Phenotypic integration between anti-predator behaviour and camouflage pattern in juvenile sticklebacks

Predation is a strong selective force that promotes the evolution of anti-predator behaviours and camouflage in prey animals. However, the independent evolution of single traits cannot explain how observed phenotypic variations of these traits are maintained within populations. We studied genetic and phenotypic correlations between anti-predator behaviours (shoaling and risk-taking) and morphology traits (pigmentation and size) in juvenile three-spined sticklebacks by using pedigree-based quantitative genetic analysis to test phenotypic integration (or complex phenotype) as an evolutionary response to predation risk. Individuals with strongly melanised (i.e. camouflaged) phenotype and genotype were less sociable to conspecifics but bolder during foraging under predation risk. Individuals with faster growing phenotype and genotype were bolder, and those with lager eyes were more fearful. These phenotypic integrations were not confounded with correlated plastic responses to predation risk because the phenotypes were measured in naïve fish born in the laboratory but originated from a natural population with predation pressure. Consistent selection for particular combinations of traits under predation pressure or pleiotropic genes might influence the maintenance of the genetic (co)variations and polymorphism in melanin colour, growth trajectory and behaviour patterns.

捕食是一种强烈的选择压力，推动猎物演化出反捕食行为（anti-predator behaviours）与伪装（camouflage）策略。然而，单一性状的独立演化无法解释种群内为何会维持这些性状的观测表型变异（phenotypic variations）。
本研究以幼年三刺鱼（three-spined sticklebacks）为研究对象，通过基于家系的数量遗传分析（pedigree-based quantitative genetic analysis），探究反捕食行为（包括集群行为（shoaling）与冒险行为（risk-taking））与形态性状（色素沉着（pigmentation）与体型）间的遗传与表型关联，以验证表型整合（phenotypic integration，或称复杂表型（complex phenotype））作为捕食压力下的演化响应这一假说。
研究发现，兼具强黑化（melanised）表型与基因型的个体，对同种个体的社交性更低，但在捕食压力下觅食时更为大胆；生长速率更快的表型与基因型个体同样更大胆，而眼径更大的个体则更为畏怯。
这些表型整合效应并未与捕食压力下的相关性塑性响应相混淆，因为实验所用表型均在实验室繁育但源自存在捕食压力的自然种群的未接触过捕食者的幼鱼（naïve fish）中测得。
捕食压力下对特定性状组合的持续选择，或多效基因（pleiotropic genes）的作用，可能会维持黑色素沉着、生长轨迹（growth trajectory）与行为模式的遗传（共）变异（genetic (co)variations）及多态性（polymorphism）。