Geographic variation in Mexican jays (Aphelocoma ultramarina): local differentiation, polyphyly or hybridization?

Studies of genetic variation within highly variable taxa can provide valuable insight into the factors influencing biological diversification. We examined six microsatellite loci, a nuclear intron and the mitochondrial control region to determine if the Mexican jay subspecies Aphelocoma ultramarina couchii and A. u. potosina have hybridized with western scrub-jays (A. californica). We suspected hybridization because these Mexican Jay populations resemble scrub-jays in several traits. We sampled six Mexican jay (N = 105) and four scrub-jay (N = 78) populations. Suspected hybrid Mexican Jay populations did not share any mitochondrial types or intron alleles with scrub-jays. All microsatellite alleles found in the suspected hybrid Mexican jay populations are also found in the control Mexican jay populations. Genetic distance-based trees from microsatellites supported reciprocal monophyly of Mexican jays and scrub-jays with bootstrap support > 80%. We randomized genotypes among populations to test scenarios consistent with hybridization. In some areas where Mexican jays and scrub-jays occur in sympatry or geographical proximity, randomization tests yield results expected under hybridization (100% support for seven of nine scenarios). However, these populations were not the primary candidates for hybridization based on phenotype. Even if low-level hybridization did occur, hybridization does not appear to be the main reason some Mexican jay populations resemble scrub-jays more than others. The scrub-jay-like traits in these populations may be due to drift, adaptation or plasticity. Alternatively, ancient hybridization, followed by selection for scrub-jay like traits in some Mexican jay populations, might have given rise to the observed variation.