The Smell of Speciation: Chickadees prefer the scent of their own species

Could odor cues promote reproductive isolation?

To understand the origin of new bird species, ornithologists have mainly studied characters such as plumage color and song. If birds look or sound different, they probably represent distinct species. But what about less obvious features? What about smell? Recent work has shown that odors play an important role in avian ecology and evolution (see this paper for a nice review). In songbirds, olfactory cues are most likely produced by the uropygial gland. This organ is situated at the base of the tail and secretes complex mixtures of chemicals (generally called preen oil). Could these oils contribute to avian speciation? A recent paper in the journal Ecology and Evolution put this idea to the test.

Poecile_atricapillus

A Black-capped Chickadee perching on someone’s hand. © Chris | Flickr

 

Chickadees

Alex Van Huynh and Amber Rice focused on two chickadee species: the Black Capped Chickadee (Poelice atricapillus) and the Carolina Chickadee (P. carolinensis). These North American passerines interbreed along a hybrid zone, which has been studied extensively (you can check out the Paridae page for an overview). Because Black-capped Chickadee and Carolina Chickadee look very similar and can learn each others songs, these characters might not be reliable indicators in mate choice. Individuals can easily pick the ‘wrong’ species. Perhaps these birds use odor to chose their partner?

 

Experiment

The researchers tested this hypothesis with an elegant experiment. They collected wild chickadees from the hybrid zone and placed them in a Y-shaped maze. Each arm of the maze contained the odor of one species (which was blown into the maze from a box containing a bird). Next, the researchers noted how much time the birds spend in the respective arms of the maze.

The results indicated that birds spend more time in the odor arm of their own species, suggesting that these birds can discriminate between species based on smell. Previous studies have reported similar results in other bird species, such Waxwings and Crimson Rosellas. But this is the first evidence of olfaction-based species discrimination in a natural hybrid zone.

chickadees_experiment.jpg

The results from the experiment for (a) males and (b) females. Birds spend more time in the odor arm that contains the smell of their own species. From: Van Huynh & Rice (2019) Ecology and Evolution

 

The scent of hybrids

The results from this experiment are consistent with a possible role of odors in avian speciation. However, more work is needed to validate this idea. For example, what is the scent of hybrids? If hybrids smell different from ‘pure’ species, they might be less attractive and unable to find a partner. This selection against hybrids could increase genetic divergence between the species. In any case, this study shows that there is more to avian speciation than meets to eye (and ear).

Carolina_Chickadee.jpg

A Carolina Chickadee © Dan Pancamo | Flickr

 

References

Caro, S. P., Balthazart, J., & Bonadonna, F. (2015). The perfume of reproduction in birds: chemosignaling in avian social life. Hormones and behavior68, 25-42.

Mihailova, M., Berg, M. L., Buchanan, K. L., & Bennett, A. T. (2014). Odour-based discrimination of subspecies, species and sexes in an avian species complex, the crimson rosella. Animal Behaviour95, 155-164.

Van Huynh, A., & Rice, A. M. (2019). Conspecific olfactory preferences and interspecific divergence in odor cues in a chickadee hybrid zone. Ecology and Evolution.

Zhang, Y. H., Du, Y. F., & Zhang, J. X. (2013). Uropygial gland volatiles facilitate species recognition between two sympatric sibling bird species. Behavioral ecology24(6), 1271-1278.

 

This paper has been added to the Paridae page.

Advertisements

Is there a hybrid zone between subspecies of the Scarlet Macaw?

A genetic study took a closer look at this colorful parrot.

Linneaus described The Scarlet Macaw (Ara macao) in 1758. The father of modern nomenclature considered it a monotypic species (i.e. no subspecies). However, a study of museum specimens led to the recognition of two subspecies: cyanoptera and macao. The former subspecies, which ranges from southern Mexico to central Nicaragua, is more robust and has a wide band of yellow feathers on its wing. The other subspecies can be found from southern Nicaragua to Brazil and has more pronounced green coloration on the wing. In 1994, David Wiedenfeld described an intergradation of these color variants in southern Nicaragua and norther Costa Rica. Could this be a hybrid zone?

Waky.jpg

The Scarlet Macaw © Morrisdelta | Wikimedia Commons

 

Hybrid Zone

A recent study in the journal Ibis used mitochondrial DNA to check whether there is a hybrid zone. Analyses of 100 samples revealed a clear distinction between the two subspecies, which probably diverged between 320,000 and 850,000 years ago. There was, however, no evidence for a hybrid zone. The subspecies were clearly separated in southern Nicaragua and did not share any mitochondrial variants.

