The effects of ancestral plasticity and genetic assimilation on convergence and the diversification of East African cichlid fishes

不同的环境意味着不同的选择机制，因此预计会影响适应性。对于这种环境挑战，具有发育可塑性的适应性反应能力有望增加生物体生存的机会。理论和实证研究表明，表型可塑性本身可以成为选择的目标，从而进化。表型可塑性已在许多生态相关性状中得到证实，并应在自然选择中缓冲生物体。但它在物种形成和快速适应性辐射中的作用尚未得到很好的理解和争议。“灵活茎模型”提出表型可塑性可以促进谱系的多样化。由可塑祖先发起的适应性辐射将导致表型多样性的突然增加，随后逐渐转化为潜在的不同的以前的可塑表型，从而导致几个谱系具有不同的和较少的可塑表型。这可能通过降低环境敏感性的机制发生，如遗传同化/渠化。当最初可塑性表达的表型经历遗传同化时，其可塑性随之降低，性状的表型遗传固定，遗传力增加。预计选择会更快地作用于这些表型，潜在地促进表型分化并加速物种形成。慈鲷是一个很有前途的进化模式系统，研究灵活的茎和遗传同化对谱系多样化和快速适应性辐射的潜在贡献。慈鲷的一个最有趣的关键创新被认为促进了它们的营养特化程度，从而允许它们无与伦比的多样化，这是它们高度修饰的咽颚装置（PJA，具有不同表型可塑性牙列的功能第二颚，不断被替换）。PJA的表型和牙齿的大小和形状具有表型可塑性，特定的PJA形态反映了物种的营养生态位。在这项研究中，我们将重点放在柔性茎最关键和最具争议的假设上：不同的种群开始经历遗传同化，从而遗传固定表型的时期。我们将研究几种慈鲷在河流（祖先/来源）和湖泊（特化/衍生）种群之间诱导可塑性的差异（通过在普通花园实验中分卵饮食处理）。我们将研究（通过RNAseq， qPCR和原位杂交）可塑性候选基因的潜在作用，其中一些我们已经获得。因为它们将表征PJA形态在实验诱导的塑性反应过程中的表达动态和功能相关性。

Different environments mean different selective regimes and are thereby expected to affect fitness. The ability to respond adaptively with developmental plasticity to such environmental challenges would be expected to increase an organism’s chances to persist. Theoretical and empirical work suggests that phenotypic plasticity itself can be the target of selection and thus evolve. Phenotypic plasticity has been documented in many ecologically relevant traits and should buffer organisms from natural selection. But its role in speciation and during rapid adaptive radiations is not well understood and controversial. The "flexible stem model", proposes that phenotypic plasticity can boost a lineage’s diversification. An adaptive radiation initiated by a plastic ancestor would lead to a sudden increase in phenotypic diversity followed by a gradual canalization into potentially different formerly plastic phenotypes and thus lead to several lineages with then distinct and less plastic phenotypes. This might happen through mechanisms that reduce environmental sensitivity such as genetic assimilation/canalization. When an initially plastically expressed phenotype undergoes genetic assimilation, its plasticity would be concomitantly reduced, the trait’s phenotype becomes genetically fixed and its heritability increased. Selection would be expected act on these phenotypes faster, potentially promoting phenotypic divergence and speeding up speciation. Cichlid fishes are a promising evolutionary model systems in which to investigate the potential contributions of a flexible stem and genetic assimilation to lineage diversification and rapid adaptive radiations. One most interesting key innovation of cichlids that is thought to have facilitated their degree of trophic specializations and thereby permitting their un-rivalled diversification is their highly modified pharyngeal jaw apparatus (PJA, a functional second jaw with different phenotypically plastic dentition that gets replaced continuously). The phenotype of the PJA and the size and shape of the teeth are phenotypically plastic and a particular PJA morphology reflects the trophic niche of the species. In this study, we focus on the most crucial and most controversially debated assumption of the flexible stem: the period when divergent populations start to undergo genetic assimilation and thus genetic fixation of a phenotype. We will investigate differences in inducible plasticity (through split-brood diet treatments in common garden experiments) between riverine (ancestral/source) and lacustrine (specialized/derived) populations in several cichlids. We will investigate (through RNAseq, qPCR and in situ hybridization) the potential role of plasticity candidate genes, some of which we already obtained. For them will characterize the expression dynamics and functional relevance during the experimentally induced plastic response to the PJA morphology.