A new developmental mechanism for the separation of the mammalian middle ear ossicles from the jaw. Monodelphis domestica

Multiple mammalian lineages independently evolved a definitive mammalian middle ear (DMME) through breakdown of Meckel’s cartilage (MC). However, the cellular and molecular drivers of this evolutionary transition remain unknown for most mammal groups. Here, we identify such drivers in the living marsupial opossum Monodelphis domestica, whose MC transformation during development anatomically mirrors the evolutionary transformation observed in fossils. Specifically, we link increases in cellular apoptosis and TGF-BR2 signalling to MC breakdown in opossums. We demonstrate that a simple change in TGF-b signalling is sufficient to inhibit MC breakdown during opossum development, indicating that changes in TGF-b signalling might be key during mammalian evolution. Furthermore, the apoptosis that we observe during opossum MC breakdown does not seemingly occur in mouse, consistent with homoplastic DMME evolution in the marsupial and placental lineages. Overall design: RNA-seq of tissues from 3 time points (N=3 at each point) to analyze differential gene expression levels.

多个哺乳类演化支系通过麦克尔软骨（Meckel’s cartilage, MC）的降解，独立演化出定型哺乳类中耳（definitive mammalian middle ear, DMME）。然而，该演化转变的细胞与分子驱动机制，在绝大多数哺乳类类群中仍未被阐明。本研究以现存有袋类短尾负鼠（Monodelphis domestica）为研究对象，成功鉴定出该驱动机制：其发育过程中麦克尔软骨的解剖学重塑，与化石记录中观察到的演化转变高度吻合。具体而言，我们将细胞凋亡水平的上调与转化生长因子β受体2（TGF-BR2）信号通路的活化，与负鼠体内麦克尔软骨的降解建立了直接关联。实验证实，仅需对转化生长因子β（TGF-β）信号通路进行简单调控，即可抑制负鼠发育阶段的麦克尔软骨降解，这表明转化生长因子β信号通路的改变可能是哺乳类演化过程中的关键调控因素。此外，我们在负鼠麦克尔软骨降解过程中观察到的细胞凋亡现象，在小鼠体内并未出现，这与有袋类与真兽类演化支系中定型哺乳类中耳的趋同演化特征相符。整体实验设计：选取3个时间点的组织样本进行RNA测序（每个时间点设置3个生物学重复，即N=3），以分析不同样本间的差异基因表达水平。