North African hybrid sparrows (Passer domesticus,P. hispaniolensis) back from oblivion – ecological segregation and asymmetric mitochondrial introgression between parental species

Abstract

A stabilized hybrid form of the house sparrow (Passer domesticus) and the Spanish sparrow (P. hispaniolensis) is known as Passer italiae from the Italian Peninsula and a few Mediterranean islands. The growing attention for the Italian hybrid sparrow and increasing knowledge on its biology and genetic constitution greatly contrast the complete lack of knowledge of the long‐known phenotypical hybrid sparrow populations from North Africa. Our study provides new data on the breeding biology and variation of mitochondrial DNA in three Algerian populations of house sparrows, Spanish sparrows, and phenotypical hybrids. In two field seasons, the two species occupied different breeding habitats: Spanish sparrows were only found in rural areas outside the cities and bred in open‐cup nests built in large jujube bushes. In contrast, house sparrows bred only in the town centers and occupied nesting holes in walls of buildings. Phenotypical hybrids were always associated with house sparrow populations. House sparrows and phenotypical hybrids started breeding mid of March, and most pairs had three successive clutches, whereas Spanish sparrows started breeding almost one month later and had only two successive clutches. Mitochondrial introgression is strongly asymmetric because about 75% of the rural Spanish sparrow population carried house sparrow haplotypes. In contrast, populations of the Italian hybrid form, P. italiae, were genetically least diverse among all study populations and showed a near‐fixation of house sparrow haplotypes that elsewhere were extremely rare or that were even unique for the Italian Peninsula. Such differences between mitochondrial gene pools of Italian and North African hybrid sparrow populations provide first evidence that different demographic histories have shaped the extant genetic diversity observed on both continents.

Genetic admixture despite ecological segregation in a North African sparrow hybrid zone (Aves, Passeriformes, Passer domesticus × Passer hispaniolensis)

“…Compared to Italian sparrow populations, Rothschild and Hartert () described a great phenotypic diversity of North African sparrow populations and distinguished 20 different head color patterns. For this study, we based individual classification of phenotypes on six phenotypic traits (crown, neck, cheek, breast, flanks, and back; as done by Belkacem et al, ). We distinguished (a) seven different crown color patterns and six different facial color patterns, and (b) six different ventral and three different dorsal plumage patterns (Figure ).…”

“…Hybrid scores were calculated and compared for males only, because unanimous assignment of females to either of the parental species is critical and highly prone to error (Elgvin et al, ; Hermansen et al, ; biometric analysis in Belkacem et al, ). In accordance with Hermansen et al (), the few females that were included in the Algerian sampling were only assigned to the urban or rural study populations but not to a particular phenotype.…”

“…The set of microsatellites used here (Table ) has been developed specifically for sparrows and has been proven to (a) reliably distinguish the parental species ( P. domesticus and P. hispaniolensis ) from each other and from the admixed Italian hybrid form (Hermansen et al, ) and (b) reveal intraspecific diversification in Australian house sparrows (Sheldon, Schrey, Andrew, Ragdsale, & Griffith, ). For the mitochondrial marker ND2 , there has been firm evidence of strongly asymmetrical introgression of the house sparrow mitogenome into both the Italian sparrow (Hermansen et al, ) and North African sparrow hybrids (Belkacem et al, ). As a third marker, we chose the sex chromosomal CHD1Z , a nuclear locus that seems to be under positive, divergent selection in the two parental sparrow species (Elgvin et al, ).…”

“…Based on the current state of knowledge outlined above, we predict that, Phenotypic hybrids in Algerian mixed populations should be identifiable as genetic hybrids. Furthermore, these should show similar variation of the three genetic marker systems as the Italian hybrid form (e.g., near fixation of the house sparrow mitogenome and the Spanish sparrow CHD1Z alleles; Belkacem et al, ; Elgvin et al, ; Hermansen et al, ; see also Materials and Methods below) Local spatial and temporal separation among Algerian house sparrows and Spanish sparrows (Belkacem et al, ) should act as a premating barrier and prevent interspecific gene flow to some degree. We thus expect admixture of the two nuclear markers to be limited in the two parental species despite strong unidirectional mitochondrial introgression in Algeria (Belkacem et al, ). Algerian hybrids should be genetically more similar to house sparrows due to increased backcrossing in mixed urban populations.…”

“…We thus expect admixture of the two nuclear markers to be limited in the two parental species despite strong unidirectional mitochondrial introgression in Algeria (Belkacem et al, ). Algerian hybrids should be genetically more similar to house sparrows due to increased backcrossing in mixed urban populations. Likewise, the signal of admixture should be weaker in rural populations due to the absence of hybrids (Belkacem et al, ).…”