Looks can be deceiving: Parallel evolution of plumage coloration in wheatears

The similar plumage patterns can be partly explained by hybridization.

Closely related species tend to look alike. So, it is no surprise that scientists have been inferring evolutionary relationships based on external characters. However, parallel evolution can complicate matters. This process – which can be defined as “similar development of a trait in distinct species which are not not closely related, but share a similar original trait in response to similar evolutionary pressure” – can lead to wrong conclusions about how species are related to one another. A recent study in Journal of Evolutionary Biology explores this issue in wheatears (Oenanthe).

 

Problematic Plumage Patterns

Consider the following four species. How would you classify them based on plumage coloration?

• Cyprus wheatear (O. cypriaca)
• Pied wheatear (O. pleschanka)
• Western black-eared wheatear (O. hispanica hispanica)
• Eastern black-eared wheatear (O. hispanica melanoleuca)

Four species of wheatear. How would you group them?

Solely using morphology, it makes sense to group Cyprus and pied wheatears and western and eastern black-eared wheatears. Genetic analyses, however, tell a different story.

 

Genetics vs. Morphology

Manuel Schweizer, Vera Warmuth, Niloofar Alaei Kahki (I mention these three authors because they contributed equally to the study) and their colleagues collected samples of these four species and performed genetic analyses using about 20,000 markers. The results are not in line with the morphological classification: Cyprus wheatear is more closely related to eastern black-eared wheatear, followed by pied wheatear and western black-eared wheatear. Not what you would expect based on their plumage patterns.

The unexpected evolutionary tree for these four wheatear species. The circles indicate the plumage patterns (from: Schweizer et al. 2018)

 

Parallel Processes

The evolution of plumage coloration seems to be the result of parallel evolution. But what is the exact mechanism? Parallel evolution can be the outcome of several biological processes. Unrelated species might use genetic material that was already present in a distant ancestor. Or traits might be transferred from one species to another by hybridization. Or similar mutations might have arisen independently.  Unfortunately, the present study cannot confidently discriminate between these scenarios.

Finally, the genetic analyses uncovered gene flow from pied into eastern black-eared wheatear. This is not that surprising, because a hybrid zone between these species has been described before. Whether hybridization has influenced the evolution of plumage patterns in these species remains to be investigated.

 

References

Schweizer, M., Warmuth, V., Kakhki, N.A., Aliabadian, M., Forschler, M., Shirihai, H., Suh, A. & Burri, R. (2018) Parallel plumage colour evolution and introgressive hybridization in wheatears. Journal of Evolutionary Biology.

 

This paper has been added to the Muscicapidae page.

Hybridization leads to emergence of a new sex chromosome

A surprising finding in a long-term hybridization experiment with swordtail fish. 

When I say sex chromosomes, you say XY (or ZW if you are a bird or butterfly person). But there are many more sex-determining systems than you can imagine. Take the fish genus Xiphophorus for example. Among the 26 species in this genus there are species with both XY and ZW sex chromosomes, but also some with other peculiar systems. The Southern Platyfish (X. maculatus) has three different sex chromosomes (X, Y and W), while the Green Swordtail (X. hellerii) has a so-called polyfactorial sex-determining system. In this species, the genes that determine the sex are scattered across the genome (the technical term is autosomal modifiers). A recent study in the journal Nature Communications makes the situation even more confusing.

A Southern Platyfish (from: http://www.wikipedia.com/)

 

Long-term Experiment

The evolutionary history of the genus Xiphophorus is littered with hybridization events. Some species – such as the Southern Mountain Swordtail (X. monticolus) and the Yellow Swordtail (X. clemenciae) – are even thought to be of hybrid origin (similar to some bird species). To better understand the impact of hybridization in this group of fish, scientists started a long-term hybridization experiment.

First, they mated a female Southern Platyfish with a male Green Swordtail. Interestingly, these two species have different sex-determining systems as described above. From the resulting hybrid offspring, the researchers selected females with two color patterns: a red coloration (caused by the gene Dr, dorsal red) and black spots (caused by the gene Sd, spotted dorsal). These genes are closely linked on the X-chromosome of the Southern Platyfish and are thus excellent markers to check if hybrids carry genetic material from this species. The selected females were then backcrossed to male Green Swordtails for 100 generations (more than 30 years!).

A Green Swordtail (from: http://www.wikipedia.com:)

 

A New Sex Chromosome

After patiently waiting for more than 30 years (I already get anxious when I have to run an analysis overnight), the researchers were in for a surprise. During the hybridization experiment, the sex-determining region of the X-chromosome from the Southern Platyfish was translocated to another chromosome in the backcrosses. It concerns a large block of about 10 Megabases (1 Mb is 1 million nucleotides) on linkage group 2. Essentially, a new sex chromosome originated through selection and hybridization.

