Data from: Hybrid speciation in sparrows I: phenotypic intermediacy, genetic admixture and barriers to gene flow

Homoploid hybrid speciation is thought to require unusual circumstances to yield reproductive isolation from the parental species, and few examples are known from nature. Here we present genetic evidence for this mode of speciation in birds. Using Bayesian assignment analyses of 751 individuals genotyped for 14 unlinked, nuclear microsatellite loci, we show that the phenotypically intermediate Italian sparrow (Passer italiae) does not form a cluster of its own, but instead exhibits clear admixture (over its entire breeding range) between its putative parental species, the house sparrow (P. domesticus) and the Spanish sparrow (P. hispaniolensis). Further, the Italian sparrow possesses mitochondrial (mt) DNA haplotypes identical to both putative parental species (although mostly of house sparrow origin), indicating a recent hybrid origin. Today, the Italian sparrow has a largely allopatric distribution on the Italian peninsula and some Mediterranean islands separated from its suggested parental species by the Alps and the Mediterranean Sea, but occurs sympatrically with the Spanish sparrow on the Gargano peninsula in southeast Italy. No evidence of interbreeding was found in this sympatric population. However, the Italian sparrow hybridizes with the house sparrow in a sparsely populated contact zone in the Alps. Yet, the contact zone is characterized by steep clines in species-specific male plumage traits, suggesting that partial reproductive isolation may also have developed between these two taxa. Thus, geographic and reproductive barriers restrict gene flow into the nascent hybrid species. We propose that an origin of hybrid species where the hybrid lineage gets geographically isolated from the parental species, as seems to have happened here, might be more common in nature than previously assumed.同倍体杂交物种形成（Homoploid hybrid speciation）通常被认为需要特殊条件，才能使其与亲本物种产生生殖隔离，目前自然界中已知的此类案例寥寥无几。本研究针对鸟类类群，提供了该物种形成模式的遗传学证据。本研究对751个经14个独立核微卫星位点（nuclear microsatellite loci）完成基因分型的个体开展贝叶斯归属分析（Bayesian assignment analyses），结果显示，表型呈中间型的意大利麻雀（*Passer italiae*）并未形成独立的遗传聚类簇，反而在其整个繁殖分布区内，均表现出与推定亲本物种——家麻雀（*P. domesticus*）和西班牙麻雀（*P. hispaniolensis*）——的明显遗传混合。此外，意大利麻雀的线粒体DNA（mtDNA）单倍型与两种推定亲本物种均一致（且多数源自家麻雀），这表明其为近期起源的杂交类群。如今，意大利麻雀主要以异域分布（allopatric distribution）模式栖息于亚平宁半岛及部分地中海岛屿，其分布区与推定亲本物种被阿尔卑斯山和地中海相隔；但在意大利东南部的加尔加诺半岛，意大利麻雀与西班牙麻雀呈同域分布（sympatric distribution）。在该同域种群中未发现杂交繁殖的迹象。不过，在阿尔卑斯山一处人口稀少的接触带（contact zone）中，意大利麻雀会与家麻雀发生杂交。但该接触带以物种特异性的雄性羽色性状存在陡峭渐变群为特征，这表明这两个类群（taxa）之间或许已演化出部分生殖隔离。综上，地理隔离与生殖隔离限制了基因流向这一新生杂交物种。本研究提出，若杂交谱系与亲本物种发生地理隔离（如本案例所示），此类杂交物种的起源方式或许比此前所认为的更为常见于自然界中。