Data from: Sharp genetic discontinuity across a unimodal Heliconius hybrid zone

Hybrid zones are powerful natural systems to study evolutionary processes to gain an understanding of adaptation and speciation. In the Cauca Valley (Colombia), two butterfly races, Heliconius cydno cydnides and Heliconius cydno weymeri, meet and hybridize. We characterized this hybrid zone using a combination of mitochondrial DNA (mtDNA) sequences, Amplified Fragment Length Polymorphisms (AFLPs), microsatellites, and sequences for nuclear loci within and outside of the genomic regions that cause differences in wing color pattern. The hybrid zone is largely composed of individuals of mixed ancestry. However, there is strong genetic discontinuity between the hybridizing races in mtDNA and, to a lesser extent, in all nuclear markers surveyed. The mtDNA clustering of H. c. cydnides with the H. cydno race from the Magdalena Valley and H. c. weymeri with the H. cydno race from the pacific coast suggests that H. c. cydnides colonized the Cauca Valley from the north, whereas H. c. weymeri did so by crossing the Andes in the southern part, implying a secondary contact origin. Colonization of the valley by H. cydno was accompanied by mimicry shift. Strong ecological isolation, driven by locally adaptive differences in mimetic wing patterns, is playing an important role in maintaining the hybrid zone. However, selection on wing pattern alone is not sufficient to explain the genetic discontinuity observed. There is evidence for differences in male mating preference, but the contribution of additional barriers need further investigation. Overall our results support the idea that speciation is a cumulative process, where the combination of multiple isolation barriers, combined with major phenotypic differences, facilitate population divergence in face of gene flow.

杂交带是探究演化进程、解析适应与物种形成机理的极具科研价值的天然实验系统。
在哥伦比亚考卡山谷中，袖蝶属（Heliconius）的两个地理宗——Heliconius cydno cydnides与Heliconius cydno weymeri——相遇并发生杂交。
本研究结合线粒体DNA（mtDNA）序列、扩增片段长度多态性（AFLPs）、微卫星标记，以及调控翅色图案差异的基因组区域内外的核基因座序列，对该杂交带开展了系统表征与分析。
该杂交带的个体大多具有混合遗传祖源。然而，线粒体DNA层面呈现出杂交两地理宗间显著的遗传不连续性；在所有检测的核标记中，该遗传不连续性的强度相对较弱。
针对mtDNA的聚类分析结果显示，H. c. cydnides与马格达莱纳河谷的H. cydno地理宗聚为一支，而H. c. weymeri则与太平洋沿岸的H. cydno地理宗聚类，这表明H. c. cydnides从北部拓殖至考卡山谷，而H. c. weymeri则经由南部的安第斯山脉完成拓殖，暗示该杂交带起源于二次接触事件。
H. cydno对该山谷的拓殖过程伴随拟态模式的转变。
由拟态翅色模式的局部适应性差异驱动的强烈生态隔离，在维持该杂交带的过程中发挥了关键作用。
但仅依靠翅色模式的选择压力，不足以解释本次研究观测到的遗传不连续性。
现有证据表明雄性交配偏好存在差异，但其他额外生殖隔离屏障的具体贡献仍需进一步探究。
综上，本研究结果支持物种形成是一个累积过程的观点：在基因流存在的背景下，多重生殖隔离屏障与显著的表型差异共同作用，推动种群在基因流的干扰下实现分化。