For two million years, genetics has been used as a weapon by a species of parasitic cuckoo that hatches its eggs by other species by mimicking their colors and patterns. But its victims are turning the weapon of genes in their favor, according to a study.
The cuckoo lays eggs that mimic those of other species
The Cuckoo-weaver is a charming little passerine bird with a bright yellow belly, endemic to southern Africa. It is also a parasite of other no less charming species of birds, the modest Prinia, with its white belly and brown wings, and the red-faced Cisticola.
The female Cuckoo-weaver has developed an extraordinary gift for mimicking the pattern and color of her host’s eggs. It is all the more interesting that the latter, if he discovers the deception, will hasten to pierce the shell of the intruder with his beak before getting rid of it. The Cisticola has thus become particularly adept at spotting a “false”.
But if the cuckoo-weaver’s maneuver pays off, its offspring will have a big advantage, because its chick is hatched a day or two before its competitors. Significantly larger than the latter, it will then capture most of the beak provided by the parent-hosts, convinced to feed their young, and guarantee the legitimate chicks death by starvation.
The female carries the specific gene that allows imitation
Everything therefore rests on the ability of the female cuckoo to imitate as perfectly as possible the appearance of the egg of its host. A British geneticist, Reginald Punnett, hypothesized in 1933 that this gift was genetically inherited through the mother.
The team led by the South African evolutionary biologist Claire Spottiswoode, professor at the University of Cambridge (United Kingdom) and that of Cape Town (South Africa), finally brings the proof, in a study published this week. in the Proceedings of the American Academy of Sciences (PNAS).
With the help of young Zambians, she studied the genetic heritage of Cuckoo-weaver populations and their hosts. The female of the parasitic bird transmits to her offspring the ability to lay eggs often resembling those of her host species. That is, to the species in whose nest it itself was born.
“He seems to know what species he was bred by”
“They are able to find the nest of the right host species, probably through a process of impregnation,” explains Prof. Spottiswoode. It’s unclear what cues the parasitic bird uses, “but it seems to know what species it was bred by,” she adds.
This association between a species and its parasite is so close that about two million years ago, the Cuckoo-weaver evolved into two branches: one associating with the Red-faced Cisticola and the other with the Prinia. modest.
The latter lays white eggs pulling more or less towards blue or towards red. The cuckoo has followed this evolution, but can only lay eggs of one color, and relies on luck to match that of its host’s eggs.
The Prinia species changes the colors of its eggs
The female inherits and therefore transmits alone the ability to produce eggs of a very particular color. A specialization that has become a handicap. Because the Prinia uses the genetic heritage of both parents to lay eggs with ever more diverse patterns and colors.
He began to lay eggs of a pretty olive green, a combination using in particular red and blue pigments, which the cuckoo, specialized in only one of the two pigments, cannot imitate. “We don’t know when this evolution towards olive-green appeared, but much less than 2 million years ago”, remarks Prof. Spottiswoode. Who also remains pensive in the face of the observation that if the cuckoo passes the detection test for its egg, its offspring is certain to be well received afterwards.
She describes the phenomenon of adaptation at the level of the egg as “magnificent”, but is surprised that when it reaches the baby bird stage, “the hosts seem completely stupid and raise a baby that looks nothing like their offspring”.