ANALYSIS OF GENETIC DIFFERENTIATION OF HOST RACES OF THE COMMON CUCKOO CUCULUS-CANORUS USING MITOCHONDRIAL AND MICROSATELLITE DNA VARIATION

It has long been argued that populations of the parasitic common cucko o Cuculus canorus consist of sympatric host-specific female races, eac h of which lays eggs that match, to varying degrees, those of their ch osen hosts. We tested this hypothesis by comparing rapidly evolving DN A markers among the cuckoo chicks reared by the three most common host s in the United Kingdom. Comparing cuckoos from different hosts, we fo und no significant differences in the number of repeats in the control region of the mtDNA nor in the allele frequencies of three microsatel lite loci. Gives that cuckoos parasitizing the three different hosts d o lay different eggs, these results suggest that either: (i) egg-colou r variation in cuckoos is facultative, which is unlikely; (ii) gene fl ow between races occurs because female cuckoos sporadically successful ly parasitize alternative hosts; or (iii) the presumably neutral marke rs in this study have not tracked the rapid and/or recent evolution of host races in this species. Studies of the laying and mating patterns of female cuckoos in marked populations in the wild will help evaluat e which of these interpretations is most likely.