 

Population Structure

Interestingly, the northern subspecies (cyanoptera) showed more population structure than the southern subspecies (macao). In the map below, you can see more mitochondrial haplotypes (represented by different colors) in cyanoptera compared to macao. The authors suggest that the cyanoptera populations were isolated into separate refugia during climatic oscillations. The macao populations, on the other hand, remained more stable during these shifts in climatic conditions.

ibi12760-fig-0002-m.jpg

Population structure in the Scarlet Macaw. The northern subspecies (cyanoptera) shows more fragmented population structure compared to the northern subspecies (macao). From: Schmidt et al. (2019) Ibis

 

References

Schmidt, K. L., Aardema, M. L., & Amato, G. (2019) Genetic analysis reveals strong phylogeographic divergences within the Scarlet Macaw Ara macaoIbis. Early View.

 

This paper was added to the Psittaciformes page.

 

Classifying Corvids: Peculiar phylogeographic patterns in the crow family

“And the crow once called the raven black.”

— George R. R. Martin

“As the crow flies” is a common expression to refer to the linear distance between two points. When you consider the current distribution of several crow species (family Corvidae), you will notice that they can fly a long way. Indeed, many corvids have wide distributions, sometimes ranging from Western Europe all the way to Eastern Siberia. This makes corvids excellent study systems for phylogeography, the study of the historical processes responsible for the contemporary geographic distributions of individuals. A recent paper in the Russian Journal of Genetics and Breeding nicely reviews the recent work on several members of the Corvidae.

Carrion-Crow.jpg

A Carrion Crow © Andreas Trepte | Wikimedia Commons

 

Carrion and Hooded Crow (x2)

Let’s start with a textbook example: the hybrid zone between Western Carrion Crow (Corvus corone) and Hooded Crow (C. cornix). This contact zone runs from Scotland, through Denmark, Germany, the Czech Republic and Hungary, to Italy. Genomic analyses of this species pair revealed that they differed in just a few genomic regions, representing less than 1 percent of the genome. One of these differentiated regions contains contains genes involved in pigmentation and visual perception which might explain why these species mate assortatively (i.e. choose a partner with the same plumage pattern).

Interestingly, a similar hybrid zone occurs in Siberia. Here, Hooded Crow interbreeds with the Eastern Carrion Crow (C. orientalis). Genomic analyses of this hybrid zone showed that different genomic regions were differentiated compared to the European situation. This finding indicates that the genetic basis of speciation is highly species-specific and context-dependent. You can read this blog post for more information about this hybrid zone comparison.

Previous work on the Siberian hybrid zone also revealed that the Collared Crow (C. pectoralis) had the same mitochondrial haplotype as the southeastern Carrion Crow population. Perhaps it acquired this haplotype through hybridization?

crow hybrid zone

The locations of the European and Siberian hybrid zones between Hooded Crows and Carrion Crows. From: Vijay et al. (2016) Nature Communications

 

Medieval Transport?

An interesting hypothesis concerns the Azure-winged Magpie (Cyanopica cyanus). This beautiful bird has a peculiar distribution: one population occurs on the Iberian Peninsula while the other population is restricted to Eastern Asia. Some researchers argued that Portuguese and Spanish sailor might have transported these birds from Asia during Medieval times. Analyses of the mitochondrial gene cytb showed that these populations diverged more than three million years ago. This finding rejects the hypothesis of human-mediated transport. These birds probably had a wider distribution that was broken up during the Pleistocene ice ages.

The two populations are currently recognized as distinct species: the Asian Azure-winged Magpie (Cyanopica cyanus) and the Iberian Azure-winged Magpie (Cyanopica cooki).

Cyanopica_cooki_Caceres_1.jpg

A pair of Iberian Azure-winged Magpies © Mario Modesto Mata | Flickr

 

Comparative Phylogeography

Azure-winged Magpies are not the only corvids that show million-year-old divergences. A comparative study uncovered similar patterns in other species, such as the Rook (Corvus frugilegus), the Jackdaw (Corvus monedula and Corvus dauuricus) and the Eurasian Magpie (Pica pica). All of these corvids show significant divergence between eastern and western populations. Such divergence was absent for other species, including the Raven (Corvus corax), the Siberian Jay (Perisoreus infaustus) and the Nutcracker (Nucifraga caryocatactes). What could explain these patterns? The author of this review – Alexey Kryukov – thinks ecological differences might hold the key:

All species of the first group preferably nest in semi-open habitats and forest edges, while the second group (single-group species) live mainly in forests. The raven (C. corax) is an ubiquist. We arrived at the conclusion that a prominent factor influencing the pattern of genetic differentiation seems to be the preference for either open to semi-open habitats (the west-east pattern) or forest dominated habitats (the single group pattern). Separated refuge areas (western and eastern) during cold periods led to accumulation of diversity.

