The common cuckoo deposits its eggs in the nests of more than 100 host species, and each egg must mimic its host's to escape rejection. Since every female lays one particular colour and pattern, biologists have long wondered why these host-specialised lineages have not fractured into separate species. An international team led by LMU evolutionary biologists Justin Merondun and Jochen Wolf, writing in Science, has found the answer in the architecture of inheritance.
Analysing around 300 European cuckoo genomes alongside 50 from the Oriental cuckoo, the researchers show that egg base colour passes almost entirely through the female line, carried on the W sex chromosome and in mitochondria, so daughters reliably lay eggs matching their mothers'. Patterning, by contrast, draws on autosomal genes from both parents. One gene candidate for coloration appears to have relocated from the autosomes onto the W chromosome.
The arrangement is evolutionarily elegant: females can mate with any male without diluting their host-specific camouflage, so gene flow continues across the rest of the genome and the Eurasian population remains nearly identical in biparentally inherited regions. The study also carries a warning, as cuckoo populations are declining across much of Europe with the loss of the habitats on which this intricate system depends.