Opsin genes of select treeshrews resolve ancestral character states within Scandentia

Treeshrews are small, squirrel-like mammals in the order Scandentia, which is nested together with Primates and Dermoptera in the superordinal group Euarchonta. They are often described as living fossils, and researchers have long turned to treeshrews as a model or ecological analogue for ancestral primates. A comparative study of colour vision-encoding genes within Scandentia found a derived amino acid substitution in the long-wavelength sensitive opsin gene (OPN1LW) of the Bornean smooth-tailed treeshrew (Dendrogale melanura). The opsin, by inference, is red-shifted by ca. 6 nm with an inferred peak sensitivity of 561 nm. It is tempting to view this trait as a novel visual adaptation; however, the genetic and functional diversity of visual pigments in treeshrews is unresolved outside of Borneo. Here we report gene sequences from the northern smooth-tailed treeshrew (Dendrogale murina) and the Mindanao treeshrew (Tupaia everetti, the senior synonym of Urogale everetti). We found that the opsin genes are under purifying selection and that D. murina shares the same substitution as its congener, a result that distinguishes Dendrogale from other treeshrews, including T. everetti. We discuss the implications of opsin functional variation in light of limited knowledge about the visual ecology of smooth-tailed treeshrews.

树鼩（Treeshrews）是树鼩目（Scandentia）下一类小型、形似松鼠的哺乳类，该类群与灵长目（Primates）、皮翼目（Dermoptera）同隶属于真灵长总目（Euarchonta）。它们常被称为活化石，长期以来被研究者用作祖先灵长类的模型类群或生态类似物。一项针对树鼩目内色觉编码基因的比较研究发现，婆罗洲滑尾树鼩（Dendrogale melanura）的长波长敏感视蛋白基因（OPN1LW）存在一处衍生氨基酸替换。据推测，该视蛋白的光谱峰值发生了约6 nm的红移，峰值敏感度为561 nm。将这一性状视为一种全新的视觉适应似乎颇具吸引力，但目前除婆罗洲地区外，树鼩视觉色素的遗传与功能多样性仍未明确。本研究报道了北滑尾树鼩（Dendrogale murina）和棉兰老树鼩（Tupaia everetti，为Urogale everetti的优先同物异名）的视蛋白基因序列。研究发现，这类视蛋白基因处于纯化选择压力之下，且北滑尾树鼩与其同属物种共享同一种氨基酸替换，这一结果将滑尾树鼩属（Dendrogale）与包括棉兰老树鼩在内的其他树鼩类群区分开来。鉴于目前学界对滑尾树鼩视觉生态学的认知仍较为有限，本文还讨论了视蛋白功能变异的相关意义。