European cuckoos lay very different eggs depending on the host species. Genetic analyses have revealed how this adaptation is inherited without leading to speciation.

Bright blue, white, greenish, speckled, or striped – cuckoo eggs exhibit an extraordinary variety. This range of colors is the result of an evolutionary race with over 100 avian host species. Cuckoos famously do not incubate their eggs, but secretly lay them in the nests of other bird species. To ensure that the host does not recognize the cuckoo egg and throw it out of its nest, the egg must closely resemble the eggs of its host parent. However, every female cuckoo is tied to lay eggs of a specific color and pattern. This suggests that various evolutionary lineages of the European cuckoo (Cuculus canorus) exist, with each of them adapted to a specific avian host species.

An international team led by LMU evolutionary biologists Justin Merondun and Jochen Wolf have now deciphered the genetic basis of these adaptations and shown how the cuckoo remains a single species. This calls for explanation, because as cuckoos evolve specialized adaptations to exploit new hosts, these populations could begin to genetically diverge to the point of forming new species. For their study, the researchers analyzed some 300 genomes of the European and 50 of the Oriental cuckoo (Cuculus optatus), its eastern sister species. Subsequently, they checked which gene variants corresponded to the egg coloration.

Inheritance via the W chromosome
“The question was: How can a cuckoo reliably pass on the right egg color?” says Wolf. “After all, a female might not know what her own egg looks like.” Presumably female cuckoos return to a nest of the type in which she was raised. For the egg color to really match, however, it needs to be encoded in the bird’s genes. As far back as the 1930s, the hypothesis was formulated that the responsible genes reside somewhere on the maternal lineage.

The current analyses now confirm that the base color of the eggs of the European cuckoo is inherited almost exclusively via the female sex chromosome – the W chromosome – and mitochondria. The patterning, by contrast, depends to a greater extent on autosomal genes, which come from both parents. In the Oriental cuckoos studied, whose eggs were all whitish-green and differed only in their patterning, the researchers found no inheritance via the maternal lineage.
Inheritance via the W chromosome ensures that daughters always lay eggs with the same base color as their mothers. For new adaptations, however, this type of inheritance is suboptimal, as the possibilities of genetic variation are limited and more strongly dependent on random mutations than in the case of DNA inherited from both parents. “As such, we were excited to observe that a gene which is possibly involved in egg coloration evidently ‘migrated’ from the autosomes [the non-sex chromosomes inherited from both parents] to the W chromosome,” says Wolf.

Gene flow is preserved
Matrilineal inheritance shapes how genetic variation is spread across a species. When traits matter for both males and females, adapting to different hosts can quickly drive populations apart – and eventually create new species. In the cuckoo, by contrast, females can freely mate with any male without losing their adaptation to their host. The flow of genetic information across the rest of the genome is preserved. “And that is precisely what we observe: The huge cuckoo population throughout Eurasia is genetically almost identical within DNA regions inherited from both parents,” emphasizes Wolf.

But this evolutionary advantage does not protect the cuckoo from the dangers of the present. In many regions of Europe, populations are significantly declining, because their habitats are disappearing. “Without intact habitats, this fascinating system risks vanishing on our doorstep,” cautions Wolf.