不同种类的齿鲸(Odontoceti)表现出多种牙齿形式和牙釉质类型。一些牙切体具有高度棱柱形的釉质,带有亨特-施雷格带,而牙釉质在其他物种中是残留的或完全缺乏的。不同的牙齿形状和釉质类型与替代的喂养策略相关,这些策略的范围从大多数海洋海豚和河豚中的牙齿咬和抓住猎物,到在许多喙鲸中不使用牙齿的情况下吸食较软的猎物。在分子水平上,以前的研究已经记录了一些缺乏复杂牙釉质的牙釉质物种的牙釉质特异性基因的失活突变。在更广泛的范围内,然而,目前尚不清楚现有牙本质全部多样性的牙釉质复杂性是否与牙釉质特异性基因的纯化选择的相对强度相关。这里,我们对七个釉质特异性基因(ACP4,AMBN,AMELX,AMTN,ENAM,KLK4,MMP20)在62种齿科中,代表所有现存科。从数据库中获得了33种牙本质物种的序列,其余29个物种的序列是本研究新生成的。我们筛选了这些比对的失活突变(例如,移码indels),并提供具有一个或多个失活牙釉质基因的物种中这些突变的全面目录。在牙釉质复杂性评分较高的Delphinidae(海洋海豚)和Platanistidae/Inioidea(河流海豚)中,失活突变很少见。相比之下,突变在诸如Monodonidae(独角鲸,beluga),Ziphiidae(喙鲸),Physeteroidea(抹香鲸),和Phocoenidae(海豚),其特征是更简单的釉质或甚至无釉质的牙齿。Further,几个更高级别的分类群(例如,Hyperoodon,Kogiidae,Monodontidae)在一个或多个釉质基因中具有共同的失活突变,这表明这些基因在每个进化枝的共同祖先中的功能丧失。我们还对这些基因的串联进行了选择(dN/dS)分析,并使用线性回归和Spearman的排序相关性来测试牙釉质复杂性和两种不同的选择强度度量(每百万年失活突变的数量,dN/dS值)。选择分析表明,宽松的净化选择在类植物中尤为突出,单齿,和phocoenids。线性回归和相关性分析显示,选择性压力(dN/dS值)与牙釉质复杂性之间存在很强的负相关。在牙釉质较复杂的分支上发现了更强的净化选择(低dN/dS),而在牙釉质较不复杂或无牙釉质的分支上发现了较弱的净化选择(较高dN/dS)。随着齿科的多样化,进入了多种喂养方式,特别是,猎物的吸力捕获,减少对牙列捕获猎物的依赖导致对牙釉质发育至关重要的基因的宽松选择。
Different species of toothed whales (Odontoceti) exhibit a variety of tooth forms and enamel types. Some odontocetes have highly prismatic enamel with Hunter-Schreger bands, whereas enamel is vestigial or entirely lacking in other species. Different tooth forms and enamel types are associated with alternate feeding strategies that range from biting and grasping prey with teeth in most oceanic and river dolphins to the suction feeding of softer prey items without the use of teeth in many beaked whales. At the molecular level, previous studies have documented inactivating mutations in the enamel-specific genes of some odontocete species that lack complex enamel. At a broader scale, however, it is unclear whether enamel complexity across the full diversity of extant Odontoceti correlates with the relative strength of purifying selection on enamel-specific genes. Here, we employ sequence alignments for seven enamel-specific genes (ACP4, AMBN, AMELX, AMTN, ENAM, KLK4, MMP20) in 62 odontocete species that are representative of all extant families. The sequences for 33 odontocete species were obtained from databases, and sequences for the remaining 29 species were newly generated for this study. We screened these alignments for inactivating mutations (e.g., frameshift indels) and provide a comprehensive catalog of these mutations in species with one or more inactivated enamel genes. Inactivating mutations are rare in Delphinidae (oceanic dolphins) and Platanistidae/Inioidea (river dolphins) that have higher enamel complexity scores. By contrast, mutations are much more numerous in clades such as Monodontidae (narwhal, beluga), Ziphiidae (beaked whales), Physeteroidea (sperm whales), and Phocoenidae (porpoises) that are characterized by simpler enamel or even enamelless teeth. Further, several higher-level taxa (e.g., Hyperoodon, Kogiidae, Monodontidae) possess shared inactivating mutations in one or more enamel genes, which suggests loss of function of these genes in the common ancestor of each clade. We also performed selection (dN/dS) analyses on a concatenation of these genes and used linear regression and Spearman\'s rank-order correlation to test for correlations between enamel complexity and two different measures of selection intensity (# of inactivating mutations per million years, dN/dS values). Selection analyses revealed that relaxed purifying selection is especially prominent in physeteroids, monodontids, and phocoenids. Linear regressions and correlation analyses revealed a strong negative correlation between selective pressure (dN/dS values) and enamel complexity. Stronger purifying selection (low dN/dS) is found on branches with more complex enamel and weaker purifying selection (higher dN/dS) occurs on branches with less complex enamel or enamelless teeth. As odontocetes diversified into a variety of feeding modes, in particular, the suction capture of prey, a reduced reliance on the dentition for prey capture resulted in the relaxed selection of genes that are critical to enamel development.