1-s2.0-S1055790307002047-gr1

Comparative phylogeography of several corvid species shows a common east-west pattern (indicated in red and blue) in some species. Other species (in black) do not show this pattern. The explanation to this phylogeograhic phenomenon might be ecological. From: Haring et al. (2007) Molecular Phylogenetics and Evolution

 

References

Kryukov, A. P. (2019). Phylogeography and hybridization of corvid birds in the Palearctic Region. Journal of Genetics and Breeding, 23(2), 232-238.

 

This paper has been added to the Corvidae page.

 

More than meets the eye: Scientists discover several cryptic storm-petrel species

Sympatric populations that breed in different seasons are genetically differentiated.

There is more diversity than we can see. Some species are morphologically indistinguishable but they differ in other aspects, such genetics, song or behavior. These so-called cryptic species are difficult to detect because humans are visual species. More detailed investigations are needed to uncover this hidden diversity of cryptic species. In 2001, for example, Darren Irwin and his colleagues showed that the morphologically similar taxa humei and inornatus (which were classified as the same species: Yellow-browed Warbler, Phylloscopus inornatus) were in fact different species. They are genetically distinct and sing different songs. A recent study in the journal Molecular Phylogenetics and Evolution checked whether there are also cryptic species among the storm-petrels (genus Hydrobates).

storm petrel.jpg

The Band-rumped Storm-petrel © Luke Seitz | eBird

 

Breeding Seasons

Storm-petrels are small seabirds with a wide distribution. They can be found on several tropical and sub-tropical islands in the Pacific and Atlantic Oceans. On some islands, there are populations with distinct breeding seasons. Previous work already showed that populations on the Azores are genetically differentiated. Moreover, birds from these different breeding populations produced different vocalizations, leading ornithologists to recognize them as distinct species.

Do similar processes occur on other islands? To answer this question, Rebecca Taylor and her colleagues collected blood samples from 754 storm-petrels, covering numerous breeding areas (see map below).

map_petrels.jpg

Locations of storm-petrel breeding areas. Red circles indicate hot seasons breeding while blue circles point to cold season breeding. The purple circle (Cape Verde) refers to year-round breeding. From: Taylor et al. (2019) Molecular Phylogenetics and Evolution

 

Genetic Clusters

The genetic analyses uncovered seven clusters, indicating that the different archipelagos are genetically distinct. Moreover, within three clusters, the researchers could even differentiate between hot and cold season breeders. These results suggest that the storm-petrels represent multiple cryptic species.

Interestingly, the level of genetic differentiation between the hot and cold season breeders varies from island to island. On the Azores, the populations are clearly distinct and can be seen as different species (as mentioned above). On the Selvagem island, the cool season breeders still clustered with the hot season breeders. This suggests that these breeding populations are still differentiating and thus represent an earlier stage in the speciation process.

PCA_petrels.jpg

A principal component analysis (PCA) showing the seven distinct genetic clusters. From: Taylor et al. (2019) Molecular Phylogenetics and Evolution

 

Isolation Mechanisms

The existence of multiple cryptic species begs the question: what is driving the genetic divergence between these populations? The differences between islands can be explained by philopatry, the tendency of birds to breed in the same location. Occasionally, an individual might switch between islands but in general birds stay loyal to their breeding grounds. In addition, differences in ocean regimes (e.g., surface temperature, ocean currents) can act as physical barriers between islands.

The genetic differentiation on islands is mainly due to allochrony: different populations breed at different times of the year. What processes lead to allochrony is still an open question. Birds might breed at different times because of competition for nest sites or food. Or perhaps predators can force individuals to start nesting at another time. More research is needed to sort this out.

 

Also check out this great blog post at the British Ornithologists’ Union (BOU): “Just what is a species?“, written by Rebecca Taylor.

References

Bolton, M., Smith, A. L., Gomez-Diaz, E., Friesen, V. L., Medeiros, R., Bried, J., Roscales, J. L. & Furness, R. W. (2008). Monteiro’s Storm‐petrel Oceanodroma monteiroi: a new species from the Azores. Ibis, 150(4):717-727.

