Emerling Christopher A, Springer Mark S, Gatesy John, Jones Zachary, Hamilton Deana, Xia-Zhu David, Collin Matt, Delsuc Frédéric
Museum of Vertebrate Zoology, University of California, Berkeley, Berkeley, CA, 94720, USA.
Institut des Sciences de l'Evolution de Montpellier (ISEM), CNRS, IRD, EPHE, Université de Montpellier, Montpellier, France.
Open Res Eur. 2021 Dec 13;1:75. doi: 10.12688/openreseurope.13795.2. eCollection 2021.
: The study of regressive evolution has yielded a wealth of examples where the underlying genes bear molecular signatures of trait degradation, such as pseudogenization or deletion. Typically, it appears that such disrupted genes are limited to the function of the regressed trait, whereas pleiotropic genes tend to be maintained by natural selection to support their myriad purposes. One such set of pleiotropic genes is involved in the synthesis ( , ) and signaling ( , ) of melatonin, a hormone secreted by the vertebrate pineal gland. Melatonin provides a signal of environmental darkness, thereby influencing the circadian and circannual rhythmicity of numerous physiological traits. Therefore, the complete loss of a pineal gland and the underlying melatonin pathway genes seems likely to be maladaptive, unless compensated by extrapineal sources of melatonin. : We examined , , and in 123 vertebrate species, including pineal-less placental mammals and crocodylians. We searched for inactivating mutations and modelled selective pressures (dN/dS) to test whether the genes remain functionally intact. : We report that crocodylians retain intact melatonin genes and express and in their eyes, whereas all four genes have been repeatedly inactivated in the pineal-less xenarthrans, pangolins, sirenians, and whales. Furthermore, colugos have lost these genes, and several lineages of subterranean mammals have partial melatonin pathway dysfunction. These results are supported by the presence of shared inactivating mutations across clades and analyses of selection pressure based on the ratio of non-synonymous to synonymous substitutions (dN/dS), suggesting extended periods of relaxed selection on these genes. The losses of melatonin synthesis and signaling date to tens of millions of years ago in several lineages of placental mammals, raising questions about the evolutionary resilience of pleiotropic genes, and the causes and consequences of losing melatonin pathways in these species.
对退化进化的研究已产生了大量实例，其中相关基础基因带有性状退化的分子特征，如基因假化或缺失。通常，此类功能受损的基因似乎仅限于退化性状的功能，而多效性基因往往通过自然选择得以保留，以支持其多种功能。一组这样的多效性基因参与了褪黑素的合成（ ， ）和信号传导（ ， ），褪黑素是脊椎动物松果体分泌的一种激素。褪黑素提供环境黑暗信号，从而影响众多生理性状的昼夜节律和年节律。因此，松果体以及基础褪黑素通路基因的完全缺失似乎可能是适应不良的，除非有松果体外的褪黑素来源进行补偿。：我们研究了123种脊椎动物中的 、 、 和 ，包括无松果体的胎盘哺乳动物和鳄鱼。我们寻找失活突变并对选择压力（dN/dS）进行建模，以测试这些基因是否仍保持功能完整。：我们报告称，鳄鱼保留了完整的褪黑素基因，并在其眼睛中表达 和 ，而在无松果体的贫齿目动物、穿山甲、海牛和鲸鱼中，所有这四个基因都已多次失活。此外，鼯猴失去了这些基因，并且几个地下哺乳动物谱系存在部分褪黑素通路功能障碍。这些结果得到了不同进化枝中共享失活突变的存在以及基于非同义替换与同义替换比率（dN/dS）的选择压力分析的支持，表明这些基因经历了长时间的松驰选择。褪黑素合成和信号传导的丧失可追溯到数千万年前的几个胎盘哺乳动物谱系，这引发了关于多效性基因进化弹性以及这些物种中褪黑素通路丧失的原因和后果的问题。
