A supergene determines morphological variation in redpolls

But are these redpolls different species or not?

“Thus, redpolls appear to function as a single species harboring ecotypic variation, rather than three distinct species.” This statement caught my attention while I was reading a recent Nature Communications paper by Erik Funk and his colleagues. Using genomic data from 73 individuals, they managed to pinpoint the genetic basis of three redpoll taxa that are currently described as separate species: the Hoary Redpoll (Acanthis hornemanni), the Common Redpoll (A. flammea), and the Lesser Redpoll (A. cabaret). The morphological variation between these three taxa could be traced back to a big inversion – about 55 million DNA-letters long – on chromosome one (see this blog post for more information on inversions). This genomic region contains almost 500 genes, several of which are involved in plumage pigmentation and beak morphology. As all these genes are tightly linked and inherited together, the researchers describe it as a “supergene”.

The taxonomically-orientated reader might now be confused by the first sentence of this blog post. These redpolls are morphologically distinct and we now know the genetic basis of these differences. Why refer to them as ecotypes, and not species? Let’s find out.

Morphological differences between the three redpolls species – or ecotypes – in the study. From: Funk et al. (2021).

Three Genotypes

Detailed analyses of the inversion revealed three clusters that do not completely correspond to the three redpoll taxa. Two clusters contain multiple species, namely Lesser Redpolls with Common Redpolls, and Hoary Redpolls with Common Redpolls. The third cluster consists almost entirely of Hoary Redpolls. The researchers assigned these clusters to three inversion genotypes: AA, AB and BB. Interestingly, these genotypes seem to align with plumage patterns:

Transitioning from the AA, to AB, to BB genotype also broadly mirrors a transition in phenotype from dark to light plumage coloration, where the AA genotype is associated with dark plumage, BB is associated with light plumage, and AB is intermediate.

Moreover, the geographical distribution of the three genotypes follows a latitudinal gradient with AA-birds at low latitudes and BB-birds at high latitudes. The heterozygotes (AB) occupy an intermediate position. These patterns suggest that the inversion might play a role in local adaptation. Indeed, bird species with white plumage and small beaks, such as the BB-redpolls, tend to occur in high-altitude regions.

The three inversion genotypes do not correspond to the three redpolls species (left figure), but do show a relationship with latitude (right figure). From: Funk et al. (2021).

Reproductive Isolation

The observation of heterozygotes (AB-birds) indicates that no combination of the supergene is lethal. This situation is different from other cases where certain combinations are often sterile or unviable due to genetic incompatibility. In Ruffs (Philomachus pugnax), for example, homozygotes with an inverted region are unviable and heterozygotes show low survival rates. The lack of lethal combinations in redpolls suggests that the inversion does not play an important role in reproductive isolation between the taxa. In fact, intermediate individuals and mixed pairs have been observed in the wild. Moreover, the genetic analyses in the current study indicated significant levels of gene flow between the redpoll taxa.

With this additional information about local adaptation and reproductive isolation, we can return to the statement about ecotypes versus species that kicked off this blog post. Here is the complete quote from the paper.

In light of the link between the redpoll supergene and phenotype and differences in breeding distribution between ecotypes, the supergene may impart local adaptation to the environment. However, given the detection of inversion heterozygotes and the presence of gene flow, the inversion likely does not influence reproductive isolation. Thus, redpolls appear to function as a single species harboring ecotypic variation, rather than as three distinct species.

It is clear that the authors are referring to “biological species” where a certain level of reproductive isolation is required. However, some ornithologists might focus on the morphological and genetic differences between the taxa, and downplay the relevance of reproductive isolation. I will not get involved in this debate and will leave the final decision with taxonomists. Honestly, I do not care whether these redpolls are classified as species, subspecies or ecotypes. I prefer to focus on the ecological and evolutionary impact of their supergene, which is obviously super-interesting.

References

Funk, E. R., Mason, N. A., Pálsson, S., Albrecht, T., Johnson, J. A., & Taylor, S. A. (2021). A supergene underlies linked variation in color and morphology in a Holarctic songbird. Nature communications, 12(1), 1-11.

Featured image: Common Redpoll (Acanthis flammea) © Jyrki Salmi | Wikimedia Commons

Why is sympatric speciation rare in birds?

A guest post by Cody Porter on a recent study in The American Naturalist.

