Stemmer Manuel, Schuhmacher Laura-Nadine, Foulkes Nicholas S, Bertolucci Cristiano, Wittbrodt Joachim
Centre for Organismal Studies (COS), Heidelberg University, 69120 Heidelberg, Germany.
Institute of Toxicology and Genetics (ITG), Karlsruhe Institute of Technology, 76131 Karlsruhe, Germany.
Development. 2015 Feb 15;142(4):743-752. doi: 10.1242/dev.114629. Epub 2015 Jan 23.
The troglomorphic phenotype shared by diverse cave-dwelling animals is regarded as a classical example of convergent evolution. One unresolved question is whether the characteristic eye loss in diverse cave species is based on interference with the same genetic program. Phreatichthys andruzzii, a Somalian cavefish, has evolved under constant conditions in complete darkness and shows severe troglomorphic characteristics, such as complete loss of eyes, pigments and scales. During early embryonic development, a complete eye is formed but is subsequently lost. In Astyanax mexicanus, another blind cavefish, eye loss has been attributed to interference during eye field patterning. To address whether similar pathways have been targeted by evolution independently, we investigated the retinal development of P. andruzzii, studying the expression of marker genes involved in eye patterning, morphogenesis, differentiation and maintenance. In contrast to Astyanax, patterning of the eye field and evagination of the optic vesicles proceeds without obvious deviation. However, the subsequent differentiation of retinal cell types is arrested during generation of the first-born cell type, retinal ganglion cells, which also fail to project correctly to the optic tectum. Eye degeneration in both species is driven by progressive apoptosis. However, it is retinal apoptosis in Phreatichthys that progresses in a wave-like manner and eliminates progenitor cells that fail to differentiate, in contrast to Astyanax, where lens apoptosis appears to serve as a driving force. Thus, evolution has targeted late retinal differentiation events, indicating that there are several ways to discontinue the development and maintenance of an eye.
多种穴居动物共有的洞穴适应性表型被视为趋同进化的经典例子。一个尚未解决的问题是，不同洞穴物种特有的眼睛退化是否基于对相同遗传程序的干扰。索马里洞穴鱼鲁氏平胸鮡在完全黑暗的恒定条件下进化，表现出严重的洞穴适应性特征，如眼睛、色素和鳞片完全丧失。在胚胎早期发育过程中，会形成完整的眼睛，但随后会消失。在另一种盲眼洞穴鱼墨西哥丽脂鲤中，眼睛退化被归因于眼场模式形成过程中的干扰。为了探究类似的途径是否被进化独立靶向，我们研究了鲁氏平胸鮡的视网膜发育，研究了参与眼模式形成、形态发生、分化和维持的标记基因的表达。与墨西哥丽脂鲤不同，眼场模式形成和视泡外翻过程没有明显偏差。然而，视网膜细胞类型的后续分化在第一代细胞类型视网膜神经节细胞产生过程中停止，这些细胞也无法正确投射到视顶盖。两种物种的眼睛退化都是由渐进性细胞凋亡驱动的。然而，鲁氏平胸鮡的视网膜凋亡以波浪状方式进行，并消除未能分化的祖细胞，而在墨西哥丽脂鲤中，晶状体凋亡似乎是驱动力。因此，进化靶向了视网膜晚期分化事件，这表明有几种方式可以停止眼睛的发育和维持。