Irwin, D. E., Alström, P. , Olsson, U., & Benowitz‐Fredericks, Z. M. (2001). Cryptic species in the genus Phylloscopus (Old World leaf warblers). Ibis, 143(2):233-247.

Friesen, V. L., Smith, A. L., Gomez-Diaz, E., Bolton, M., Furness, R. W., González-Solís, J., & Monteiro, L. R. (2007). Sympatric speciation by allochrony in a seabird. Proceedings of the National Academy of Sciences, 104(47):18589-18594.

Taylor, R. S., Bolton, M., Beard, A., Birt, T., Deane-Coe, P., Raine, A. F., Gonzalez-Solis, J., Lougheed, S. C. & Friesen, V. L. (2019). Cryptic species and independent origins of allochronic populations within a seabird species complex (Hydrobates spp.). Molecular Phylogenetics and Evolution, 139:106552.

 

This paper has been added to the Procellariiformes page.

 

How many times did Steamer Ducks lose the ability to fly?

Answering that question might be more difficult than expected.

Several bird species have become flightless. The ability to fly is mostly seen as a binary character: a species is either flying or flightless. However, reality is often not that straightforward. Take, for example, the steamer ducks of the genus Tachyeres. These four South American species originated within the last two million years and show different degrees of flight ability. Most individuals of the Flying steamer duck (T. patachonicus) can still fly, only the heaviest males cannot take to the skies any longer. The other three species – Fuegian steamer duck (T. pteneres), Chubut steamer duck (T. leucocephalus) and Falkland steamer duck (T. brachypterus) – are largely flightless. This situation raises the question: how many times did these steamer ducks lose the ability to fly?

Tachyeres_pteneres.jpg

Two Fuegian steamer ducks © Николай Усик | Wikimedia Commons

 

Candidate Genes

To answer this question, Leonardo Campagna and his colleagues sequenced the genomes of these species and pinpointed the genomic regions underlying the loss of flight. A genome-wide association analyses (GWAS) uncovered two regions on chromosome 1 which contain 28 genes with known functions. Flightlessness is probably influenced by several genes, although one candidate gene seems especially promising: DYRK1A. In humans, this gene is involved in Down Syndrome and experiments in mice revealed that high levels of DYRK1A lead to skeletal abnormalities. The authors hypothesize that differential expression of DYRK1A could explain the morphological differences in steamer-ducks.

GWAS_ducks

Results from the GWAS analyses shows two large peaks on chromosome 1. These regions contain several genes of which DYRK1A (in blue circle) is a promising candidate for further research. From: Campagna et al. (2019) Evolution.

 

Flighted Alleles

Phylogenetic analyses of the genome-wide data set revealed that the three species of flightless steamer ducks are not monophyletic. This result, which is consistent with previous work using mitochondrial DNA, suggests that the ability to fly was lost independently three times. There is, however, a second possibility. Two flightless species – pteneres and leucocephalus – have similar genotypes in the candidate regions. Perhaps the genetic variants for flightlessness were already present in the ancestor of these species. This would point to a single genetic origin of “flightless alleles” which consequently segregated in the different species. In some species, these alleles go to fixation, leading to flightless birds. In other species, these alleles do not completely replace the “flighted alleles”, resulting in a few individuals that can still fly.

This latter situation might explain the occurrence of some flying brachypterus ducks. Interestingly, flying individuals of this species possess the same genotypes as the flying patachonicus ducks. Possibly, brachypterus obtained these “flighted alleles” through introgressive hybridization with patachonicus ducks. This scenario is supported by the observation that several patachonicus birds possess mitochondrial DNA from brachypterus. Hybridization is relatively common in birds, often resulting in the exchange of genetic material. The possible acquisition of “flighted alleles” through hybridization has an interesting implication. It provides a plausible mechanism for regaining flight, an evolutionary transition that has been deemed unlikely.

scenarios.jpg

Two scenarios for the loss of flight in steamer ducks. Blue color indicated flighlessness, while red points to flighted birds. Color gradients represent polymorphism of “flighted alleles”. (A) three independent losses (indicated by the three black arrows) or (B) a single genetic loss (black arrow), followed by transfer of “flighted alleles” from T. patachonicus to T. brachypterus (grey arrow). From: Lele & Ottenburghs (2019) Evolution.

