School of Biological and Chemical Sciences, Queen Mary, University of London, London, United Kingdom.
Department of Vertebrate Zoology, Smithsonian National Museum of Natural History, Washington, DC.
Mol Biol Evol. 2020 Jul 1;37(7):2069-2083. doi: 10.1093/molbev/msaa070.
The transition to an aquatic lifestyle in cetaceans (whales and dolphins) resulted in a radical transformation in their sensory systems. Toothed whales acquired specialized high-frequency hearing tied to the evolution of echolocation, whereas baleen whales evolved low-frequency hearing. More generally, all cetaceans show adaptations for hearing and seeing underwater. To determine the extent to which these phenotypic changes have been driven by molecular adaptation, we performed large-scale targeted sequence capture of 179 sensory genes across the Cetacea, incorporating up to 54 cetacean species from all major clades as well as their closest relatives, the hippopotamuses. We screened for positive selection in 167 loci related to vision and hearing and found that the diversification of cetaceans has been accompanied by pervasive molecular adaptations in both sets of genes, including several loci implicated in nonsyndromic hearing loss. Despite these findings, however, we found no direct evidence of positive selection at the base of odontocetes coinciding with the origin of echolocation, as found in studies examining fewer taxa. By using contingency tables incorporating taxon- and gene-based controls, we show that, although numbers of positively selected hearing and nonsyndromic hearing loss genes are disproportionately high in cetaceans, counts of vision genes do not differ significantly from expected values. Alongside these adaptive changes, we find increased evidence of pseudogenization of genes involved in cone-mediated vision in mysticetes and deep-diving odontocetes.
在鲸类（鲸鱼和海豚）向水生生活方式的转变中，它们的感觉系统发生了根本性的变化。有齿鲸获得了与回声定位进化相关的专门的高频听觉，而须鲸则进化出了低频听觉。更一般地说，所有的鲸目动物都表现出适应水下听觉和视觉的特征。为了确定这些表型变化在多大程度上是由分子适应驱动的，我们对 179 个感觉基因在鲸目中进行了大规模的靶向序列捕获，包括来自所有主要分支的多达 54 种鲸类以及它们最近的亲属河马。我们在与视觉和听觉相关的 167 个基因中筛选了正选择，并发现鲸类的多样化伴随着这两组基因的普遍分子适应，包括几个与非综合征性听力损失有关的基因座。然而，尽管有这些发现，我们没有发现正选择的直接证据，与回声定位起源时的齿鲸类的基础一致，正如在研究较少分类单元时发现的那样。通过使用包含分类单元和基因控制的列联表，我们表明，尽管在鲸类中，正选择的听觉和非综合征性听力损失基因的数量不成比例地高，但视觉基因的数量与预期值没有显著差异。除了这些适应性变化之外，我们还发现了更多的证据表明神秘鲸类和深潜齿鲸类中与视锥细胞介导的视觉相关的基因发生了假基因化。
