张亚平研究组Cell Res揭示家鸡视觉退化的遗传机制

　　

　　经过长期的驯化和人工选择作用，家鸡与其野生祖先红原鸡除了在行为、体型、生殖周期和生长速度等存在显著差异外，其视觉功能也显著衰退。目前研究理论认为这种视觉退化是由于选择压力放松导致，但是至今这种视觉退化的遗传机制并不清楚。

　　中国科学院昆明动物研究所张亚平研究团队基于家鸡和红原鸡的群体基因组数据，并结合家鸡和红原鸡的视网膜、大脑皮层、纹状体、视叶和小脑蚓部的转录组数据，揭示了家鸡视觉退化的遗传机制。群体基因组数据分析意外发现大量视觉相关基因在家鸡驯化过程中受到正选择作用，而非选择压力放松。进一步的群体历史模拟分析亦排除了选择压力放松和搭乘效应的可能性。为了进一步探讨家鸡驯化过程中视觉变弱的遗传机制，研究人员对家鸡和红原鸡的脑组织和视网膜的转录组进行比较分析，发现大量表达显著差异的视觉通路基因，例如RHO， GUCA1A，PDE6B和NR2E3等。同时发现正选择基因的表达量在视网膜中变化显著，研究人员推测正选择可能通过影响视网膜中基因的表达来影响家鸡视觉的退化。进一步的分析发现VIT基因在家鸡中受强烈的正选择作用，且其在家鸡的视网膜中表达显著下调，但是目前该基因的具体生物学功能并不清楚。研究人员推测VIT很可能是视觉相关基因，为验证此推测，研究人员基于大量表达数据发现小鼠在不同的视觉刺激下，VIT基因在其眼睛中的表达水平显著变化。在斑马鱼中抑制VIT基因表达，其视网膜成像和视觉功能显著受损，视网膜上大量的感光细胞呈现凋亡，验证了VIT基因在视觉上的重要作用。

　　此项研究揭示正选择而非选择压力放松促使家鸡视觉退化，正选择可能通过影响视网膜中基因的表达来影响家鸡视觉的退化。研究人员推测早期视觉弱的鸡更加容易被驯化和饲养管理，人们对家鸡视觉的选择是一种无意识的选择。此研究为无意识选择在家养动物驯化中的重要作用提供了一个有趣的范例。另外，该工作也发现了一个新的视觉相关基因VIT，为视觉相关进化、疾病的研究提供新的靶点。

　　该研究结果以Positive selection rather than relaxation of functional constraint drives theevolution of vision during chicken domestication 为题发表在Cell Research 上，博士研究生王明山为第一作者，中科院院士张亚平和研究员吴东东为共同通讯作者。研究工作得到了上海生命科学研究院神经科学研究所研究员杜久林的大力支持，并得到了中国科学院重大突破专项、国家自然科学基金重大研究计划“微进化过程的多基因作用机制”培育项目的经费支持。

 

VIT抑制表达导致斑马鱼视觉行为显著受损

 

昆明动物所揭示家鸡视觉退化的无意识选择作用机制

原文摘要：

Positive selection rather than relaxation of functional constraint drives theevolution of vision during chicken domestication

As noted by Darwin, chickens have the greatest phenotypic diversity of all birds, but an interesting evolutionary difference between domestic chickens and their wild ancestor, the Red Junglefowl, is their comparatively weaker vision. Existing theories suggest that diminished visual prowess among domestic chickens reflect changes driven by the relaxation of functional constraints on vision, but the evidence identifying the underlying genetic mechanisms responsible for this change has not been definitively characterized. Here, a genome-wide analysis of the domestic chicken and Red Junglefowl genomes showed significant enrichment for positively selected genes involved in the development of vision. There were significant differences between domestic chickens and their wild ancestors regarding the level of mRNA expression for these genes in the retina. Numerous additional genes involved in the development of vision also showed significant differences in mRNA expression between domestic chickens and their wild ancestors, particularly for genes associated with phototransduction and photoreceptor development, such as RHO (rhodopsin), GUCA1A, PDE6B and NR2E3. Finally, we characterized the potential role of the VIT gene in vision, which experienced positive selection and downregulated expression in the retina of the village chicken. Overall, our results suggest that positive selection, rather than relaxation of purifying selection, contributed to the evolution of vision in domestic chickens. The progenitors of domestic chickens harboring weaker vision may have showed a reduced fear response and vigilance, making them easier to be unconsciously selected and/or domesticated.