 

Another digest

I also wrote a digest about this study (you can read it here). Digests are short news articles about selected original research included in the journal Evolution written by students or postdocs. The story behind this digest is quite interesting, so I decided to cover it in more detail on my personal website. You can check it out here.

 

References

Lele, A. & Ottenburghs, J. (2019) Digest: A single genetic origin and a role for bone development pathways in repeated losses of flight in steamer ducks. Evolution.

Campagna, L., McCracken, K. G., & Lovette, I. J. (2019). Gradual evolution towards flightlessness in Steamer‐Ducks. Evolution.

 

The papers have been added to the Anseriformes page.

Genetic study reveals how four different sparrow species adapted to life in the salt marshes

If you put butter and salt on it, it tastes like salty butter.”

– Terry Pratchett (Moving Pictures)

Salt marshes are a relatively new feature in the North American landscape. The expansion of these habitats occurred only a few thousand years ago. In this short time span, several bird species have quickly adapted to living in these salty conditions. Adaptations include a larger bill to facilitate heat exchange in these open environments and an altered kidney structure to cope with the salty drinking water. Members of the bird family Passerellidae seem to have an inordinate fondness for salt marshes, four lineages have independently colonized this habitat. A recent study in the journal Evolution Letters investigated whether these birds used the same genetic tricks to thrive in this new environment.

Passerculus_sandwichensis_crop.jpg

A singing Savannah Sparrow © Cephas | Wikimedia Commons

 

Parallel Evolution

Jennifer Walsh and her colleagues compared the genomes of four salt marsh specialists with their upland cousins (see list below). They wanted to know whether the salt-adapted subspecies survive in the salt marshes due to parallel evolution, which they define as “shared [genomic] regions of elevated differentiation between multiple upland-salt marsh pairs.” Alternatively, differentiated regions that are only found in one subspecies pairs are an example of lineage-specific evolution.

  • Savannah Sparrows (Passerculus sandwichensis nevadensis and Passerculus sandwichensis beldingi)
  • Nelson’s sparrows (Ammospiza nelsoni nelsoni and Ammospiza nelsoni subvirgatus)
  • Song Sparrows (Melospiza melodia gouldii and Melospiza melodia pusillula)
  • Swamp Sparrows (Melospiza georgiana georgiana and Melospiza georgiana nigrescens).

The researchers found no regions of elevated differentiation that were shared by all four subspecies pairs. However, some differentiated regions were shared among two or three pairs. In total, the analyses yielded 33 candidate regions that contained 16 genes with a putative role in adaptation to salt marshes.

Nelson's_Sparrow_(10301730173).jpg

A Nelson’s Sparrow © Andy Reago & Chrissy McClarren | Wikimedia Commons

 

Osmoregulation

Most of the candidate genes were involved in osmoregulation (i.e. maintaining the fluid balance and the concentration of salts to keep the body fluids from becoming too diluted or concentrated). This makes sense because salty environments can disturb the osmotic balance of cells. Interestingly, different osmoregulation genes were under selection in different subspecies pairs.

In Savannah Sparrows, the researchers found WNK2 under selection. This gene plays an important role in regulating cell volume in response to osmotic stress. In Nelson’s Sparrows, MMP17 showed signs of selection. This gene has been linked to drinking behavior and kidney function in mice. In Song Sparrows, yet another gene popped up: MYOF, which is differentially expressed when fish are transferred from fresh to salt water. And in Swamp Sparrows, SLC9A3 has been under selection, a gene that is involved in sodium transport across the cell membrane.

These examples suggest that “these [osmoregulation] pathways are common targets of selection, but the specific genic targets of selection within the pathways differ among species.”

Melospiza-melodia-001.jpg

A Song Sparrow © Mdf | Wikimedia Commons

 

Gene Flow

The researchers also reconstructed the demographic histories of all subspecies pairs. They did this to rule out any demographic factors that might influence their search for differentiated regions. These analyses revealed continuous gene flow between each pair of subspecies. This observation raises the question whether introgression of beneficial alleles might have facilitated adaptation to salt marshes. Indeed, a previous study on Nelson’s Sparrows identified several candidate genes for adaptive introgression. Whether this also occurred in the other lineages remains to be determined.

Melospiza_georgiana_MN2.jpg

A Swamp Sparrow © Cephas | Wikimedia Commons

 

References

Walsh, J., Benham, P. M., Deane‐Coe, P. E., Arcese, P., Butcher, B. G., Chan, Y. L., … & Shriver, W. G. (2019). Genomics of rapid ecological divergence and parallel adaptation in four tidal marsh sparrows. Evolution Letters.

