Data from: Dissecting the variation of a visual trait: the proximate basis of UV-Visible reflectance in crab spiders (Thomisidae)

1. The astounding diversity of animal colouration is indicative of a wide variety of selection pressures. Despite great interest in adaptive function, detailed understanding of the constituent elements of colour traits is lacking for many systems. Such information is important in allowing more accurate appraisals of colour variation and its potential production costs. 2. In this study, we ‘dissect’ the dorsal colour of crab spiders (Thomisidae) to examine the mechanistic basis of a polyphenic colour trait. These spiders possess the ability to alter reflectance in the ultraviolet (UV), violet and blue wavelengths, changing their colour within days. We investigate and compare the proximate mechanistic basis of colour production in multiple phenotypes of three species using histology and spectrophotometry. 3. Our analyses indicate that the spider cuticle is not equivalently transparent to light across the spectrum (300-700 nm) – as previously argued – and contributes to colour variation. UV light is reflected from guanine crystals, present in storage cells ventral to the hypodermis. The crystals are exposed through a partially UV-transmitting hypodermis and cuticle. Variation from white to yellow is likely mediated through pigments/crystals present in different oxidative stages in the hypodermal cells. 4. Simple mechanistic changes are therefore necessary to produce the observed variation, and likely underlie the evolutionary and ontogenetic lability of this trait. Our findings imply that either a UV-reflective abdomen was the ancestral state for crab spiders, or, if pre-dated by UV-absorbent hypodermal pigments, the evolution of UV-reflection has only involved the exposure of underlying guanine crystals through an otherwise clear hypodermis.

Usage Notes
R FunctionsR functions for summarising and analysing spectral data.functions.RSpectrum filesSpectra folder contains all spectral datum files collected from spiders.spectra.zip

动物色彩的惊人多样性反映了广泛的选择压力。尽管对适应性功能的兴趣极大，但对于许多系统而言，对颜色性状构成要素的详细理解尚显不足。此类信息对于更准确评估颜色变异及其潜在生产成本至关重要。在本研究中，我们剖析了蟹蛛（Thomisidae）的背色，以探究多相颜色性状的机制基础。这些蜘蛛具备改变紫外（UV）、紫色和蓝色波长反射率的能力，在数日内改变其颜色。我们利用组织学和分光光度法，对三个物种的多个表型的颜色产生近端机制进行比较研究。我们的分析表明，蜘蛛的表皮并非在整个光谱（300-700 nm）范围内对光线具有等效的透明性——正如先前所论证的——并且这有助于颜色变异。紫外光从位于真皮下方储存细胞中的尿酸晶体反射出来。这些晶体通过部分透紫外线的真皮和表皮暴露出来。从白色到黄色的变化很可能是通过真皮细胞中不同氧化阶段存在的色素/晶体介导的。因此，为了产生观察到的变异，必要的简单机制改变是必要的，并且可能构成了这一性状的进化和发育易变性的基础。我们的发现暗示，要么紫外反射腹部是蟹蛛的原始状态，要么，如果先于紫外吸收真皮色素，紫外反射的进化仅涉及通过原本清晰的真皮暴露出下方的尿酸晶体。