海洋哺乳动物信息素嗅觉的分子进化

研究感觉基因的进化规律是动物进化领域长期探索的重要问题.哺乳动物通常具有2套嗅觉系统:主要嗅觉系统(MOS)和犁鼻器系统(VNS).其中,VNS主要感知动物个体释放的信息素分子,而信息素在动物的生殖和社会行为中起重要调节作用.为了研究动物信息素嗅觉进化的背后推动力,对海洋哺乳动物的代表物种进行了Trpc2基因(VNS功能的分子标记)的序列测定和进化分析.以前的研究表明,Trpc2基因仅在VNS中表达,其序列完整/缺失与VNS的功能完整/退化完全一致.本研究结果显示,鲸类和海牛类的Trpc2为假基因,鳍脚类的1个分支类群(海豹类)和水獭类的Trpc2也是假基因,提示VNS功能丢失,即信息素嗅觉功能退化;而北极熊和鳍脚类的另一个分支类群(海狮类)保留了1个完整的Trpc2,并且这个基因仍受强烈的净化选择和功能限制,提示信息素嗅觉功能仍然保留.进一步分析表明,信息素嗅觉退化的海兽主要在水中交配,而信息素嗅觉保留的海兽主要在陆地上交配.本研究提出了一个新的科学假说:交配场所的选择可能推动了海洋哺乳动物信息素嗅觉的进化.

Molecular Evolution of Pheromonal Olfaction in Marine Mammals

The evolution of senosry genes is one of the most important questions in the field of animal evolution. Mammals typically possess two olfactory systems: the main olfactory system (MOS) and the vomeronasal system (VNS). The VNS mainly detects intraspecific pheromonal cues released by animal individuals, and pheromone molecules play critical roles in regulating their reproductive and social behaviors. To understand the driving force underlying the evolution of pheromonal olfaction in marine animals, we analyzed the Trpc2 gene, an essential genetic component of pheromonal olfaction mediated by the VNS in four additional marine mammals alongside the published taxa that represent all five major groups of marine mamamls. Previous studies showed that Trpc2 is only expressed in the VNS, and the presence or absence of intact Trpc2 is fully consistent with the VNS functionality. Our results show that the two groups (cetaceans and sirenians) appear to possess a Trpc2 pseudogene, and otters and one clade of pinnipeds (seals) also have a Trpc2 pseudogene, whereas polar bear and the other clade of pinnipeds (sea lions) retain an intact Trpc2 that is still under strong purifying selection, suggesting independent losses of VNS function (pheromonal olfaction) in multiple groups of marine mammals. We observed a clear correlation between mating substrates and Trpc2 functionality. Specifically, three marine mammals with an intact Trpc2 largely copulate on land, while the remaining species without a functional Trpc2 copulate underwater, suggesting that mating substrates may play an important role during the evolution of pheromonal olfaction in marine mammals. This hypothesis will be tested in future if more taxa, mating observations and whole genome sequences of marine mammals are available.