Jiangsu Key Laboratory for Biodiversity and Biotechnology, College of Life Sciences, Nanjing Normal University, 1 Wenyuan Road, Nanjing 210023, China.
BMC Evol Biol. 2013 Sep 9;13:189. doi: 10.1186/1471-2148-13-189.
Osmoregulation was a primary challenge for cetaceans during the evolutionary transition from a terrestrial to a mainly hyperosmotic environment. Several physiological mechanisms have been suggested to maintain the water and salt balance in cetaceans, but their genetic and evolutionary bases remain poorly explored. The current study investigated the genes involved in osmoregulation in cetaceans and compared them with their counterparts in terrestrial mammals to test whether adaptive evolution occurred during secondary aquatic adaptation.
The present study analyzed the molecular evolution of 11 osmoregulation-related genes in 11 cetacean species, which represented all of the major cetacean clades. The results demonstrated positive selection acting on angiotensin converting enzyme (ACE), angiotensinogen (AGT), SLC14A2, and aquaporin 2 (AQP2). This evidence for the positive selection of AQP2 and SLC14A2 suggests that the adaptive evolution of these genes has helped to enhance the capacity for water and urea transport, thereby leading to the concentration of urine, which is an efficient mechanism for maintaining the water balance. By contrast, a series of positively selected amino acid residues identified in the ACE and AGT (two key members of the renin-angiotensin-aldosterone system, RAAS) proteins of cetaceans suggests that RAAS might have been adapted to maintain the water and salt balance in response to a hyperosmotic environment. Radical amino acid changes in positively selected sites were distributed among most internal and terminal branches of the cetacean phylogeny, which suggests the pervasively adaptive evolution of osmoregulation since the origin of cetaceans and their subsequent diversification.
This is the first comprehensive analysis of the molecular evolution of osmoregulation-related genes in cetaceans in response to selection pressure from a generally hyperosmotic environment. Four genes, i.e., AQP2, SLC14A2, ACE, and AGT were subject to positive selection in cetaceans, which suggests that cetaceans may have adapted to maintain their water and salt balance. This also suggests that cetaceans may have evolved an effective and complex mechanism for osmoregulation.
从陆地到主要高渗环境的进化过渡中，渗透压调节是鲸类动物面临的主要挑战。已经提出了几种生理机制来维持鲸类动物的水盐平衡，但它们的遗传和进化基础仍未得到充分探索。本研究调查了鲸类动物中与渗透压调节相关的基因，并将其与陆地哺乳动物中的对应基因进行比较，以检验在二次水生适应过程中是否发生了适应性进化。
本研究分析了 11 种鲸类物种中 11 种与渗透压调节相关的基因的分子进化，这些物种代表了所有主要的鲸类分支。结果表明，血管紧张素转换酶（ACE）、血管紧张素原（AGT）、SLC14A2 和水通道蛋白 2（AQP2）受到正选择作用。AQP2 和 SLC14A2 正选择的证据表明，这些基因的适应性进化有助于增强水和尿素的转运能力，从而导致尿液浓缩，这是维持水平衡的有效机制。相比之下，鲸类 ACE 和 AGT（肾素-血管紧张素-醛固酮系统，RAAS 的两个关键成员）蛋白中鉴定的一系列正选择氨基酸残基表明，RAAS 可能已经适应了高渗环境以维持水盐平衡。正选择位点的激进氨基酸变化分布在鲸类系统发育的大多数内部和末端分支中，这表明自鲸类起源及其随后的多样化以来，渗透压调节的适应性进化是普遍存在的。
这是首次全面分析鲸类动物在高渗环境选择压力下与渗透压调节相关基因的分子进化。AQP2、SLC14A2、ACE 和 AGT 这四个基因在鲸类动物中受到正选择，这表明鲸类动物可能已经适应了维持其水盐平衡。这也表明鲸类动物可能已经进化出了一种有效的、复杂的渗透压调节机制。