How do new bird species form? Clearly, there are many dimensions to this question, but a nearly universal requirement for bird speciation is geographic isolation. Most avian sister species, especially those that diverged recently, have largely or entirely non-overlapping geographic ranges. Furthermore, sister species of birds rarely co-occur in small, circumscribed areas like oceanic islands. This and other lines of evidence suggest that most bird populations need to be physically separated for long periods of time for speciation to occur. It is probably no accident that Ernst Mayr, who vehemently argued against the possibility of sympatric speciation for most of his career, was an ornithologist.

In some respects, this is a strange phenomenon. Classic theoretical models indicate that there are two basic requirements for sympatric speciation: ecological divergence and assortative mating. There is a wealth of evidence for both criteria in birds, including groups in the early stages of divergence (Darwin’s finches are an obvious example, but far from the only one). So why do all lines of evidence point to allopatric speciation as being responsible for the diversity of Darwin’s finches and most other birds and vertebrates?

Bills and Pine Cones

An important caveat to the aforementioned models is that most assume reproduction coincides with periods of strong performance tradeoffs associated with alternative resources. If this occurs, ecologically divergent populations may become “automatically” reproductively isolated from each other. For example, populations might breed in different habitats where alternative food resources occur that each is adapted to. This occurs in some organisms such as some herbivorous insects, where sympatric host races reproduce on different host plants (e.g., apple and hawthorn races of Rhagoletis pomonella). Perhaps unsurprisingly, the strongest evidence for sympatric speciation generally comes from herbivorous insects. By contrast, most birds (indeed, most vertebrates) time reproduction to coincide with abundant food resources that multiple species can easily exploit. For example, during the breeding season, different species of Darwin’s finches converge in their use of arthropods — a very different situation than in the nonbreeding season, when each specializes on a different subset of resources. These patterns suggest that weak tradeoffs and resulting ecological convergence during the breeding season may preclude sympatric speciation in many organisms. But can we test this hypothesis directly?

This was the central goal of my dissertation research on the Red Crossbill (Loxia curvirostra) complex. Previous work has shown that bill morphology of sympatric ecotypes has diversified in response to tradeoffs in the ability to efficiently feed on different conifers. For example, the western hemlock crossbill has a tiny bill that allows it to rapidly remove seeds from the small cones of Western Hemlock (Tsuga heterophylla). On the other hand, the large bill of ponderosa pine crossbills are unwieldy when feeding on hemlock, but is great for prying open the large, woody cones of Ponderosa Pine (Pinus ponderosa), which western hemlock crossbills cannot feed on. During most breeding seasons, sympatric ecotypes primarily feed on their respective “key conifers” that impose such tradeoffs. Every few years or so, however, conifers with easily accessible seeds (e.g., Engelmann Spruce, Picea engelmanni) produce large cone crops that multiple ecotypes feed on during breeding. We thus have multiple ecotypes breeding in sympatry in some years when there are strong feeding tradeoffs and other years when there are weak feeding tradeoffs: exactly the conditions necessary to test those classic theoretical models. Moreover, because most crossbills are nomadic, birds are constantly dispersing long distances and opportunistically breeding during different resource conditions in different locations. Thus, we have something akin to a natural experiment in crossbills.

This video initially shows two western hemlock crossbills feeding on western hemlock cones. The bird on the right consumed three seeds in 10.78 seconds. The larger, lone bird later in the clip is a ponderosa pine crossbill feeding on western hemlock cones. This bird consumed just one seed in 10.48 seconds, meaning its feeding rate is ~66% lower than that of the western hemlock crossbills.

Reproductive Isolation

My colleagues and I collected data on two sympatric ecotypes (Lodgepole Pine and Ponderosa Pine crossbills) that co-occur throughout the Rocky Mountains. Based on data from 10 breeding seasons, we found striking support for those classic sympatric speciation models. When feeding tradeoffs are strong, reproductive isolation between ecotypes is nearly complete, which is necessary for sympatric speciation. Specifically, the two ecotypes largely breed in different forest types where their respective conifers dominate and are thus unlikely to encounter each other as potential mates. Moreover, the few “migrants” that do attempt to breed in the “wrong” habitat type struggle to reproduce, further reducing opportunities for mating between ecotypes. We even found that birds are more likely to mate assortatively when tradeoffs are strong. Playback experiments and lots of data on flock composition indicate that this arises because crossbills are more likely to flock assortatively when tradeoffs are stronger. Because crossbills choose mates from within flocks, stronger assortative flocking leads to stronger assortative mating. Interestingly, previous work has shown that crossbills gain feeding efficiency benefits by assortative flocking only when tradeoffs are strong, thus explaining the flocking data. When feeding tradeoffs are weak, all of these barriers to gene flow are much weaker – not nearly strong enough for sympatric speciation to occur. We even found that resource availability (for example, how abundant food resources are) affects reproductive isolation, with higher availability leading to lower isolation. This latter result suggests important extensions to those classic theoretical models.

