Opsins are light-sensitive proteins involved in many photoreceptive processes, including, but not limited to, vision and regulation of circadian rhythms. Arthropod (e.g., insects, spiders, centipedes, and crabs) opsins have been extensively researched, but the relationships and function of opsins found in lineages that are evolutionarily closely related to the arthropods remains unclear. Multiple, independent, opsin duplications are known in Tardigrada (the water bears), evidencing that protostome opsin duplications are not limited to the Arthropoda. However, the relationships, function, and expression of these new opsins are still unknown. Here, we use two tardigrade transcriptomes with deep coverage to greatly expand our knowledge of the diversity of tardigrade opsins. We reconstruct the phylogenetic relationships of the tardigrade opsins and investigate their ontogenetic expression. We found that while tardigrades have multiple opsins that evolved from lineage-specific duplications of well-understood arthropod opsins, their expression levels change during ontogeny such that most of these opsins are not co-temporally expressed. Co-temporal expression of multiple opsins underpins color vision in Arthropoda and Vertebrata. Our results clearly show duplications of both rhabdomeric and ciliary opsins within Tardigrada, forming clades specific to both the Heterotardigrada and Eutardigrada in addition to multiple independent duplications within genera. However, lack of co-temporal, ontogenetic, expression suggests that while tardigrades possess multiple opsins, they are unlikely to be able to distinguish color.

中文翻译：

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新的缓步动物视蛋白和差异表达分析显示光感知的个体变异

视蛋白是参与许多感光过程的光敏蛋白，包括但不限于视觉和昼夜节律的调节。节肢动物（例如昆虫、蜘蛛、蜈蚣和螃蟹）视蛋白已被广泛研究，但在进化上与节肢动物密切相关的谱系中发现的视蛋白的关系和功能仍不清楚。在缓步动物（水熊类）中已知多个独立的视蛋白重复，这证明原生体视蛋白重复不限于节肢动物。然而，这些新视蛋白的关系、功能和表达仍然未知。在这里，我们使用两个具有深度覆盖的缓步动物转录组来极大地扩展我们对缓步动物视蛋白多样性的了解。我们重建了缓步动物视蛋白的系统发育关系并研究了它们的个体发育表达。我们发现，虽然缓步动物具有多种视蛋白，这些视蛋白是从广为人知的节肢动物视蛋白的谱系特异性重复进化而来的，但它们的表达水平在个体发育过程中会发生变化，因此这些视蛋白中的大多数不会同时表达。多种视蛋白的同时表达支持节肢动物和脊椎动物的色觉。我们的结果清楚地显示了缓步动物内的横纹视蛋白和睫状体视蛋白的重复，除了属内的多个独立重复外，还形成了对 Heterotardigrada 和 Eutardigrada 特异的进化枝。然而，缺乏同时的、个体发育的表达表明，虽然缓步动物拥有多种视蛋白，但它们不太可能区分颜色。