Sparrows are a small family of passerine birds. Hybridization is restricted to the genus Passer. Two cases of hybridization have been studied extensively. First, interbreeding between House Sparrow (P. domesticus) and Eurasian Tree Sparrow (P. montanus). And second, hybridization between House Sparrow and Spanish Sparrow (P. hispaniolensis), that probably resulted in the formation of a hybrid species, the Italian Sparrow (P. italiae).
Hybrids between House Sparrow and Eurasian Tree Sparrow have been documented numerous times (Albrecht, 1983; Cheke, 1969; Cordero, 1990a; Cordero, 1990b; Cordero & Summers-Smith, 1993; Järvinen, 1987; Monsees, 1962; Ruthke, 1930; Solberg & Ringsby, 1996). These hybrids may be fertile (Solberg et al., 2000), but DNA analyses showed that extra-pair fertilization may have confounded this conclusion (Solberg et al., 2006).
Hybrids between House Sparrow and Spanish Sparrow have been bred in captivity (Alonso, 1984; Macke, 1965), but these hybrids have also been found in nature (Alonso López, 1985). Experimental work showed that the post-copulatory prezygotic barrier between these species is insufficient to establish reproductive isolation (Cramer et al., 2014). The resemblance of these hybrids to the Italian Sparrow led to the hypothesis that this sparrow might be a hybrid taxon. Although this hypothesis was first rejected (Topfer, 2006), new morphological and genetic evidence suggests that the Italian Sparrow is indeed of hybrid origin (Hermansen et al. 2011, Elgvin et al. 2017). There are some constraints on the formation of hybrid genomes: not all combinations are possible. Some genes come exclusively from one parental species. The genes from Spanish Sparrow affect external phenotypes, whereas the House Sparrow genes are involved in DNA-repair and mitonuclear functions (Runemark et al., 2018).
A detailed study proposes a particular role for sex-chromosomes in this case of hybrid speciation (Elgvin et al., 2011). But for the Italian Sparrow to be a true hybrid species, it has to be reproductively isolated from his parental species, but a hybrid zone between Italian Sparrow and one of its parental species, House Sparrow, has been described in Italy (Lockley, 1992; Lockley, 1996). The Italian Sparrow also lives sympatrically with its other parent, the Spanish Sparrow. Genomic analyses of several hybrid zones suggested that conflict between nuclear and sex-linked genes with mitochondrial genes might lead to reproductive isolation between Italian Sparrow and its parental species (Trier et al., 2014). A subsequent genomic study suggests that intrinsic barriers isolating the Italian Sparrow from its parent species have mainly developed through the sorting of pre-existing sex-linked parental incompatibilities and that isolation is driven in part by mitonuclear conflict involving the Z chromosome (Hermansen et al., 2014). Cline analysis of the hybrid zone in the Alps suggests that differences in crown colour act as an important isolation mechanism between the hybrid Italian species and its Spanish parent species (Bailey et al., 2015). Competition between these two species did not result in morphological changes (e.g., beak morphology), but was noticeable on a genetic level (Saetre et al. 2017).
The evolution of beak morphology of the Italian Sparrow might not be influenced by its mixed genetic background, in contrast to for example Darwin’s Finches (Eroukhmanoff et al., 2013; Eroukhmanoff et al., 2014). However, a more recent study showed that adaptation to local conditions is influenced by genomic composition in island populations of the Italian Sparrow (Runemark et al. 2018).
In Northern Africa, House Sparrow and Spanish Sparrow also interbreed (Summers‐Smith & Vernon, 1972). In Algeria, House Sparrows and hybrids mainly reside in urban areas, while Spanish Sparrows live in cultivated areas. The latter also breed later and raise only two clutches (House Sparrows and hybrids raise three clutches). The mitogenome of the House Sparrow has almost completed introgressed into the Italian Sparrow (see above). In Algeria, however, a small percentage of the hybrids has a Spanish haplotype (Belkacem et al., 2016). Moreover, there is extensive introgression between both species, resulting in a mosaic hybrid zone (Päckert et al., 2019).
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