These data may help explain why some groups, like herbivorous insects, are more prone to speciating in sympatry than others, such as most birds. It is worth noting that there may be some crossbill lineages that have undergone sympatric speciation (e.g., the Cassia Crossbill, L. sinesciuris). What’s interesting is that these crossbills specialize on extremely stable seed resources that do not fluctuate from year-to-year. Thus, unlike most crossbills, these lineages never experience periods of abundant food and weak tradeoffs. This likely explains why barriers to gene flow in such systems are remarkably strong and stable from year-to-year, which in turn may explain why these lineages are genomically divergent from sympatric ecotypes.

We took advantage of nomadism and opportunistic breeding by two crossbill ecotypes to determine how feeding tradeoffs affect reproductive isolation. A) Both ecotypes breed in forests of mixed lodgepole and ponderosa pines. The dotted curve represents the relationship between performance and bill size on lodgepole pine and the solid curve represents the relationship on ponderosa pine. In mixed pine forests, tradeoffs are large. B) In forests of spruce the two ecotypes have more similar feeding abilities, reflecting weak tradeoffs. The locations where these crossbills breed vary yearly, because of spatiotemporal variation in cone crops that crossbills track with nomadic movements.

This blog post was written by Cody Porter. Check out more of his research on his personal website. Want to write a guest post for the Avian Hybrids Project? Get in touch with me!

References

Porter, C.K. & Benkman, C.W. (2021) Performance tradeoffs and resource availability drive variation in reproductive isolation between sympatrically diverging crossbills. The American Naturalist.

Featured image: Red Crossbill (Loxia curvirostra) © Frank Vassen | Wikimedia Commons

How many rosy-finch species are there in North America?

Phylogenomic analyses try to solve this taxonomic puzzle.

Ornithologists have created several world bird lists to summarize avian taxonomy, such as the International Ornithological Community (IOC) World Bird List or the Howard and Moore Checklist. These checklists do not always agree on the classification of specific birds (see for example this study on raptors), leading to heated debates between taxonomists. Take, for instance, the rosy-finches of the genus Leucosticte in North America. The American Ornithologists’ Union recognizes three species: the gray-crowned rosy-finch (L. tephrocotis), the brown-capped rosy-finch (L. australis), and the black rosy-finch (L. atrata). In the Howard and Moore checklist, however, they are lumped into one species.

The lumped arrangement is based on a 2009 study in Molecular Phylogenetics and Evolution where Sergei Drovetski and his colleagues could not discriminate between the three American taxa using the mitochondrial gene ND2 and two autosomal genes. They attributed the lack of genetic differentiation to gene flow between neighboring populations. However, can we base a taxonomic decision on a handful of genes? These taxa clearly look distinct. Perhaps these morphological differences can be traced back to a few genomic regions, similar to golden-winged warbler (Vermivora chrysoptera) and blue-winged warbler (V. pinus) where a few “plumage genes” are responsible for their distinct plumage patterns. To test this idea, Erik Funk and his colleagues used genomic data to study the evolution of the rosy-finches in North America. Their findings recently appeared in the journal Systematic Biology.

Three Species?

Using whole genome sequencing data from 68 individuals, the researchers reconstructed the phylogenetic relationships between the different taxa. The analyses provided support for three American species: the black rosy-finch, the brown-capped rosy-finch, and the Alaska island rosy-finch. The first two are already considered distinct species by the AOU, while the populations on the Alaskan islands are currently classified as two subspecies within the grey-crowned rosy-finch (griseonucha and umbrina). The remaining subspecies of the grey-crowned rosy-finch are intermixed in the phylogeny, rendering this taxon paraphyletic (check this blog post for an explanation of paraphyly). A taxonomic revision might thus be necessary.

