The phenotypic determinants of diet variation between divergent lineages of threespine stickleback

Lineages with independent evolutionary histories often differ in both their morphology and diet. Experimental work has improved our understanding of the links between the biomechanics of morphological traits and foraging performance (trait-utility). However, because the expression of foraging-relevant traits and their utility can be highly context-specific, it is often unclear how dietary divergence arises from evolved phenotypic differences. Here, we explore the phenotypic causes of dietary divergence between two genetically and phenotypically divergent lineages of threespine stickleback (Gasterosteus aculeatus) with independent evolutionary histories of freshwater colonization and adaptation. First, using individuals from a line-cross breeding design, we conducted 150 common-garden foraging trials with a community of multiple prey species and performed morphological and behavioural analyses to test for prey-specific trait-utility. Second, we tested if the traits that explain variation in foraging performance among all individuals could also explain the dietary divergence between the lineages. Overall, we found evidence for the utility of several foraging traits, but these traits did not explain the observed dietary divergence between the lineages in common garden. This work suggests that evolved dietary divergence results not only from differences in morphology but also from divergence in behaviours that underlie prey capture success in species-rich prey communities.

Methods
We conducted a common-garden feeding experiment, in order to understand the functional basis of dietary divergence between two divergent lineages of European stickleback (Gasterosteus aculeatus). In the experiment, 150 lab-raised individuals, derived from a line-cross breeding design, foraged freely on a diverse prey community. The feeding trials were video recorded and analyzed using the event-logging software BORIS to obtain data on the foraging behavior (number and targets of feeding strikes, non-foraging behaviors). After the conclusion of the feeding trials, fish were sacrificed and dissected for gut content analyses. We counted, identified and weighed the prey items contained in the guts. Finally, we used the software phenopype to measure the functional morphology.

具有独立演化历史的类群，其形态与食性往往存在差异。已有实验研究深化了我们对形态性状生物力学与觅食表现（性状效用，trait-utility）之间关联的认知。然而，由于与觅食相关的性状表达及其效用具有极强的情境特异性，我们往往难以明确演化产生的表型差异如何引发食性分化。本研究聚焦于两个具有独立淡水定殖与适应演化历史的遗传和表型分化三刺棘鱼（Gasterosteus aculeatus）类群，探究二者食性分化的表型成因。首先，我们利用品系杂交育种设计获得的实验个体，开展了150组针对多猎物群落的同质园觅食实验，并通过形态学与行为学分析，检验猎物特异性的性状效用；其次，我们验证了能够解释所有个体间觅食表现差异的性状，是否同样可以解释两类群之间的食性分化。总体而言，我们发现了若干觅食性状具有效用的证据，但这些性状无法解释同质园实验中观察到的两类群间食性分化。本研究表明，演化而来的食性分化不仅源于形态差异，还源于物种丰富的猎物群落中决定猎物捕获成功率的行为分化。

方法
为阐明欧洲三刺棘鱼（Gasterosteus aculeatus）两个分化类群间食性分化的功能基础，我们开展了同质园摄食实验。本实验中，150尾通过品系杂交育种设计获得的实验室繁育个体，可自由取食多样化的猎物群落。实验过程对摄食实验进行录像，并通过事件记录软件BORIS进行分析，以获取觅食行为数据（摄食攻击次数、攻击目标及非觅食行为）。摄食实验结束后，对实验鱼进行处死与解剖，开展胃含物分析：统计、鉴定并称量胃内的猎物个体。最后，我们使用phenopype软件对功能形态学特征进行测量。