Several studies have uncovered a highly heterogeneous landscape of genetic differentiation across the genomes of closely related species. Specifically, genetic differentiation is often concentrated in particular genomic regions (“islands of differentiation”) that might contain barrier loci contributing to reproductive isolation, whereas the rest of the genome is homogenized by introgression. Alternatively, linked selection can produce differentiation islands in allopatry without introgression. We explored the influence of introgression on the landscape of genetic differentiation in two hybridizing goose taxa: the Taiga Bean Goose ( Anser fabalis ) and the Tundra Bean Goose ( A. serrirostris ). We re-sequenced the whole genomes of 18 individuals (9 of each taxon) and, using a combination of population genomic summary statistics and demographic modeling, we reconstructed the evolutionary history of these birds. Next, we quantified the impact of introgression on the build-up and maintenance of genetic differentiation. We found evidence for a scenario of allopatric divergence (about 2.5 million years ago) followed by recent secondary contact (about 60,000 years ago). Subsequent introgression events led to high levels of gene flow, mainly from the Tundra Bean Goose into the Taiga Bean Goose. This scenario resulted in a largely undifferentiated genomic landscape (genome-wide F ST = 0.033) with a few notable differentiation peaks that were scattered across chromosomes. The summary statistics indicated that some peaks might contain barrier loci while others arose in allopatry through linked selection. Finally, based on the low genetic differentiation, considerable morphological variation and incomplete reproductive isolation, we argue that the Taiga and the Tundra Bean Goose should be treated as subspecies.

中文翻译：

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最近泰加豆鹅和苔原豆鹅之间的基因渗入导致遗传分化的基本同质景观

几项研究揭示了密切相关物种基因组中遗传分化的高度异质性。具体来说，遗传分化通常集中在特定的基因组区域（“分化岛”），这些区域可能包含有助于生殖隔离的屏障基因座，而基因组的其余部分则通过基因渗入而同质化。或者，连锁选择可以在异源性中产生分化岛而无需基因渗入。我们探讨了基因渗入对两种杂交鹅类群遗传分化的影响：针叶鹅 (Anser fabalis) 和苔原豆鹅 (A. sarrirostris)。我们对 18 个个体（每个分类单元 9 个）的全基因组进行了重新测序，并且，结合人口基因组汇总统计和人口统计模型，我们重建了这些鸟类的进化历史。接下来，我们量化了基因渗入对遗传分化的建立和维持的影响。我们发现了异域分歧情景的证据（大约 250 万年前），然后是最近的二次接触（大约 6 万年前）。随后的基因渗入事件导致了高水平的基因流动，主要从苔原豆鹅流入针叶鹅。这种情况导致了很大程度上未分化的基因组景观（全基因组 F ST = 0.033），其中有一些显着的分化峰分散在染色体上。汇总统计表明，一些峰可能包含屏障基因座，而其他峰可能通过连锁选择出现在异源性中。最后，