Genetic and geographic variation in rejection behavior of cuckoo eggs by European magpie populations: An experimental test of rejecter-gene flow

Host responses toward brood parasitism have been shown to differ among popu lations depending on the duration of sympatry between host and parasite, al though populations not currently parasitized show rejection behavior agains t parasitic eggs. The persistence of rejection behavior in unparasitized ho st populations and rapid increases of rejection rate in parasitized ones ha ve sometimes been explained as the result of gene flow of rejecter genes fr om sympatry to allopatry (rejecter-gene flow hypothesis). We present data o n the rejection behavior of magpies (Pica pica), the main European host of the great spotted cuckoo (Clamator glandarius), in 15 populations (nine sym patric, six allopatric) across their distribution range in Europe. Rejectio n rates of mimetic and nonmimetic model eggs were significantly higher in s ympatric than in allopatric magpie populations, although differences in rej ection rate of both mimetic and nonmimetic model eggs between magpie popula tions were significantly correlated even after controlling for phylogenetic effects, with differences between sympatric and allopatric magpie populati ons being larger for mimetic than for nonmimetic model eggs. Differences in rejection of mimetic model eggs were related to both genetic and geographi c distances between populations, but differences in rejection rate of nonmi metic model eggs were unrelated to these distances. However, when comparing only sympatric populations, differences in rejection rate of both mimetic and nonmimetic model eggs were related to geographic distances. A multiple autocorrelation analysis revealed that differences among populations in rej ection rates of mimetic model eggs had a strong geographic component, where as the main component of rejection rate of nonmimetic model eggs was geneti c rather than geographic. These results support the rejecter-gene flow hypo thesis. We discuss differences in rejection rates of mimetic and nonmimetic model eggs that suggest the egg-recognition ability of the host is genetic ally based, but is affected by a learning process for fine tuning of recogn ition.