 

This paper has been added to the Passerillidae page.

Adventures in the Andes: The Tantalizing Tale of the Torrent Duck

Molecular and morphological analyses uncover three distinct lineages in the Torrent Duck.

The Torrent Duck (Merganetta armata) surely lives up to its name. These beautiful ducks – the males have a striking black and white head and a fire-red bill – fearlessly dive into the rapid waters of the Andes. They have a wide distribution, ranging from southern Chile all the way up to Venezuela. Their widespread occurrence and affinity for wild waters raises an important question: How do the Andean river networks shape the population structure of this species? On the one hand, rivers could promote dispersal and gene flow between distant populations. On the other hand, rivers could act as barriers to gene flow, leading to population differentiation. A recent study in the journal Zoologica Scripta tackled this question and assessed the population structure of this widespread species.

 

Three Groups

Natalia Gutiérrez-Pinto and her colleagues analyzed the mitochondrial control region of 198 Torrent Ducks. These analyses uncovered three distinct lineages that correspond to previously described subspecies. A Northern Andes group (colombiana) with birds from Colombia, Ecuador, and northern Peru. A Central Andes group (leucogenis) that covers Peru and northwestern Bolivia. And a Southern Andes group (armata) with individuals living in southeastern Bolivia and Argentina.

zsc12367-fig-0002-m.jpg

Analyses of the mitochondrial control region uncovered three distinct lineages that correspond to previously described subspecies of the Torrent Duck. From: Gutiérrez-Pinto et al. (2019) Zoologica Scripta

 

Barriers

The distribution of these three groups seems to match the geographic regions delineated by Jon Fjeldså: Páramo, Puna and Southern Andes. The main geographical barriers between these regions are the North Peruvian Low (between Páramo and Puna) and the Bolivian Altiplano (between Puna and Southern Andes). In case of the Torrent Ducks, the Bolivian Altiplano appears to be an effective barriers between the Central and Southern Andes groups. The high elevations of the Tunari mountain range separate both groups.

The role of North Peruvian Low, however, is less clear. This region was not densely sampled in the study, preventing the researchers from assessing potential gene flow across this barrier. Moreover, the North Peruvian Low has a complex topography which might provide Torrent Ducks with small watersheds that connect the Northern with the Central Andes group. More research in this area is needed to characterize this barrier.

Merganetta_armata

A pair of Torrent Ducks in Colombia © Alejandro Bayer Tamayo | Wikimedia Commons

 

Morphology

As mentioned above, the three mitochondrial lineages correspond to three previously described subspecies. A morphological analysis of these birds showed that the Northern Andes group (colombiana) is highly differentiation from the other two groups. The differences between the Central (leucogenis) and the Southern (armata) groups is more subtle. Possibly, these birds represent the extremes of a range of phenotypes. There might even be a hybrid zone at some location. Denser sampling is required to figure this out. If it turns out that they hybridize, you will definitely read it on this blog.

 

References

Gutiérrez‐Pinto, N., McCracken, K. G., Tubaro, P., Kopuchian, C., Astie, A., & Cadena, C. D. (2019). Molecular and morphological differentiation among Torrent Duck (Merganetta armata) populations in the Andes. Zoologica Scripta.

 

This paper has been added to the Anseriformes page.

These two White-eye species should be hybridizing, but they don’t…

Despite their recent divergence, Solomons White-eye and Kolombangara White-eye don’t interbreed.

When closely related species come into secondary contact, they often hybridize. In 2002, Trevor Price and Michelle Bouvier estimated that after, on average, 5 million years of divergence birds tend to produce sterile hybrids. After about ten million years of independent evolution, the hybrids are not viable anymore. Hence, it would be surprising to find species pairs markedly younger than 5 million years to show complete reproductive isolation. But that is exactly what two researchers describe in a recent paper in the journal Evolution.

white-eyes

The two species of White-eye in this study. © Sarah Cowles | https://sarahacowles.weebly.com/research.html

 

White-eyes

Sarah Cowles and Albert Uy investigated a contact zone between the Solomons White-eye (Zosterops kulambangrae) and the Kolombangara White-eye (Z. murphyi) on Kolombangara Island in the Solomon Archipelago. These species diverged about two million years ago and are thus expected to be able to hybridize. However, the genetic analyses (based on 20,000 SNPs) revealed no evidence for gene flow. This suggests that the reproductive isolation between these two passerines is complete.

