Evolutionary dynamics of hybridization and introgression following the recent colonization of Glossy Ibis (Aves: Plegadis falcinellus) into the New World

Geographic range shifts can cause secondary contact and hybridization between closely related species, revealing mechanisms of species formation and integrity. These dynamics typically play out in restricted geographic regions, but highly vagile species may experience major distributional changes resulting in broad areas of contact. The Glossy Ibis (Plegadis falcinellus) is a mobile waterbird of the Old World and Australia that colonized eastern North America in the early 19th Century and came into contact with the native White-faced Ibis (P. chihi). Putative hybrids between the two species have been observed across North America. To examine the population genomic consequences of this natural invasion, we sequenced 4,616 ultraconserved elements from 66 individuals sampled across the distributions of falcinellus, chihi, and the Puna Ibis (P. ridgwayi) of South America. We found genomic differentiation among the three species. Loci with high sequence divergence were often shared across all pairwise species comparisons, were associated with regions of high nucleotide diversity, and were concentrated on the Z chromosome. We detected signals of genetic admixture between chihi and falcinellus in individuals both near and far from the core area of sympatry. Genomic cline analyses revealed evidence of greater introgression into falcinellus from chihi, but we found little evidence for selection against hybrids. We also found signals of admixture between ridgwayi and South American populations of chihi. Our results indicate vagile species can experience pervasive introgression upon secondary contact, although these dynamics may be more ephemeral than the stable hybrid zones often observed in less dispersive organisms.地理分布范围的变迁可引发近缘物种间的二次接触与杂交现象，进而揭示物种形成与物种完整性维持的内在机制。这类动态通常局限于有限的地理区域内，但扩散能力极强的物种可经历大规模分布范围变化，从而形成大范围的物种接触区域。彩鹮（Glossy Ibis, *Plegadis falcinellus*）作为分布于旧大陆与澳大利亚的流动性水鸟，于19世纪早期扩散至北美东部，并与本土的白脸鹮（White-faced Ibis, *P. chihi*）产生了接触。目前已在北美全境观测到这两个物种间的疑似杂交个体。为探究此次自然入侵所带来的种群基因组学效应，我们对采自彩鹮、白脸鹮以及南美普纳鹮（Puna Ibis, *P. ridgwayi*）分布范围内的66个个体，完成了4616个超保守元件（ultraconserved elements）的测序。研究发现三个物种间存在显著的基因组分化：序列分化程度较高的基因座通常在所有物种两两比较中均存在共享信号，且与高核苷酸多样性区域相关，并集中分布于Z染色体上。我们在距同域分布（sympatry）核心区域远近不同的个体中，均检测到了白脸鹮与彩鹮之间的遗传混合信号。基因组梯度分析（genomic cline analyses）结果表明，白脸鹮向彩鹮的基因渐渗（introgression）程度更高，但我们几乎未发现针对杂交个体的选择淘汰信号。我们还检测到普纳鹮与南美白脸鹮种群之间存在遗传混合信号。本研究结果表明，扩散能力极强的物种在二次接触后可发生广泛的基因渐渗，不过这类动态相较于在扩散能力较弱的生物中常观测到的稳定杂交带而言，可能更为短暂。