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 cuckoo Cuculus canorus consist of sympatric host-specific female races, each of which lays eggs that match, to varying degrees, those of their chosen hosts. We tested this hypothesis by comparing rapidly evolving DNA markers among the cuckoo chicks reared by the three most common hosts in the United Kingdom. Comparing cuckoos from different hosts, we found no significant differences in the number of repeats in the control region of the mtDNA nor in the allele frequencies of three microsatellite loci. Given that cuckoos parasitizing the three different hosts do lay different eggs, these results suggest that either: (i) egg-colour variation in cuckoos is facultative, which is unlikely; (ii) gene flow between races occurs because female cuckoos sporadically successfully parasitize alternative hosts; or (iii) the presumably neutral markers in this study have not 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 evaluate which of these interpretations is most likely.