The researchers searched the literature for other such cases. They found that these White-eyes represent the youngest known case of complete reproductive isolation in sympatric birds tested within a genomic context.

white-eyes_results.jpg

The genetic analyses showed that these White-eyes are clearly distinct. From: Cowles & Uy (2019) Evolution

 

Isolation Mechanisms

But what is preventing these birds from interbreeding? It could be that these species do not recognize each other as potential partners. They produce different songs and calls. And the size of the white eye-ring is significantly different. Whether the birds use these traits in species recognition remains to be investigated. Another possibility is that the hybrids are not viable due to genetic mismatches. A genomic study in 2015 suggested that White-eyes have elevated rates of genomic evolution, which could result in the rapid accumulation of such genetic mismatches. Clearly, more research is needed here.

white-eyes_song.jpg

Both species produce clearly different song and calls. Could this prevent hybridization? From: Cowles & Uy (2019) Evolution

 

The Paradox of the Great Speciator

This study might provide a solution to a paradox raised by Jared Diamond and his colleagues. They wondered how these White-eyes (the family Zosteropidae) can diversify so quickly while colonizing a vast geographical range. You would expect that incipient species come into secondary contact during their colonization. Then hybridization and consequent gene flow would prevent further divergence and effectively reverse the speciation process. However, in just two million years, the White-eyes diversified into more than 100 species while spreading to Africa, Asia, Australia and throughout the Indo-Pacific. How can this happen?

Solomons White-eye and Kolombangara White-eye might hold the answer to this paradox: lineages with high dispersal capacities can have high speciation rates if they evolve complete reproductive isolation more rapidly than other lineages. Testing this idea with other pairs of White-eyes will be necessary to see if the solution is correct.

ZosteropsRendovaeKeulemans.png

A drawing of Solomons White-eye. © John Gerrard Keulemans | Wikimedia Commons

 

Digest

I wanted to cover this story earlier. The paper was gathering dust at the top of my writing list for some time now. But I waited for the publication of another paper, namely my digest of this study. The journal Evolution provides the opportunity to write short news articles (so-called digests) about selected original research. I wrote one about the White-eye study, which you can read here (although it is a more technical story that covers the same ground as this blog post). Apart from my Avian Hybrids blog, these digests are a great way to spread the latest findings about hybridization in birds. Even if the species under investigation don’t hybridize…

 

References

Cowles, S. A., & Uy, J. A. C. (2019). Rapid, complete reproductive isolation in two closely‐related Zosterops White‐eye bird species despite broadly overlapping ranges. Evolution.

Ottenburghs, J. (2019) Digest: White‐eye birds provide possible answer to the paradox of the great speciator. Evolution

 

This paper has been added to the Zosteropidae page.

Genetic study uncovers a deep split within the Hooded Pitta species complex

A clear split between eastern and western groups with signatures of gene flow between different subspecies.

In the 18th century, the French naturalist Georges-Louis Leclerc, Comte de Buffon published Planches Enluminées D’Histoire Naturelle, a collection of drawings from animals all around the world. One of these drawings depicted the “Merle des Philippines” (the Blackbird of the Philippines). Modern ornithologists know this species as the Hooded Pitta (Pitta sordida). This small passerine, which occurs from India to New Guinea, was generally considered a single species, but the phenotypic variation among different populations suggests this might not be the case. A recent study in the journal BMC Evolutionary Biology provides some genetic support for this idea.

hooded-pitta_Buffon.png

Buffons drawing of the Hooded Pitta, which he called the “Blackbird of the Philippines”.

 

Plumage Variation

The Hooded Pitta complex is comprised of numerous populations on different Indo-Pacific islands. These populations differ in several plumage traits, such as the color of the forehead and the crown, the amount of red, black and blue on the flanks and the belly, and the size of a white wing patch. Per Ericson and his colleagues tried to reconstruct the evolutionary history of these populations. They sampled the 13 recognized subspecies across the range of this species complex and sequenced their DNA. For interested readers, the genetic analyses were based on mitogenomes, 23 nuclear genes and about 2.1 million SNPs.

Pitta_sordida.jpg

A Hooded Pitta in Thailand © J.J. Harrison | Wikimedia Commons

 

Wallace Line

Reconstructing the evolutionary history of these colorful birds revealed two distinct groups that diverged about 2 million years ago. This split coincides to the Wallace Line, which highlights the striking differences in fauna and flora in the eastern and western parts of the Indo-Australasian archipelago. However, the origin of Wallace Line is much older than the split between the groups within the Hooded Pitta. The most likely scenario is that some birds dispersed across the Wallace Line during the Pleistocene (between 2.5 million and 11.000 years ago) when sea levels were lower.

