Evolutionary mystery of egg mimicry cracked

Easter approaches and in the human ecosystem egg hunts are in full swing. But in the avian world surprise eggs of an unwelcome nature are being found.

Many of Earth’s bird species avoid the effort of raising their young by laying their eggs in the nests of others. This evolutionary trait of tricking other species to accept foreign eggs, known as “brood parasitism,” is achieved by through mimicry.

Not only are egg forgers able to copy the colours and patterns of their victims’ eggs, but some parasitic species are able to simultaneously mimic the eggs of several different bird species.

An international team has done genetic research to answer the question of how egg mimicry is passed down through generations, verifying a near-century old theory. The scientists’ findings also indicate brood parasites’ methods, developed over two million years, may be in jeopardy as the evolutionary tug-of-war continues.

Published today in PNAS, the study focused on the cuckoo finch (Anomalospiza imberbis), which uses brood parasitism to take advantage of the parenting skills of many warbler species throughout Africa. The research revealed that cuckoo finch mothers are responsible for passing on egg mimicry through the female-specific W chromosome (analogous to the Y chromosome, specific to male mammals).

The “maternal inheritance” displayed in the cuckoo finch means that there is no risk of the wrong mimicry genes being passed on by a father raised by a different host. This “uniparental inheritance” also allows for the diversity seen in different female cuckoo finch lineages that mimic the eggs of different warbler species.

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DNA samples were collected from 196 cuckoo finches from 141 host nests by the team in southern Zambia. The cuckoo finches exploit four different grass-warbler species in the area.

“While maternal inheritance has allowed cuckoo finches to exploit multiple host species, it’s likely to slow their ability to evolve counter-adaptations as their hosts evolve new defences,” says lead researcher Professor Claire Spottiswoode, from the University of Cambridge, UK. “In particular, parasites face a daunting challenge because some host species have in return evolved an astonishing diversity of egg colour and pattern ‘signatures’, that help hosts to distinguish their own eggs from parasitic mimics.”

Grass-warblers are gaining the upper hand in the fightback against egg forgery. The host species are becoming more adept at quality control, harnessing personalised “signature” patterns and colourations on their own eggs. While cuckoo finches can mimic at least some of these signatures, they are stifled by their maternally inherited forgery traits, which cannot be recombined between family lines.

For example, an earlier study by Spottiswoode found an increasing share of tawny-flanked prinia (Prinia subflava) eggs – one grass-warbler species manipulated by the cuckoo finch – coloured olive green. Two cuckoo finch family lines specialise in blue and red eggs, but are unable to mix their genetics to create the precise mixture of pigments to produce olive-green eggs. Indeed, this earlier study found that the host species are using bi-parental inheritance, unlike the cuckoo finches, to produce these unforgeable eggs.

“Cuckoo finches are missing out on a powerful source of evolutionary novelty and that could prove costly in this ongoing arms race,” says Spottiswoode. “The way they inherit their ability to mimic host eggs has a downside by likely making the grass-warblers’ defences more effective, and constraining the parasite’s ability to respond.

“We may see the emergence of unforgeable egg signatures which could force cuckoo finches to switch to other naïve host species. Or the parasitic birds might become increasingly dependent on young host individuals that haven’t yet learned their own signatures and are bad at spotting mismatched eggs.”