Grey cuckoo, red cuckoo: unveiling the genomic secrets of color polymorphism in female cuckoo birds
ADVANCED SCIENCE RESEARCH CENTER, GC/CUNY
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FEMALE ADULT COMMON CUCKOOS APPEAR IN TWO COLOR MORPHS, SHOWN HERE AS HALF OF THE RUFOUS (HEPATIC) FORM (LEFT) AND HALF OF THE GRAY MORPH (MALE-LIKE, RIGHT).
view moreCREDIT: BIANKA REGINA JASKA
NEW YORK, April 24, 2024 — Sexual dimorphism—the visible difference between females and males—can be seen in diverse animals, including humans. One intriguing aspect of this phenomenon is sex-limited polymorphism, where one sex displays greater variations in a particular trait than the other. In a recent study published in Science Advances, a team of researchers delve into the genetic underpinnings behind the color polymorphism observed in adult females of the brood parasitic Cuculus, more widely known as cuckoo birds, shedding light on the evolution and functional significance of this phenomenon.
Several species of cuckoos, a genus of birds distributed across Africa and Eurasia, exhibit sex-limited polychromatism (color differences) in their typically striped and hawk-mimetic plumage. Specifically, while adult male cuckoos feature a gray plumage, adult females can display either gray or rufous (reddish-brown) plumage. Furthermore, the prevalence of each female color morph varies between cuckoo species as well as between different populations within the same species.
The internationally collaborative study was initiated by Mark E. Hauber, executive director of the CUNY Advanced Science Research Center (CUNY ASRC) and a psychology professor at the CUNY Graduate Center, in collaboration with a group of Hungarian field ornithologists. For their study, the team employed a genome-wide association analysis on nuclear DNA data, which was obtained from Common and Oriental cuckoos in the wild and from museum specimens. Their aim was to identify candidate genes responsible for the observed polychromatism in adult female cuckoos, the mechanisms of pigmentation underlying the coloration, and the evolutionary history of the mutation.
Genes that affect males and females differently often reside on the sex chromosomes. In mammals, including humans, males represent the heterogametic sex, characterized by their XY sex chromosomes. As such, genes on the Y chromosome exclusively manifest in males, distinguishing them from females with XX chromosomes. Conversely, in birds, females exhibit heterogamy with ZW sex chromosomes, distinct from the ZZ chromosomes of male birds. This disparity led the researchers to hypothesize that the variation in color observed in female cuckoos is likely linked to gene(s) on the female sex chromosome.
“We found that, instead of a single gene or set of genes, nearly all variations in the gray or rufous coloration were associated with the full length of the female-limited W chromosome,” said Hauber. “That was really unexpected.”
Further exploration into the biochemical basis of coloration encoded on the W chromosome uncovered a commonality across the two closely related cuckoo species: both and the Common and Oriental cuckoos utilize two shared forms of melanin-based pigmentation, the same chemical found in human skin pigmentation, to control the intensity of brown and gray coloration in their plumage.
The researchers also utilized genomic data from the two species to examine the evolutionary history of color polymorphism in female cuckoos. Their analysis revealed that a single mutation for female-only polychromatism originated over 1 million years ago in the shared ancestor of these cuckoos, predating species divergence which occurred roughly 140,000 years ago.
The study's findings prompt intriguing questions about the benefits of female’s evolving multiple morphs. Female cuckoos of the less common morph may gain advantages in mimicking rufous juvenile cuckoos to evade the attention of harassing males interested in mating. Additionally, the often-rarer rufous plumage may be strategic for these brood parasite birds, which lay their eggs in the nests of other birds, forcing the hosts to care for their young. The rufous plumage may help females avoid recognition as parasites by potential host species parents, which often attack and occasionally kill adult cuckoos near their nests.
The study transforms our understanding of sex-limited polymorphism and also adds to knowledge regarding sexual dichromatism, highlighting the adaptive advantages that can arise from such intricate evolutionary adaptations.
Article DOI: 10.1126/sciadv.adl5255
About the Advanced Science Research Center at the CUNY Graduate Center
The Advanced Science Research Center at the CUNY Graduate Center (CUNY ASRC) is a world-leading center of scientific excellence that elevates STEM inquiry and education at CUNY and beyond. The CUNY ASRC’s research initiatives span five distinctive, but broadly interconnected disciplines: nanoscience, photonics, neuroscience, structural biology, and environmental sciences. The center promotes a collaborative, interdisciplinary research culture where renowned and emerging scientists advance their discoveries using state-of-the-art equipment and cutting-edge core facilities.
JOURNAL
Science Advances
METHOD OF RESEARCH
Data/statistical analysis
SUBJECT OF RESEARCH
Animals
ARTICLE TITLE
Evolution of female-limited coloration in cuckoos
ARTICLE PUBLICATION DATE
24-Apr-2024
Color variants in cuckoos: the advantages of rareness
Rufous or gray: LMU evolutionary biologists show that the color variants of female cuckoos are based on ancient mutations
Every cuckoo is an adopted child – raised by foster parents, into whose nest the cuckoo mother smuggled her egg. The cuckoo mother is aided in this subterfuge by her resemblance to a bird of prey. There are two variants of female cuckoos: a gray morph that looks like a sparrowhawk, and a rufous morph. Male cuckoos are always gray.
“With this mimicry, the bird imitates dangerous predators of the host birds, so that they keep their distance instead of attacking,” says Professor Jochen Wolf from LMU Munich. Together with researchers at CIBIO (Centro de Investigação em Biodiversidade e Recursos Genéticos, Portugal), the evolutionary biologist has investigated the genetic foundations of the variant coloring, which is limited to females and emerged over the long evolutionary arms race between host and cuckoo.
Hypotheses from behavioral research propose that the rarer color morph always has an advantage, because the host birds gradually get wise to the trick. If, for example, there are many sparrowhawk-like cuckoos or many sparrowhawks, the host birds learn over time to distinguish whether the bird in front of them is a sparrowhawk or a cuckoo. “Then the advantage of the rufous morph comes into play, as it is less common and has not been learned by the hosts,” says Wolf. Which variant the host birds learn depends on the abundance both of the cuckoos and of the birds of prey.
Only the female line counts
Only female cuckoos who have to get close to the host nests have these color morphs. “We would therefore expect these color variants – so-called polymorphisms – to be fixed somewhere in the female genome,” says Wolf. Whereas in humans it is males who carry a sex-specific chromosome, the Y chromosome, in birds it is females who have a sex chromosome, the W chromosome. With his team, Wolf has now demonstrated that the mutations for the color morphs are in fact located either on the W chromosome or in mitochondria, which are also passed down exclusively along the maternal line.
The researchers also showed that a sister species of the cuckoo, the Oriental cuckoo, possesses the same color morphs and mutations. “Consequently, the mutations already existed in a common ancestor and are therefore older than the speciation,” says Wolf. The evolutionary biologist sees this as a strong indicator that the rarer variant does actually always have an advantage: Where there are two variants, the better one normally wins out. But if the rarer of two variants always has an advantage, the system oscillates and settles into a certain frequency. “This causes the genetic variation to be maintained for a long time,” says Wolf. “Our results support the intriguing possibility that numerous other traits that are sex-specific but more difficult to investigate are genetically encoded on the matrilinear genome like the color polymorphism of the female cuckoo.”
JOURNAL
Science Advances
ARTICLE TITLE
Evolution and genetic architecture of sex-limited polymorphism in cuckoos
ARTICLE PUBLICATION DATE
24-Apr-2024
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