 

Pitta

The distribution of the Hooded Pitta subspecies and their evolutionary relationships. Notice the deep split between eastern and western populations. From: Ericson et al. (2019) BMC Evolutionary Biology

 

Gene Flow

The researchers could not find consistent patterns within the two groups. It seems that recent gene flow between different populations from the same subspecies has partly erased any geographic patterns in the genetic data. During the Pleistocene, fluctuations in sea levels resulted in occasional land-bridges between the different islands, allowing birds to hop from one island to the next. The consequent bursts of gene flow prevented island populations from diverging from one another.

There has also been some gene flow between different subspecies. One individual from the subspecies forsteni, which lives in the eastern part, showed signatures of gene flow from the western part. In addition, samples in the western clade, bangkana from Bangka Island and palawanensis from Palawan, exhibited signs of admixture. The bangkana individual appears to be admixed between the migratory population on the Asian mainland (cucullata) and nearby populations on Sumatra and Borneo (mulleri). The subspecies palawanensis harboured DNA from sordida and mulleri.

Hooded_Pitta_Singapore.jpg

A Hooded Pitta in Singapore © Darren Bellerby | Wikimedia Commons

 

References

Ericson, P. G., Qu, Y., Rasmussen, P. C., Blom, M. P., Rheindt, F. E., & Irestedt, M. (2019). Genomic differentiation tracks earth-historic isolation in an Indo-Australasian archipelagic pitta (Pittidae; Aves) complex. BMC Evolutionary Biology19(1), 151.

 

This paper has been added to the Pittidae page.

High levels of gene flow between different populations of North American Scoters

A genetic study finds weak population structure in three Scoter species.

Some bird species are difficult to study. Take, for instance, the Scoters (genus Melanitta). These black ducks spend most of their time at sea where they aggregate in big flocks. Several breeding colonies and wintering areas have been found, but ornithologists don’t know how these populations are structured. Do birds switch between breeding areas or do they stay loyal to one location? These questions can be answered using genetic data, as shown by a recent study in the journal Ecology and Evolution.

Melanitta_perspicillata.jpg

A Surf Scoter © Alan D. Wilson | NaturesPicsOnline

 

Five (or three) species

Sarah Sonsthagen and her colleagues focused on three North American: the Black Scoter (M. americana), the Surf Scoter (M. perspicillata) and the White-winged Scoter (M. deglandi). They also analyzed samples from two European species: the Common Scoter (M. nigra, which is sometimes considered conspecific with the Black Scoter) and the Velvet Scoter (M. fusca, which is sometimes seen as conspecific with the White-winged Scoter). So, depending on your taxonomic preference, there are three or five species of Scoter in this study. The researchers used a combination of ddRAD-seq and microsatellites to probe the genetic population structure of these Scoters.

Melanitta_deglandi

A White-winged Scoter © Matt MacGillivray | Flickr

 

Dispersal

The three North American species showed weak population structure, indicating high levels of gene flow between different locations. In birds, this pattern is mostly attributed to male-biased dispersal. Males choose a partner at the wintering grounds and follow her back to the breeding area. However, female Surf Scoters and White-winged Scoters can occasionally switch between migration routes. For example, the White-winged Scoters that nest in central Canada are a mixture of birds wintering on the Atlantic and the Pacific coast. There is evidence for a female using different wintering areas in different years.

Black Scoters showed some population genetic structure that coincides with their breeding distribution. There appears to be a barrier – behavioral of physical – between birds from Alaska and the Atlantic region.

Melanitta_americana.jpg

A Black Scoter © Peter Massas | Flickr

 

Gene Flow

The three North American species were clearly differentiated from their European cousins. There was no evidence for gene flow between different continents. The pattern is most likely driven by the use of distinct wintering areas.

In addition, the researchers found no signs of gene flow between species. Hybrids between several species (e.g., White-winged Scoter x Surf Scoter) have been observed. However, such hybridization events are probably too rare to influence the genetic structure of these species. The courtship and copulation displays of different Scoter species are quite distinct and could serve as a behavioral barrier.

 

References

Sonsthagen, S. A., Wilson, R. E., Lavretsky, P., & Talbot, S. L. (2019). Coast to coast: High genomic connectivity in North American scoters. Ecology and Evolution9(12), 7246-7261.

 

The paper has been added to the Anseriformes page