Data from: The role of inhibitory dynamics in the loss and re-emergence of macropodoid tooth traits

The reversibility of phenotypic evolution is likely to be strongly influenced by the ability of underlying developmental systems to generate ancestral traits. However, few studies have quantitatively linked these developmental dynamics to traits which re-evolve. In this study we assess how changes in the inhibitory cascade, a developmental system that regulates relative tooth size in mammals, influenced the loss and reversals of the posthypocristid, a molar tooth crest, in the kangaroo superfamily Macropodoidea. We find that posthypocristid loss is linked with reduced levels of posterior molar inhibition, potentially driven by selection for lophodont, higher-crowned molar teeth. There is strong support for two posthypocristid reversals, each occurring after more than 15 million years of absence, in large-bodied species of Macropus, and two giant extinct species of short-faced sthenurine kangaroo (Procoptodon). We find that whereas primitive posthypocristid expression is linked to higher levels of posterior molar inhibition, re-emergence is tied to a relative increase in third molar size associated with increasing body mass, producing molar phenotypes similar to those in mouse where the ectodysplasin pathway is up-regulated. We argue that although shifts in the inhibitory cascade may enable re-emergence, dietary ecology may limit the frequency of phylogenetic reversal.

表型演化的可逆性很大程度上可能受潜在发育系统生成祖先性状的能力所影响，但目前鲜有研究将这类发育动力学与重新演化出现的性状进行定量关联。本研究针对袋鼠超科（Macropodoidea）展开分析，探讨调控哺乳动物相对牙齿大小的发育系统——抑制级联反应（inhibitory cascade）的变化，如何影响臼齿嵴结构下后尖脊（posthypocristid）的丢失与反转。

研究发现，下后尖脊的丢失与臼齿后部抑制水平降低存在关联，这一现象可能源于选择偏向于脊形齿、高冠臼齿的演化方向。结果强烈支持两次下后尖脊的反转事件：二者均在该结构消失超过1500万年之后，分别出现在体型较大的袋鼠属（Macropus）物种，以及两种隶属于巨袋鼠属（Procoptodon）的已灭绝短面史图伦亚科巨型袋鼠。

我们还发现，原始状态下的下后尖脊表达与更高的臼齿后部抑制水平相关，而该结构的重新出现则与体重增加所伴随的第三臼齿相对尺寸增大有关，由此产生的臼齿表型与外胚层素通路（ectodysplasin pathway）上调的小鼠中的表型相似。

本研究认为，尽管抑制级联反应的改变可能促使下后尖脊重新出现，但饮食生态特征可能会限制系统发育反转的发生频率。