In line with the study by Sergei Drovetski et al. (2009), the researchers also found signatures of gene flow between different populations. This finding can explain why analyses based on a few genes were unable to discriminate between the North American species. The use of genomic data often results to the detection of minor differences between populations, which leads to an important warning. Just because you can discriminate between certain populations with genomic data does not mean they should automatically be considered separate species. Indeed, in a PNAS paper Jeet Sukumaran and Lacey Knowles nicely described this issue: “Until new methods are developed that can discriminate between structure due to population-level processes and that due to species boundaries, genomic-based results should only be considered a hypothesis that requires validation of delimited species with multiple data types, such as phenotypic and ecological information.” The taxonomic story of the American rosy-finches is just getting started.

The distribution (figure a) and phylogenetic relationships between the different rosy-finch taxa (figure d). From: Funk et al. (2020) Systematic Biology.

References

Funk, E. R., Spellman, G. M., Winker, K., Withrow, J. J., Ruegg, K. C., Zavaleta, E., & Taylor, S. A. (2021). Phylogenomic data reveal widespread introgression across the range of an alpine and arctic specialist. Systematic Biology, 70(3), 527-541.

Featured image: Gray-crowned Rosy-Finch (Leucosticte tephrocotis) © Alan D. Wilson | Wikimedia Commons

This paper has been added to the Fringillidae page.

With or without gene flow? Studying speciation in the Grey-headed Bullfinch species complex

Has there been past gene flow between populations on the Asian mainland and Taiwan?

In 2008, Albert Phillimore and his colleagues concluded that “allopatric speciation is the dominant geographic mode of speciation in birds.” Allopatric speciation refers to the situation where two populations become geographically isolated and genetically diverge, ultimately giving rise to new species. In recent years, however, the focus of speciation research has shifted from a purely geographical perspective (e.g., allopatry, parapatry and sympatry) to the consideration of gene flow. Geographical isolation is still an important component of speciation, but diverging populations often continue to exchange genes during the process. The high incidence of divergence-with-gene-flow examples has led to the idea that this might be the dominant mode of speciation in birds.

Land Bridges

The presence of some gene flow during speciation appears to have become the null hypothesis that many ornithologists start from. For instance, a recent study in the journal Molecular Phylogenetics and Evolution reconstructed the evolutionary history of Grey-headed Bullfinch (Pyrrhula erythaca) species complex. Different populations can be found on the Asian mainland (subspecies erythaca and erythrocephala) and on the island Taiwan (subspecies owstoni). During the Pleistocene ice ages, land bridges connected Taiwan with the mainland, potentially allowing gene flow between these populations. We can thus expect to find some signatures of past gene flow between the Grey-headed Bullfinch populations.

However, genetic analyses clearly separated the mainland and island populations. This separation does not completely rule out past gene flow, so the researchers performed coalescent modelling to test different speciation scenarios. This exercise pointed to a strictly allopatric speciation model. It seems that there has been no gene flow between the Asian mainland and Taiwan.

Sampling locations of the different populations in the Grey-headed Bullfinch species complex. Genetic analyses revealed a clear separation between island and mainland populations. From: Dong et al. (2020) Molecular Phylogenetics and Evolution.

Sky Islands

Why did these populations not exchange DNA despite the land bridges between Taiwan and the Asian mainland? The answer lies in their habitat preferences. Reconstructing the past distributions of these populations revealed that they never overlapped. The mainland populations resided in the mountainous areas and could not expand their ranges to the lowlands. Birds in these “sky islands” were thus isolated from lowland populations, such as the birds expanding from Taiwan. The researchers conclude that “unlike lowland species, incipient sky island species might have had
limited opportunities for intermittent secondary contact and gene flow during late Pleistocene sea-level fluctuations.” Indeed, other highland species, such as the Vinaceous Rosefinch (Carpodacus vinaceus) and the Taiwan Rosefinch (C. formosanus) also diverged without gene flow. The allopatric speciation model is alive and kicking!

References

Dong, F., Li, S. H., Chiu, C. C., Dong, L., Yao, C. T., & Yang, X. J. (2020). Strict allopatric speciation of sky island Pyrrhula erythaca species complex. Molecular Phylogenetics and Evolution, 153, 106941.

Featured image: Grey-headed Bullfinch (Pyrrhula erythaca) © Robert tdc | Wikimedia Commons

Splitting the Long-tailed Rosefinch into a Chinese and a Siberian species

Genetic and morphological data support the recognition of two distinct species.

