Department of Genetics, University of Georgia, Athens, GA, 30602, USA.
Present address: Department of Biology, Bucknell University, Lewisburg, PA, 17837, USA.
BMC Evol Biol. 2018 Aug 31;18(1):129. doi: 10.1186/s12862-018-1251-9.
Genes underlying signal production and reception are expected to evolve to maximize signal detection in specific environments. Fireflies vary in their light signal color both within and between species, and thus provide an excellent system in which to study signal production and reception in the context of signaling environments. Differences in signal color have been hypothesized to be due to variation in the sequence of luciferase, the enzyme that catalyzes the light reaction. Similarly, differences in visual sensitivity, which are expected to match signal color, have been hypothesized to be due to variation in the sequence of opsins, the protein component of visual pigments. Here we investigated (1) whether sequence variation in luciferase correlates with variation in signal color and (2) whether sequence variation in opsins correlates with inferred matching visual sensitivity across populations of a widespread North American firefly species, Photinus pyralis. We further tested (3) whether selection has acted on these loci by examining their population-level differentiation relative to the distribution of differentiation derived from a genome-wide sample of loci generated by double-digest RADseq.
We found virtually no coding variation in luciferase or opsins. However, there was extreme divergence in non-coding variation in luciferase across populations relative to a panel of random genomic loci.
The absence of protein variation at both loci challenges the paradigm that variation in signal color and visual sensitivity in fireflies is exclusively due to coding variation in luciferase and opsin genes. Instead, flash color variation within species must involve other mechanisms, such as abdominal pigmentation or regulation of light organ physiology. Evidence for selection at non-coding variation in luciferase suggests that selection is targeting luciferase regulation and may favor differ expression levels across populations.
在特定环境中，信号产生和接收的基因有望进化以最大限度地提高信号检测能力。萤火虫的光信号颜色在种内和种间都存在差异，因此为研究信号产生和接收在信号环境中的情况提供了一个极好的系统。信号颜色的差异据推测是由于荧光素酶（催化光反应的酶）序列的变异。同样，视觉敏感性的差异（预期与信号颜色相匹配）也被假设是由于视觉色素蛋白成分视蛋白的序列变异所致。在这里，我们研究了（1）荧光素酶的序列变异是否与信号颜色的变化有关，以及（2）视蛋白的序列变异是否与广泛分布于北美萤火虫 Photinus pyralis 种的种群中推断出的匹配视觉敏感性有关。我们进一步测试了（3）这些基因座是否受到了选择的作用，通过检查它们相对于来自双酶切 RADseq 生成的全基因组样本的分化分布的种群水平分化来检验。
我们发现荧光素酶或视蛋白几乎没有编码变异。然而，在与一组随机基因组位点相比时，荧光素酶的非编码变异在种群间存在极端分歧。
这两个基因座的蛋白质变异缺失挑战了这样一种观点，即萤火虫的信号颜色和视觉敏感性的变化完全是由于荧光素酶和视蛋白基因的编码变异所致。相反，种内闪光颜色的变化必须涉及其他机制，例如腹部色素沉着或光器官生理学的调节。荧光素酶非编码变异的选择证据表明，选择是针对荧光素酶的调节，并可能有利于种群间表达水平的差异。
