Lemopoulos Alexandre, Montoya-Burgos Juan I
Department of Genetics and Evolution University of Geneva Geneva Switzerland.
iGE3 Institute of Genetics and Genomics of Geneva Geneva Switzerland.
Evol Lett. 2021 Mar 23;5(3):240-250. doi: 10.1002/evl3.219. eCollection 2021 Jun.
Actinopterygians (ray-finned fishes) are the most diversified group of vertebrates and are characterized by a variety of protective structures covering their integument, the evolution of which has intrigued biologists for decades. Paleontological records showed that the first mineralized vertebrate skeleton was composed of dermal bony plates covering the body, including odontogenic and skeletogenic components. Later in evolution, the exoskeleton of actinopterygian's trunk was composed of scale structures. Although scales are nowadays a widespread integument cover, some contemporary lineages do not have scales but bony plates covering their trunk, whereas other lineages are devoid of any such structures. To understand the evolution of the integument coverage and particularly the transition between different structures, we investigated the pattern of scale loss events along with actinopterygian evolution and addressed the functional relationship between the scaleless phenotype and the ecology of fishes. Furthermore, we examined whether the emergence of trunk bony plates was dependent over the presence or absence of scales. To this aim, we used two recently published actinopterygian phylogenies, one including >11,600 species, and by using stochastic mapping and Bayesian methods, we inferred scale loss events and trunk bony plate acquisitions. Our results reveal that a scaled integument is the most frequent state in actinopterygians, but multiple independent scale loss events occurred along their phylogeny with essentially no scale re-acquisition. Based on linear mixed models, we found evidence supporting that after a scale loss event, fishes tend to change their ecology and adopt a benthic lifestyle. Furthermore, we show that trunk bony plates appeared independently multiple times along the phylogeny. By using fitted likelihood models for character evolution, we show that trunk bony plate acquisitions were dependent on a previous scale loss event. Overall, our findings support the hypothesis that integument cover is a key evolutionary trait underlying actinopterygian radiation.
辐鳍鱼是脊椎动物中最多样化的类群，其特征是体表覆盖着各种保护结构，几十年来，这些结构的进化一直吸引着生物学家。古生物学记录表明，最早的矿化脊椎动物骨骼由覆盖身体的真皮骨板组成，包括牙源性和骨骼生成成分。在进化后期，辐鳍鱼躯干的外骨骼由鳞片结构组成。虽然如今鳞片是一种广泛存在的体表覆盖物，但一些现代谱系没有鳞片，而是有覆盖其躯干的骨板，而其他谱系则没有任何此类结构。为了了解体表覆盖物的进化，特别是不同结构之间的转变，我们研究了鳞片丢失事件的模式以及辐鳍鱼的进化，并探讨了无鳞表型与鱼类生态之间的功能关系。此外，我们研究了躯干骨板的出现是否依赖于鳞片的有无。为此，我们使用了最近发表的两个辐鳍鱼系统发育树，其中一个包含超过11600个物种，并通过随机映射和贝叶斯方法，推断出鳞片丢失事件和躯干骨板的获得情况。我们的结果表明，有鳞体表是辐鳍鱼中最常见的状态，但在它们的系统发育过程中发生了多次独立的鳞片丢失事件，基本上没有重新获得鳞片。基于线性混合模型，我们发现有证据支持在鳞片丢失事件之后，鱼类倾向于改变其生态并采用底栖生活方式。此外，我们表明躯干骨板在系统发育过程中多次独立出现。通过使用适合性状进化的似然模型，我们表明躯干骨板的获得依赖于先前的鳞片丢失事件。总体而言，我们的研究结果支持这样的假设，即体表覆盖物是辐鳍鱼辐射背后的关键进化特征。
