The molecular basis of phenotypic plasticity and genetic assimilation in rapidly evolving lineages of East African cichlid fishes

生物体对环境刺激做出表型调整的能力称为表型可塑性。传统上，它经常被视为阻碍和制约进化。然而，这一观点存在争议，或者，West-Eberhard的柔性茎假说提出了相反的建议，即确切的表型可塑性(以及她所说的发育重组)促进了适应性辐射的茎谱系的快速进化和物种形成，这些物种后来构成了适应性辐射。基于这一假设，发育可塑性被认为是导致进化变化的原因，即使在缺乏初始环境刺激的情况下，这种变化也是通过遗传同化在不同的表型中表现出来的。在这里，我们从维多利亚湖和马拉维湖的适应性辐射以及两条河流的基本谱系中精心挑选了四种具有代表性的慈鱼物种，对柔性茎假说(FSH)的预测进行了测试。慈鱼在东非的几个湖泊中经历了爆炸性的适应性辐射，填补了大量的生态位，因此是检验这一假说的理想模型。在以前的工作和初步实验中，我们可以根据喂给鱼的食物的硬度(坚硬的蜗牛与碾碎的蜗牛)，在分裂的雏鸟中诱导不同类型的牙齿和咽下巴形态。有可能在一种非特有的河豚类中产生截然不同的摄食形态。根据FSH，我们预计来自维多利亚湖和马拉维湖适应性辐射的两对专门化物种和派生物种的反应标准都将小得多。在之前的RNA-SEQ实验和后来的工作中，我们发现了几组共表达的基因，它们在颌骨向食性(压碎蜗牛)的颌骨转化的不同阶段都是活跃的。我们计划对所有六个物种进行饲养实验，并进行RNA-SEQ分析，这些分析将在已知的系统发育框架内进行分析和解释。我们预计，与FSH预测的那样，适应性辐射的四个派生的和专门的地方性成员(年龄不同)在形态和转录方面的差异都比两个基本物种的变化要小。进一步的生物信息学分析将调查与适应性表型反应有关的基因启动子的变异和变化，并与其他中性基因进行比较。

An organisms ability to adjust its phenotype in response to environmental stimuli is termed phenotypic plasticity. Traditionally it is often seen to hamper and constrain evolution. However, this view is debated and alternatively the flexible-stem hypothesis of West-Eberhard suggest the opposite, that exactly phenotypic plasticity (and what she calls developmental recombination) facilitates in stem lineages of adaptive radiations their rapid evolution and speciation into many specialized and less plastic species that then make up adaptive radiations. Based on this hypothesis developmental plasticity is seen to lead evolutionary change that later, even in lack of the initial environmental stimulus, is phenotypically expressed in alternative phenotypes through genetic assimilation. Here, we set out test the predictions of the flexible-stem hypothesis (FSH) in a set of four carefully chosen representative cichlid fish species from the adaptive radiations of Lake Victoria and Lake Malawi and two riverine basal lineages. Cichlid fishes have undergone explosive adaptive radiations in several East African lakes, filling a vast array of ecological niches, and thus represent an ideal model in which to test this hypothesis. In previous work and in preliminary experiments we could induce in split-broods different types of teeth and pharyngeal jaw morphologies based on the hardness of the diets that was fed to the fish (hard snails vs. crushed snails). It was possible to produce drastically different feeding morphologies in one of the non-endemic riverine cichlids. In accordance with the FSH we expect that the reaction norms will be much smaller in both of the two pairs of specialized and derived species from the adaptive radiations of Lakes Victoria and Malawi. In previous RNA-seq experiments and later work we identified several sets of co-expressed genes that were active during different phases of the jaw transformation towards a durophagous (snail-crushing) jaw. We plan to do conduct feeding-experiments with all six species as well as RNA-seq analyses that will be analyzed and interpreted within a known phylogenetic framework. We anticipate that both the variation in morphology as well as in transcriptional will be smaller in the four derived and specialized endemic members of the adaptive radiations (that are of different ages) than in the two basal species as would be predicted by the FSH. Further bioinformatics analyses will investigate variation and changes in promoters of genes involved in the adaptive phenotypic response and compared with other sets of neutral genes.