A nocturnal activity pattern is central to almost all hypotheses on the adaptive origins of primates. This enduring view has been challenged in recent years on the basis of variation in the opsin genes of nocturnal primates. A correspondence between the opsin genes and activity patterns of species in Euarchonta—the superordinal group that includes the orders Primates, Dermoptera (colugos), and Scandentia (treeshrews)—could prove instructive, yet the basic biology of the dermopteran visual system is practically unknown. Here we show that the eye of the Sunda colugo (Galeopterus variegatus) lacks a tapetum lucidum and has an avascular retina, and we report on the expression and spectral sensitivity of cone photopigments. We found that Sunda colugos have intact short wavelength sensitive (S-) and long wavelength sensitive (L-) opsin genes, and that both opsins are expressed in cone photoreceptors of the retina. The inferred peak spectral sensitivities are 451 and 562 nm, respectively. In line with adaptation to nocturnal vision, cone densities are low. Surprisingly, a majority of S-cones coexpress some L-opsin. We also show that the ratio of rates of nonsynonymous to synonymous substitutions of exon 1 of the S-opsin gene is indicative of purifying selection. Taken together, our results suggest that natural selection has favored a functional S-opsin in a nocturnal lineage for at least 45 million years. Accordingly, a nocturnal activity pattern remains the most likely ancestral character state of euprimates.

中文翻译：

---

短波长敏感视蛋白在疣鼻猴中的表达和进化：为灵长类起源争论提供信息的夜间谱系。


夜间活动模式是几乎所有关于灵长类动物适应性起源的假设的核心。近年来，由于夜间灵长类动物视蛋白基因的变异，这种持久的观点受到了挑战。 Euarchonta（包括灵长目、皮翅目（疣猴）和树鼩科）物种的视蛋白基因和活动模式之间的对应关系可能具有启发性，但皮翅目视觉系统的基本生物学实际上是未知的。在这里，我们展示了巽他（ Galeopterus variegatus ）的眼睛缺乏透明层并具有无血管视网膜，并且我们报告了视锥细胞感光色素的表达和光谱敏感性。我们发现巽他具有完整的短波长敏感（S-）和长波长敏感（L-）视蛋白基因，并且这两种视蛋白都在视网膜的视锥细胞中表达。推断的峰值光谱灵敏度分别为 451 和 562 nm。视锥细胞密度较低，与夜间视觉的适应一致。令人惊讶的是，大多数 S 锥体共表达一些 L-视蛋白。我们还表明，S-视蛋白基因外显子 1 的非同义替换与同义替换的比率表明了纯化选择。总而言之，我们的结果表明，至少 4500 万年来，自然选择一直偏爱夜间谱系中的功能性 S-视蛋白。因此，夜间活动模式仍然是真灵长类动物最有可能的祖先特征状态。