Population genetics of a hybrid zone between the Red-legged partridge and Rock partridge

We studied the genetic structure of a hybrid zone between Red-legged (Alectoris rufa) and Rock (A, graeca) partridges in the southern French Alps using six allozyme loci. Allele frequencies showed sharp clinal variation across the hybrid zone shifting by 60% over a distance of about 60 km on average. Single-locus dines were coincident, but only partially concordant, with intermediate allele frequencies in the hybrid population Percent polymorphic loci, heterozygosity, and linkage disequilibria were higher in the hybrids and in some populations near the hybrid zone than in allopatric populations. Nonrandom associations favored parental allele combinations. The hybrids and most of the allopatric populations were in Hardy-Weinberg equilibrium, but six populations near the hybrid zone showed a significant deficit of heterozygotes. The partridges in the hybrid zone included F-1 hybrids, backcrosses, and other recombinant genotypes. This hybrid zone could result from secondary contact of formerly allopatric populations following deglaciation of the Alps, probably not before 6,000 to 8,000 years ago. The width of the observed multilocus dine (70 to 160 km) is much shorter than expected (1,120 to 2,750 km) by neutral diffusion of allelic variants since the time of secondary contact of the two species (2,000 to 3,000 generations). Linkage disequilibria suggest that this hybrid zone is maintained by crossing of genetically divergent parentals and natural selection against hybrid genotypes. Although parapatric interspecific populations can exchange their genes through the hybrid zone, natural selection might constrain gene flow such that two species continue to evolve independently.