Science. Victims of a cuckoo species rebel thanks to genetics

For two million years, genetics has been used as a weapon by a species of parasitic cuckoo that hatches its eggs by other species mimicking their colors and patterns. But its victims are turning the gun of the genes in their favor, according to a study.

The cuckoo lays eggs that mimic those of other species

The cuckoo weaver is a small passerine bird with a bright yellow belly, endemic to southern Africa. It is also a parasite of other no less charming bird species, 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 even more interesting that the latter, if he discovers the deception, will rush to pierce the intruder’s shell with his beak before getting rid of it. Thus, Cystic Fibrosis has become particularly capable of detecting a “fake”.

But if the cuckoo weaver’s maneuver pays off, his offspring will have a big advantage, because his chick will come out a day or two ahead of his competitors. Significantly larger than the latter, it will capture most of the beak provided by the host parents, convinced to feed their young and ensure the legitimate death of the chicks by starvation.

The female carries the specific gene that allows imitation

Everything is based, therefore, on the ability of the female cuckoo to imitate as perfectly as possible the appearance of its host’s egg. A British geneticist, Reginald Punnett, hypothesized in 1933 that this gift was inherited genetically through the mother.

The team led by South African evolutionary biologist Claire Spottiswoode, a professor at the University of Cambridge (UK) and the University of Cape Town (South Africa), is finally providing evidence, in a study published this week in Proceedings of the American. Academy of Sciences (PNAS).

With the help of young Zambians, he studied the genetic inheritance of cuckoo weaver populations and their hosts. The female of the parasitic bird transmits to her offspring the ability to lay eggs often similar to those of her host species. That is, to the species in whose nest it was born.

“He seems to know what species he was raised on”

“They are able to find the right host species nest, probably through an impregnation process,” explains Professor Spottiswoode. It is unclear what evidence the parasitic bird is using, “but it seems to know by what species it was bred,” he 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 associated with the red-faced cystic and the other with the modest plum.

The latter lays white eggs, throwing more or less towards blue or red. The cuckoo has followed this evolution, but can only lay eggs of one color and depends on luck to match that of its guest’s eggs.

The Prinia species changes the colors of its eggs

The female inherits and therefore alone transmits the ability to produce eggs of a very particular color. A specialization that has become a handicap. Because Prinia uses the genetic inheritance of both parents to lay eggs with increasingly diverse patterns and colors.

He began to lay eggs of a beautiful olive green, a combination that used especially red and blue pigments, which the cuckoo, which specializes in only one of the two pigments, cannot imitate. “We don’t know when this evolution to olive green appeared, but it is much less than 2 million years ago,” says Professor Spottiswoode. Whoever is also thoughtful about the observation that if the cuckoo passes the test of detection of its egg, surely its offspring will be well received later.

She describes the egg-level adaptation phenomenon as “magnificent”, but is surprised that when she reaches the bird stage, “the guests look completely stupid and raise a baby that looks nothing like their descendants “.

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