Melanocortin 5 receptor (MC5R), which is expressed in the terminally differentiated sebaceous gland, is a G protein-coupled receptor (GPCR). MC5R exists mostly in mammals but is completely lost in whales; only the relic of MC5R can be detected in manatees, and phenotypically, they have lost sebaceous glands. Interestingly, whales and manatees are both aquatic mammals but have no immediate common ancestors. The loss of MC5R and sebaceous glands in whales and manatees is likely to be a result of convergent evolution. Here, we find that MC5R in whales and manatees are lost by two different mechanisms. Homologous recombination of MC5R in manatees and the insertion of reverse transcriptase in whales lead to the gene loss, respectively. On one hand, in manatees, there are two “TTATC” sequences flanking MC5R, and homologous recombination of the segments between the two “TTATC” sequences resulted in the partial loss of the sequence of MC5R. On the other hand, in whales, reverse transcriptase inserts between MC2R and RNMT on the chromosome led to the loss of MC5R. Based on these two different mechanisms for gene loss in whales and manatees, we finally concluded that MC5R loss might be the result of convergent evolution to the marine environment, and we explored the impact on biological function that is significant to environmental adaptation.

黑皮质素 5 受体 ( MC5R ) 在终末分化的皮脂腺中表达，是一种 G 蛋白偶联受体 (GPCR)。MC5R主要存在于哺乳动物中，但在鲸鱼中完全消失；在海牛身上只能检测到MC5R的遗迹，而且在表型上，它们已经失去了皮脂腺。有趣的是，鲸鱼和海牛都是水生哺乳动物，但没有直接的共同祖先。鲸鱼和海牛MC5R和皮脂腺的丧失很可能是趋同进化的结果。在这里，我们发现鲸鱼和海牛中的MC5R通过两种不同的机制丢失。MC5R的同源重组在海牛中和在鲸鱼中插入逆转录酶分别导致基因丢失。一方面，在海牛中，MC5R 侧翼有两个“TTATC”序列，两个“TTATC”序列之间的片段同源重组导致 MC5R 序列部分丢失。另一方面，在鲸鱼中，染色体上MC2R和RNMT之间的逆转录酶插入导致MC5R丢失。基于这两种不同的鲸鱼和海牛基因丢失机制，我们最终得出结论，MC5R损失可能是海洋环境趋同进化的结果，我们探索了对环境适应具有重要意义的生物功能的影响。