Data from: Selection on a genetic polymorphism counteracts ecological speciation in a stick insect

The interplay between selection and aspects of the genetic architecture of traits (such as linkage, dominance, and epistasis) can either drive or constrain speciation. Despite accumulating evidence that speciation can progress to “intermediate” stages—with populations evolving only partial reproductive isolation—studies describing selective mechanisms that impose constraints on speciation are more rare than those describing drivers. The stick insect Timema cristinae provides an example of a system in which partial reproductive isolation has evolved between populations adapted to different host plant environments, in part due to divergent selection acting on a pattern polymorphism. Here, we demonstrate how selection on a green/melanistic color polymorphism counteracts speciation in this system. Specifically, divergent selection between hosts does not occur on color phenotypes because melanistic T. cristinae are cryptic on the stems of both host species, are resistant to a fungal pathogen, and have a mating advantage. Using genetic crosses and genome-wide association mapping, we quantify the genetic architecture of both the pattern and color polymorphism, illustrating their simple genetic control. We use these empirical results to develop an individual-based model that shows how the melanistic phenotype acts as a “genetic bridge” that increases gene flow between populations living on different hosts. Our results demonstrate how variation in the nature of selection acting on traits, and aspects of trait genetic architecture, can impose constraints on both local adaptation and speciation.

选择与性状遗传结构（genetic architecture）的多个维度（如连锁（linkage）、显性（dominance）与上位性（epistasis））之间的相互作用，既可以推动物种形成进程，也可能对其施加制约。尽管越来越多的证据表明，物种形成可进展至“中间阶段”——即种群仅演化出部分生殖隔离，但阐释对物种形成形成制约的选择机制的研究，仍远少于探讨其推动因素的相关研究。竹节虫Timema cristinae即为典型系统：适应不同寄主植物环境的种群间已演化出部分生殖隔离，这在一定程度上源于作用于斑纹多态性（pattern polymorphism）的歧化选择。本研究阐明了针对绿色/黑化型体色多态性（color polymorphism）的选择如何在该系统中抵消物种形成进程。具体而言，体色表型并未受到寄主间的歧化选择：黑化型T. cristinae在两种寄主植物的茎干上均具有隐蔽性，可抵御真菌病原体，且拥有交配优势。本研究通过遗传杂交与全基因组关联定位（genome-wide association mapping）技术，量化了斑纹与体色多态性的遗传结构，证实二者均受简单遗传机制调控。基于上述实验结果，我们构建了基于个体的模型（individual-based model），该模型揭示了黑化型表型如何充当“遗传桥梁”，提升了不同寄主种群间的基因流动水平。本研究结果表明，作用于性状的选择性质的变异，以及性状遗传结构的相关维度，如何对局部适应与物种形成均形成制约。