six populations of P. vivipara along a predation gradient and its relationship with allometric trajectories within and among populations.ResultsWe found substantial body size variation and correlated shape changes among populations. Multivariate regression analysis showed that size variation among populations accounted for 66% of shape variation in females and 38% in males, suggesting that size is the most important dimension underlying shape variation among populations of P. vivipara in this system. Changes in the relative sizes of the caudal peduncle and cranial regions were only partly in line with predictions from divergent natural selection associated with predation regime.ConclusionsOur results suggest the possibility that adaptive shape variation among populations has been partly constrained by allometry in P. vivipara. Processes governing body size changes are therefore important in the diversification of this species. We conclude that in species characterized by substantial among-population differences in body size, ignoring allometric effects when investigating divergent natural selection?s role in phenotypic diversification might not be warranted.

背景：种群间的表型多样性，可源于直接作用于性状的趋异自然选择，或是通过对其他性状变化的相关响应产生。最常见的相关响应模式之一，是与生长或绝对体型变化相关的解剖性状尺寸按比例改变，该现象被称为异速生长（allometry）。已有研究证实，处于捕食压力梯度中的胎生鱼类，在高捕食环境下会反复演化出更大的尾柄与更小的颅区。胎生花鳉（Poecilia vivipara）是巴西里约热内卢州北部沿海潟湖的常见胎生鱼类。与其他捕食梯度系统的观测结果类似，栖息有该物种的潟湖在食鱼鱼类的存在与否上存在差异；但与其他花鳉科（poeciliid）类群不同，该物种的种群间体型差异显著，这为体型对形状变异产生强烈异速影响提供了可能。本研究针对沿捕食梯度分布的6个胎生花鳉种群，探究了其体型形状的分化情况，以及种群内和种群间异速生长轨迹与形状变异的关联。结果：本研究发现种群间存在显著的体型变异与相关形状变化。多元回归分析显示，种群间的体型差异可解释雌性66%的形状变异、雄性38%的形状变异，表明体型是该系统中胎生花鳉种群间形状变异的核心影响维度。尾柄与颅区的相对尺寸变化，仅部分符合与捕食压力相关的趋异自然选择预测。结论：本研究结果表明，胎生花鳉种群间的适应性形状变异，部分受到异速生长的约束。因此，调控体型变化的过程在该物种的性状分化中发挥着重要作用。我们认为，对于种群间体型差异显著的物种，在探究趋异自然选择在表型分化中的作用时，忽略异速生长效应可能并不恰当。