Genetic evidence for female host-specific races of the common cuckoo

The common cuckoo Cuculus canorus is divided into host-specific races (gent es)(1). Females of each race lay a distinctive egg type that tends to match the host's eggs, for instance, brown and spotted for meadow pipit hosts or plain blue for redstart hosts(2-4). The puzzle is how these gentes remain distinct. Here, we provide genetic evidence that gentes are restricted to f emale lineages, with cross mating by males maintaining the common cuckoo ge netically as one species. We show that there is differentiation between gen tes in maternally inherited mitochondrial DNA, but not in microsatellite lo ci of nuclear DNA. This supports recent behavioural evidence that female, b ut not male, common cuckoos specialize on a particular host(5), and is cons istent with the possibility that genes affecting cuckoo egg type are locate d on the female-specific W sex chromosome(6). Our results also support the ideas that common cuckoos often switched hosts during evolution(7,8), and t hat some gentes may have multiple, independent origins, due to colonization by separate ancestral lineages.