How parasitic cuckoos lay host-matching eggs while remaining a single species

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

Bright blue, white, greenish, speckled or striped, cuckoo eggs present an extraordinary variety. This color range is the result of an evolving breed with over 100 avian host species. It is well known that cuckoos do not incubate their eggs, but secretly lay them in the nests of other bird species.

So 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, each female cuckoo is required to lay eggs of a specific color and pattern. This suggests that there are diverse evolutionary lineages of the European cuckoo (Cuculus canorus), each of which is adapted to a specific avian host species.

An international team led by LMU evolutionary biologists Justin Merondun and Jochen Wolf has now deciphered the genetic basis of these adaptations and shown how the cuckoo remains a single species. This calls for an explanation, because as cuckoos evolve specialized adaptations to exploit new hosts, these populations could begin to diverge genetically to the point of forming new species.

For their study, the researchers analyzed some 300 genomes from the European cuckoo and 50 from the oriental cuckoo (Cuculus optatus), its eastern sister species. Subsequently, they checked which genetic variants matched the egg coloring.

The article is published in the journal Science.

Inheritance via the W chromosome

“The question was: How can a cuckoo reliably convey the correct egg color?” said Wolf. “After all, a female may not know what her own egg looks like.”

Presumably, a female cuckoo will return to a nest of the type in which she was raised. However, for the egg color to truly match, it must be encoded in the bird’s genes. As early as the 1930s, the hypothesis was formulated that the responsible genes reside somewhere in the maternal lineage.

Current analyzes now confirm that the basic egg color of the European cuckoo is inherited almost exclusively via the female sex chromosome – the W chromosome – and the mitochondria. Patterning, on the other hand, 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 patterns, the researchers found no inheritance via the maternal line.

Inheritance via the W chromosome ensures that daughters always lay eggs of the same basic color as their mothers. However, for new adaptations, this type of inheritance is not optimal, because the possibilities for genetic variation are limited and depend more heavily on random mutations than in the case of DNA inherited from both parents.

“As such, we were excited to observe that a gene potentially involved in egg coloring has clearly ‘migrated’ from autosomes. [the non-sex chromosomes inherited from both parents] to the W chromosome,” Wolf explains.

Gene flow is preserved

Matrilineal inheritance shapes how genetic variation spreads within a species. When traits are important to both males and females, adaptation to different hosts can quickly separate populations and eventually create new species. In the cuckoo, on the other hand, females can mate freely with any male without losing their adaptation to their host. The flow of genetic information through the rest of the genome is preserved.

“And this is precisely what we observe: the huge population of cuckoos throughout Eurasia is almost genetically identical within the 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 parts of Europe, populations are experiencing significant decline due to loss of habitats. “Without intact habitats, this fascinating system risks disappearing on our doorstep,” Wolf warns.