Evolution des familles multigéniques, Laboratoire Evolution Génome et Spéciation, UPR CNRS, Gif-sur-Yvette, France.
BMC Evol Biol. 2011 Oct 18;11:307. doi: 10.1186/1471-2148-11-307.
Teeth and tooth-like structures, together named odontodes, are repeated organs thought to share a common evolutionary origin. These structures can be found in gnathostomes at different locations along the body: oral teeth in the jaws, teeth and denticles in the oral-pharyngeal cavity, and dermal denticles on elasmobranch skin. We, and other colleagues, had previously shown that teeth in any location were serially homologous because: i) pharyngeal and oral teeth develop through a common developmental module; and ii) the expression patterns of the Dlx genes during odontogenesis were highly divergent between species but almost identical between oral and pharyngeal dentitions within the same species. Here we examine Dlx gene expression in oral teeth and dermal denticles in order to test the hypothesis of serial homology between these odontodes.
We present a detailed comparison of the first developing teeth and dermal denticles (caudal primary scales) of the dogfish (Scyliorhinus canicula) and show that both odontodes develop through identical stages that correspond to the common stages of oral and pharyngeal odontogenesis. We identified six Dlx paralogs in the dogfish and found that three showed strong transcription in teeth and dermal denticles (Dlx3, Dlx4 and Dlx5) whereas a weak expression was detected for Dlx1 in dermal denticles and teeth, and for Dlx2 in dermal denticles. Very few differences in Dlx expression patterns could be detected between tooth and dermal denticle development, except for the absence of Dlx2 expression in teeth.
Taken together, our histological and expression data strongly suggest that teeth and dermal denticles develop from the same developmental module and under the control of the same set of Dlx genes. Teeth and dermal denticles should therefore be considered as serial homologs developing through the initiation of a common gene regulatory network (GRN) at several body locations. This mechanism of heterotopy supports the 'inside and out' model that has been recently proposed for odontode evolution.
牙齿和牙齿状结构，统称为牙原基，是被认为具有共同进化起源的重复器官。这些结构可以在沿身体不同位置的有颌类动物中找到：口腔中的牙齿在颌骨中，口腔-咽腔中的牙齿和齿状突，以及鲨鱼皮肤中的真皮齿状突。我们和其他同事之前曾表明，任何位置的牙齿都是系列同源的，因为：i）咽和口腔牙齿通过共同的发育模块发育；ii）在牙发生过程中 Dlx 基因的表达模式在物种之间差异很大，但在同一物种的口腔和咽齿之间几乎相同。在这里，我们检查了口腔牙齿和真皮齿状突中的 Dlx 基因表达，以检验这些牙原基之间的系列同源假说。
我们详细比较了狗鲨（Scyliorhinus canicula）的第一颗发育中的牙齿和真皮齿状突（尾侧初级鳞片），并表明这两种牙原基通过相同的阶段发育，这些阶段与口腔和咽牙齿发生的共同阶段相对应。我们在狗鲨中鉴定出了六个 Dlx 基因的同源物，发现其中三个在牙齿和真皮齿状突中具有强烈的转录（Dlx3、Dlx4 和 Dlx5），而 Dlx1 在真皮齿状突和牙齿中的表达较弱，Dlx2 在真皮齿状突中的表达较弱。牙齿和真皮齿状突发育之间的 Dlx 表达模式差异很小，除了 Dlx2 在牙齿中不表达之外。
总的来说，我们的组织学和表达数据强烈表明，牙齿和真皮齿状突是从相同的发育模块发育而来，并受同一组 Dlx 基因的控制。因此，牙齿和真皮齿状突应该被视为通过在身体的几个位置启动相同的基因调控网络（GRN）而发育的系列同源物。这种异位发生的机制支持了最近提出的牙原基进化的“内-外”模型。