“We echo calls for integrative taxonomy in which genomic and phenotypic data are considered on equal footing when delimiting species.” This concluding remark was stated by Carlos Daniel Cadena and Felipe Zapata in their recent review paper on species delimitation. I wholeheartedly agree with this statement. In fact, I have advocated this integrative approach to taxonomy in other blog posts (see for example here). The rationale behind this approach is quite straightforward: different taxonomic concepts and methods are combined in drawing species limits. Within this context, two general frameworks can be used: integration by congruence and integration by cumulation. The congruence approach entails that different data sets, such as molecular and morphological characters, support the decision to recognize certain taxa as valid and distinct species. In the cumulation approach, evidence from different data sets is gathered, concordances and conflicts are explained within the specific evolutionary context of the taxa under study, and based on the available evidence a decision is made.

 

Five Subspecies

The best way to highlight the importance and usefulness of integrative taxonomy is to see it in action. Luckily, a recent study in the Journal of Ornithology applied this strategy to the Long-tailed Rosefinch (Carpodacus sibiricus). The distribution of this species extends across a large part of eastern Asia. Based on its discontinuous distribution and some subtle differences in plumage, five subspecies have been proposed: the northern sibiricus, ussuriensis and sanguinolentus, and Chinese endemics henrici and lepidus. Simin Liu, Chentao Wei and their colleagues collected samples from all five subspecies and performed detailed genetic, morphological and acoustic analyses to determine how distinct these subspecies are. Perhaps some can be elevated to species rank?

The discontinuous distribution of the Long-tailed Rosefinch. The symbols correspond to different sampling locations for particular subspecies. Colors refer to resident (green) wintering (blue) and breeding (yellow) areas. From: Liu et al. (2020) Journal of Ornithology.

 

Two Clades with Similar Songs

Genetic analyses of the mitochondrial gene COI uncovered two main clades that correspond to the northern and southern grouping of subspecies. The northern clade consists of sibiricus, ussuriensis and sanguinolentus, while the southern clade comprises henrici and lepidus. Interestingly, the subspecies ussuriensis and sanguinolentus form a mixed group, suggesting that they should be considered one subspecies instead of two. The other three subspecies are clearly distinct lineages.

In contrast to the genetic analyses, there were no clear differences in songs. The authors write that “Overall, there was much overlap among the taxa and no clear separation was found, and there was no clear division between the songs of the northern and southern groups.” The lack of song divergence might be due to the recent origin of these groups (ca. 1.36 million years ago) or their allopatric distribution. The birds might not need species-specific songs because they rarely encounter other subspecies. There might thus be no strong selection that could lead to different songs.

The genetic analyses uncovered two main groups (left figure). All subspecies form distinct lineages, except for ussuriensis and sanguinolentus. There were no clear differences in songs between the subspecies (right). From: Liu et al. (2020) Journal of Ornithology.

 

Particular Plumage Patterns

What about morphological differences? Previous studies indicated subtle variation in plumage patterns. Although the authors did not perform a quantitative morphological analysis, they do describe notable differences between the northern and southern groups.

While adult males from the northern group have the entire forehead and crown silvery-pink, in the southern group the feathers of the forehead are silvery-pink, whereas the crown is reddish. The mantle of northern birds is pink to deep red with moderately broad brown streaking, while the mantle of southern birds is more brownish and less reddish. The northern taxa have broader median and greater covert wing bars, while the wing bars of southern birds are relatively narrow. Finally, the three outermost tail feathers are extensively white in northern birds, while in southern birds only the outermost feathers are extensively white, and the white part only covers less than half the length of the second outermost tail feathers.

Based on the gathered evidence, the researchers propose to split the Long-tailed Rosefinch into two species, namely the Siberian Long-tailed Rosefinch (C. sibiricus comprising the subspecies sibiricus and sanguinolentus), and Chinese Long-tailed Rosefinch (C. lepidus comprising the subspecies henrici and epidus). Would you agree?

The different subspecies show some subtle differences in plumage patterns. From: Liu et al. (2020) Journal of Ornithology.

 

Evolutionary History

Regardless of whether the northern and southern group will be recognized as distinct species, it is worthwhile to have a closer look at their evolutionary history. This north-south division between a Siberian-Japanese clade and a Chinese clade has been documented in other bird species and can probably be attributed to the mountains of northern and central China. The combined effects of the geographical isolation of these mountains and the changing vegetation during the Pleistocene turned this area into a formidable barrier for passerines, resulting in genetic divergence between separated populations.  Similarly, these two taxa in the southern group likely diverged in separate mountain ranges: henrici in the Hengduan mountains and lepidus in the Qinling Mountains and Yan Mountains.

