Apparently inefficient structure of vertebrate eyes: microlens in photoreceptor cells as a possible cause

2020 Fiscal Year Final Research Report

• Project/Area Number: 18K19362
• Research Category: Grant-in-Aid for Challenging Research (Exploratory)
• Allocation Type: Multi-year Fund
• Review Section: Medium-sized Section 45:Biology at organismal to population levels and anthropology, and related fields
• Research Institution: Kyoto University
• Principal Investigator: Koga Akihiko 京都大学, 霊長類研究所, 教授 (80192574)
• Project Period (FY): 2018-06-29 – 2021-03-31
• Keywords: 眼 / 視細胞 / 夜行性 / ゲノム / ヘテロクロマチン / 進化 / 脊椎動物

Outline of Final Research Achievements

In photoreceptor cells of many nocturnal mammals, heterochromatin localizes to the central region of the nucleus and serves as a lens that leads to elevated night vision. This situation of the nucleus is called the nocturnal type. Photoreceptor cells of this type have hitherto been observed only in mammals. It is therefore widely believed that the nocturnal type emerged in the mammalian lineage after its divergence from reptiles. Considering that the nocturnal type is an adaptation to dark environments, it is a possible hypothesis that the nocturnal type already existed at the time point of the eye acquisition by an early vertebrate. This hypothesis is supported if the nocturnal types is found in a taxon other than mammals. Photoreceptor cells of 8 species, which belonged to reptiles, amphibians, or fishes, were examined by immunostaining experiments. One of these species was found to have photoreceptor cells that exhibited a situation close to the nocturnal type.

Free Research Field

進化遺伝学

Academic Significance and Societal Importance of the Research Achievements

脊椎動物の眼では、光は視細胞の光受容部位に達する前に、視神経の層を通過し、さらに視細胞の核の部分を通過する。その間に情報の減衰や散逸が起こることが想像され、不合理な構造であるといえる。いったん成立した発生過程を変えられないための制約と、考えられる。しかし、夜行性型の核を現出するためには、この構造は必要である。この点を考慮すると、脊椎動物が眼を獲得した時点ではこの構造はむしろ合理的であったとの仮説が、成り立つ。得られた結果は、この仮説を積極的に支持しないとはいえ、否定するものではない。仮説をさらに追求するための足掛かりが得られたといえる。外洋の深海魚を調べることが効率的であるとの示唆である。