Extensive hybridization between two Andean warbler species with shallow divergence in mtDNA

Studying processes acting on differentiated populations upon secondary contact, such as hybridization, is important to comprehensively understand how species are formed and maintained over time. However, avian speciation studies in the tropical Andes have largely focused on the role of topographic and ecological barriers promoting divergence in allopatry, seldom examining hybridization and introgression. We describe a hybrid zone involving 2 closely related Andean warblers (Parulidae), the Golden-fronted Redstart (Myioborus ornatus) and the Spectacled Redstart (Myioborus melanocephalus). Geographic ranges of these species abut near the Colombia-Ecuador border and many specimens from the region exhibit intermediate phenotypes, but a formal description of phenotypic variation in the contact zone were heretofore lacking. We collected specimens across a transect encompassing the area where ranges abut and areas where only “pure” parental phenotypes of M. ornatus chrysops and M. melanocephalus ruficoronatus occur. We described variation in plumage traits including patterns of head and ventral coloration and tail markings based on 321 specimens. To describe genetic variation in the contact zone and over a broader phylogeographic context, we used sequences of the mitochondrial ND2 gene for 219 individuals across the transect and the entire range of both species, including all subspecies, from Venezuela to Bolivia. We documented a hybrid zone ~200 km wide based on head coloration, where intermediate plumage phenotypes are most common and “pure” forms do not overlap geographically, consistent with extensive hybridization. Across the range of the M. ornatus–M. melanocephalus complex, mitochondrial genetic structure was shallow, with genetic breaks only coinciding clearly with one topographic feature. Such low genetic structure is striking given the high diversity in plumage phenotypes and current taxonomy of the group. Our phenotypic data suggest that barriers to hybridization are not strong, and allow us to postulate hypotheses to be tested using forthcoming genomic data.

Methods
The dataset was collected from museum specimens collected in our study and previously available in museum collections. Specimen information is on Supplementary Table 1 and Supplementary Table 2.

The head coloration data was obtained by using the scoring system described in the Table 1 of the paper. Was entered on Excel, and then processed on R. The data table included here was exported from R.

The underpart coloration data was obtained by using a reflectance spectrometer. Then, the colorimetric variables were calculated on R, using functions implemented in the package pavo. The data table included here was exported from R.

The tail coloration data was entered on Excel and then processed in R. Data table included here was exported from R.

研究次生接触情境下作用于分化种群的过程（如杂交事件），对于全面解析物种随时间演化的形成与维持机制具有重要意义。但目前热带安第斯地区的鸟类物种形成研究，大多聚焦于地形与生态屏障在异域分化过程中的驱动作用，极少关注杂交与基因渐渗现象。本研究报道了一类涉及两种近缘安第斯森莺（Parulidae）的杂交带，分别为金前额红尾鸲（Myioborus ornatus）与眼镜红尾鸲（Myioborus melanocephalus）。二者的分布范围在哥伦比亚与厄瓜多尔边境附近接壤，该区域的多数标本呈现中间表型，但此前尚未有针对该接触区表型变异的正式研究报道。我们在横跨两物种分布接壤区，以及仅存在纯种M. ornatus chrysops与M. melanocephalus ruficoronatus亲本表型的区域设置样带并采集标本，共计321号，据此描述了其羽色性状的变异，包括头部、腹面着色模式与尾羽斑纹。为解析该接触区及更广系统地理背景下的遗传变异，我们针对样带内与两物种全分布范围（涵盖从委内瑞拉至玻利维亚的所有亚种）的219个个体，获取了线粒体ND2基因的序列数据。基于头部着色特征，我们记录到一条宽约200 km的杂交带：该区域内中间表型最为常见，且纯种亲本表型不存在地理重叠，这与广泛杂交的现象相符。在M. ornatus-M. melanocephalus复合群的全分布范围内，线粒体遗传结构较为浅弱，仅存在一处与地形特征明确对应的遗传分化断点。鉴于该类群丰富的羽色表型多样性与现行分类系统，这种低水平的遗传结构颇为引人关注。我们的表型数据表明，杂交障碍并不强烈，据此可提出可供后续基因组数据验证的研究假说。

研究方法
本数据集来源于本研究采集的标本以及博物馆馆藏的已公开标本。标本信息详见补充表1与补充表2。

头部着色数据采用本文表1所述的评分系统获取，先在Excel软件中录入，随后在R软件中进行处理，本文附带的数据表即从R中导出。

腹面着色数据通过反射光谱仪获取，随后利用pavo软件包中的内置函数在R软件中计算色度变量，本文附带的数据表即从R中导出。

尾羽着色数据先在Excel软件中录入，随后在R软件中进行处理，本文附带的数据表即从R中导出。