African cuckoo finches sneak their eggs into other birds' nests to avoid parenting duties. New research in Nature Ecology & Evolution shows this deceptive habit carries a heavy genetic cost. Scientists found that harmful mutations hitch a ride on the female W chromosome, causing significant damage to the birds' cellular energy.
African cuckoo finches (Anomalospiza imberbis) are very deceptive birds, or at least the mothers are. They sneak into the nests of other birds, such as prinias and cisticolas, to lay their eggs, then slip away and leave the hosts to raise the chicks.
For this to work, the eggs have to look similar in color, pattern and speckling to the host eggs. And they generally do, thanks to many generations of adaptation. But this triggers an arms race where hosts reject unfamiliar eggs and finches evolve better disguises.
However, as new research published in the journal Nature Ecology & Evolution reveals, this deceptive approach to mothering appears to come with a hidden genetic cost.
Researchers led by Michael Sorenson at Boston University suspected that strong selection of a maternally inherited trait might trigger genetic hitchhiking.
When harmful mutations hitch a ride
Usually, during sexual reproduction in animals, genetic material is reshuffled through a process called recombination. One of the benefits of this is that potentially damaging mutations can be separated from good ones before they can spread. But with hitchhiking, reduced genetic mixing or a lack of it means harmful mutations are linked to beneficial ones and dragged along for the ride.
To test whether adapting to host eggs causes these unwanted changes to accumulate, the scientists compared the genomes of eight songbird species, concentrating on the cuckoo finch and its closest relatives.
They combined this information with population DNA data and created 3D protein structural models to track where genetic changes occurred in proteins involved in cellular energy production.
And the results revealed strong evidence consistent with genetic hitchhiking.
A heavy genetic price
The W chromosome (the female sex chromosome in birds) had suffered extensive gene loss and deterioration, with 17 of its genes either lost or affected by changes that could stop them from working properly. The remaining active genes on the chromosome also showed a faster rate of genetic changes, including potentially harmful mutations.
The damage did not stop there. Because the W chromosome and mitochondrial DNA are passed down together through females, the genetic material in these cellular energy factories also accumulated potentially harmful mutations more rapidly.
To help counter this buildup of bad mutations, adaptation appears to have taken place elsewhere. Nuclear genes evolved quickly to compensate, apparently patching over the defects to keep the cells running.
"Our results link species-level coevolution to both sex chromosome degeneration and mitonuclear coevolution," commented the study authors. "More generally, they highlight the genomic consequences of adaptation without recombination."
Female finches may win in the short term by getting other birds to raise their young, but this evolutionary strategy is not without its genetic costs.