This is a 90-year-old hypothesis that has just been confirmed by the team led by Professor Claire Spottiswoode from the Department of Zoology at Cambridge (United Kingdom), Michaël Sorenson from Boston University (United States) and by the Institute of Ornithology in Cape Town (South Africa). In 1933, a British geneticist, Reginald Punnett, known for having developed a tool to find the ancestors of an offspring, had put forward the hypothesis that the parasitism developed by certain species of birds was transmitted by the mother. By analyzing the genome of 196 parasitic anomalospizes (Anomalospiza imberbis) captured in 141 nests of herbaceous warblers, small passerines strongly affected by the intruder, the researchers were able to confirm this hypothesis in this species from southern Africa. Their results have just been published in the PNAS.
The relationships between the various species of parasitic birds and the hosts forced to raise their offspring go back at least two million years. It is a well-known and well-studied example of coevolution borrowing from the Red Queen theory, a term borrowed from Alice in Wonderland by Lewis Carroll. When a species develops a mutation allowing it to gain an advantage, the species that evolves with it will develop a defense that restores balance. This is what happens to Alice when she runs while the landscape moves at the same speed as her: she has the impression of standing still. For parasitic birds, it is therefore essential to preserve this ability to imitate the shell of the eggs of parasitized species, otherwise they will be spotted by their victims and chased from the nest. There is therefore no question that the genes responsible for this ability will be lost if one of the fathers was raised by a host different from that of the mother, a high risk since these parasitic species do not form faithful pairs. Researchers have demonstrated that anomalospizes transmit this ability through the female-specific W chromosome similar to the male-specific Y chromosome in humans. Thanks to this transmission, like Alice, the parasites run at the same speed as the landscape.
An arms race that turns in favor of the victims
But that could well change. “While this maternal heritage has enabled anomalospizes to exploit a large number of species, it is however likely to lose the ability to develop a counter-adaptation if their host opposes new defenses”explains Claire Spottiswoode on the site of the University of Cambridge. Parasites thus seem to face a daunting problem because some parasitized species have evolved an amazing variety of egg colors that helps them distinguish parasite eggs from their own.”