Within the northern group, we see a east-west divide between sibiricus and ussuriensis/sanguinolentus. This pattern can be explained by historical glacial isolation in Siberia. It is possible that the eastern and western groups were connected by gene flow in the past. Indeed, one ussuriensis (in yellow) individual is nested within the sibiricus (in blue) group. Whether this concerns a misidentified individual or a signature of gene flow remains to be determined. I am secretly hoping for the latter explanation.

 

References

Liu, S., Wei, C., Leader, P. J., Carey, G. J., Jia, C., Fu, Y., Alström, P & Liu, Y. (2020). Taxonomic revision of the Long-tailed Rosefinch Carpodacus sibiricus complex. Journal of Ornithology, 161(4), 1061-1070.

Featured image: Long-tailed Rosefinch (Carpodacus sibiricus) © Попов Евгений | Wikimedia Commons

Solving the genetic mystery of the mosaic canary

Which genes are responsible for this peculiar plumage pattern?

Good scientific research resembles a thrilling mystery novel. Gathering clues, testing potential leads and critical thinking enable both detectives and scientists to solve the challenging questions. A recent study in the journal Science nicely illustrated this approach. The mystery: the genetic basis of mosaic plumage coloration in captive canaries. These red canaries are the result of crossing the Common Canary (Serinus canaria) with the Red Siskin (Spinus cucullatus). Bird breeders have selected for this color pattern – commonly known as the mosaic phenotype – by consecutive backcrossing the hybrids with “pure” Common Canaries. Over time, the genome of resulting red canaries is largely Common Canary-DNA with a dash of Red Siskin. And this dash of DNA probably contains the genes responsible for the red plumage color.

Consecutive backcrossing between the Common Canary x Red Siskin hybrid and “pure” Common Canaries results in a genome that mainly consists of Common Canary DNA (light green) with a bit of Red Siskin-DNA (dark green). From: Gazda et al. (2020) Science.

Zooming in

Using a series of genomic techniques, the researchers zoomed in on the Red Siskin-DNA in the mosaic canaries. They narrowed the search down to a genomic region (on scaffold NW_007931177) with 52 genetic variants that were different between canaries with yellow and red feathers. The mosaic phenotype is a recessive trait, meaning that mosaic birds have the same genetic variant on both chromosomes (in other words, they are homozygous). This insight provides another important clue to solve the mystery. Which of the 52 genetic variants are homozygous for the Red Siskin in the mosaic canaries? Focusing on these homozygous variants pointed to a genomic region of about 36,000 DNA-letters, containing three genes: PTS (6-pyruvoyltetrahydropterin synthase), BCO2 (b-carotene oxygenase 2), and TEX12 (testis-expressed protein 12).

The genetic basis of red coloration is homozygous for Red Siskin DNA. This knowledge allowed the researchers to zoom in on a particular region with homozygous variants (highlighted in the black box). From: Gazda et al. (2020) Science.

Gene Expression

Now that we have three main suspects (PTS, BCO2 and TEX12) we can explore a next lead: differential gene expression. Mosaic males and females show distinct plumage patterns. Males accumulate more red pigment in their feathers than females. Hence, we can expect that the genes controlling red color are differently expressed in males and females. The researchers took a closer look at the expression patterns of PTS, BCO2 and TEX12 in regenerating feather follicles. One gene showed decreased expression in males compared to females: BCO2. Did we find the culprit?!

We know that BCO2 codes for an enzyme that plays an essential part in the degradation of carotenoids, the pigments responsible for red coloration. In mosaic males, this enzyme is not very active and does not break down many carotenoids, resulting in the accumulation of red pigment in the feathers. The mosaic phenotype is thus the outcome of sex-specific differences in BCO2-activity, suggesting that it is controlled by other regulatory sequences (the genetic on-and-off switches). These regulatory elements remain to be identified. We might have found the murderer, but we are still looking for the brains behind the crime.

Two genes (PTS and TEX12) did not show significant differences in gene expression between males and females (top boxes). The third gene (BCO2), however, was less active in males compared to females. Interestingly, the difference in gene expression was only apparent in feather follicles (lower left box) and not in the liver (lower right box). From: Gazda et al. (2020) Science.

References

Gazda, M. A. et al. (2020). A genetic mechanism for sexual dichromatism in birds. Science, 368(6496), 1270-1274.

Featered image: A mosaic canary © Fernando Zamora Vega | Shutterstock