Department of Natural Sciences, University of Michigan-Dearborn, Dearborn, MI 48128, USA.
Department of Molecular and Cell Biology, University of California, Berkeley, CA 94720, USA.
Integr Comp Biol. 2021 Nov 17;61(5):1933-1945. doi: 10.1093/icb/icab039.
Extant anurans (frogs and toads) exhibit reduced dentition, ranging from a lack of mandibular teeth to complete edentulation, as observed in the true toads of the family Bufonidae. The evolutionary time line of these reductions remains vague due to a poor fossil record. Previous studies have demonstrated an association between the lack of teeth in edentulous vertebrates and the pseudogenization of the major tooth enamel gene amelogenin (AMEL) through accumulation of deleterious mutations and the disruption of its coding sequence. In this study, we have harnessed the pseudogenization of AMEL as a molecular dating tool to correlate loss of dentition with genomic mutation patterns during the rise of the family Bufonidae. Specifically, we have utilized AMEL pseudogenes in three members of the family as a tool to estimate the putative date of edentulation in true toads. Comparison of AMEL sequences from Rhinella marina, Bufo gargarizans and Bufo bufo, with nine extant, dentulous frogs, revealed mutations confirming AMEL inactivation in Bufonidae. AMEL pseudogenes in modern bufonids also exhibited remarkably high 86-93% sequence identity among each other, with only a slight increase in substitution rate and relaxation of selective pressure, in comparison with functional copies in other anurans. Moreover, using selection intensity estimates and synonymous substitution rates, analysis of functional and pseudogenized AMEL resulted in an estimated inactivation window of 46-60 million years ago in the lineage leading to modern true toads, a time line that coincides with the rise of the family Bufonidae.
现存的无尾两栖类（青蛙和蟾蜍）表现出牙齿数量减少，从下颌牙齿缺失到完全无齿，就像蟾蜍科的真蟾蜍那样。由于化石记录不佳，这些减少的进化时间线仍然模糊不清。以前的研究表明，无齿的有颌脊椎动物与主要牙齿釉质基因 amelogenin (AMEL) 的假基因化之间存在关联，其通过积累有害突变和破坏其编码序列而发生。在这项研究中，我们利用 AMEL 的假基因化作为分子定年工具，将牙齿缺失与蟾蜍科兴起过程中的基因组突变模式相关联。具体来说，我们利用蟾蜍科的三个成员中的 AMEL 假基因作为工具，估计真蟾蜍无齿的假定日期。与 9 种现存的有齿青蛙相比，对 Rhinella marina、Bufo gargarizans 和 Bufo bufo 的 AMEL 序列进行比较，发现突变证实了蟾蜍科 AMEL 的失活。现代蟾蜍科的 AMEL 假基因彼此之间也表现出惊人的高 86-93%的序列同一性，与其他无尾两栖动物的功能拷贝相比，只有替代率略有增加，选择压力略有放松。此外，使用选择强度估计和同义替换率，对功能和假基因化的 AMEL 进行分析，导致在导致现代真蟾蜍的谱系中，大约在 4600 万至 6000 万年前发生了失活，这一时间线与蟾蜍科的兴起相吻合。