Based on the outcome of this experiment and the widespread occurrence of hybridization during the evolution of Xiphophorus fish, the researchers conclude that “hybridization may be a key contributor to the evolutionary history of this group of fishes.”

 

References

Franchini, P., Jones, J.C., Xiong, P., Kneitz, S., Gompert, Z., Warren, W.C., Walter, R.B., Meyer, A. & Schartl, M. (2018) Long-term experimental hybridisation results in the evolution of a new sex chromosome in swordtail fish. Nature Communications, 9:5136.

A hybrid zone between subspecies of the Common Ringer Plover?

Genetic analyses support subspecies classification in the Common Ringed Plover.

“Of what use are subspecies?” asked biologist Ernst Mayr in a 1982 paper. The concept of a subspecies has generally been used to subdivide the geographical distribution of a species into meaningful units that differ in some morphological characters. Most subspecies, however, have not been assessed with genetic data (but see e.g., Wagtails). A recent study in the journal Ardea checks how well subspecies of Common Ringed Plover (Charadrius hiaticula) are supported by genetics.

 

How Many Subspecies?

The number of Common Ringed Plover subspecies ranges from two to seven, but most authors recognize three subspecies:

• hiaticula (southern Scandinavia and the Baltic)
• tundrae (northern Scandinavia and Russia)
• psammodromus (Canada, Greenland, Iceland and Faeroe Islands)

Birds from Chukotka, in the Russian far east, differ from the other subspecies in this region (tundrae) and might represent a distinct subspecies: kolymensis.

A Common Ringed Plover of the subspecies tundrae (from: http://www.wikipedia.com/)

 

No Population Structure

Leon Thies (University of Graz) and his colleagues collected samples across the range of the Common Ringed Plover and genotyped them using microsatellites. In general, the genetic analyses revealed no pronounced population structure. This observation can be explained by the dispersal patterns of these birds: juveniles do not breed at the site where they were born (i.e. low natal philopatry). The dispersal between different breeding sites results in gene flow that counteracts population differentiation.

However, when prior knowledge about the sampling location is included in the models, the three main subspecies are supported by the genetic data. There was no evidence for the putative kolymensis subspecies from Chukotka.

 

A Hybrid Zone

Admixture analyses (using the software package Structure) suggests a hybrid zone between all three subspecies, running from Northern Scandinavia to Belarus. The authors speculate that the subspecies diverged in allopatry during the ice ages and came into secondary contact. However, a more thorough genetic analysis is needed to characterize this putative hybrid zone.

A juvenile Common Ringed Plover (from: http://www.hbw.com/)

 

North vs. South

Finally, one paragraph in the discussion caught my attention:

In Dunlins Calidris alpina and Purple Sandpipers Calidris maritima, two sandpipers that breed at similar latitudes as Ringed Plovers, subspecies delineation based on phenotypic characters is poorly supported by genetic markers. In contrast, in several temperate or tropic waders, sub species delineation is in agreement with patterns of genetic differentiation.

Why do mismatches between genetic and phenotypic data occur more frequently in high latitude species? The classical explanation is that climatic oscillations had a stronger impact at higher latitudes. But is that really so?

 

References

Thies, L., Tomkovich, P., dos Remedios, N, Lislevand, T., Pinchuk, P., Wallander, J., Dänhardt, J., þórisson, B., Blomqvist, D & Küpper, C. (2018) Population and Subspecies Differentiation in a High Latitude Breeding Wader, the Common Ringed Plover Charadrius hiaticula. Ardea 106(2), 163-176.

 

This paper has been added to the Charadriiformes page.

How many hybrid bird species are there?

A review paper on hybrid speciation in birds proposes new way to classify hybrid species.

This week, I published my first single-author paper in a scientific journal: a review on hybrid speciation in birds. It feels strange to write a blog post about your own work, but I will do it anyway. First, I will briefly summarize the main points of the paper. Then I will provide some insights into the origin of this review.

 

Putative Hybrid Species

The last couple of years, ornithologists have speculated that several bird species have a hybrid origin, namely:

• Italian sparrow (Passer italiae)
• Audubon’s warbler (Setophaga auduboni)
• Genovesa mockingbird (Mimus parvulus bauri)
• Hawaiian duck (Anas wyvilliana)
• red‐breasted goose (Branta ruficollis)
• golden‐crowned manakin (Lepidothrix vilasboasi)
• “Big Bird” (Geospiza spp.)

The evidence supporting these claims varies for each species. Some cases (e.g., the Italian sparrow and “Big Bird”) are quite solid, while other putative hybrid species (e.g., red-breasted goose and Genovesa mockingbird) need more research. In each case, there is convincing evidence for hybridization, but not all studies could confidently discriminate between hybrid speciation and (recurrent) introgressive hybridization.

I have written about several of these hybrid species before (see here for “Big Bird” and golden-crowned manakin, here for the Italian sparrow and here for red-breasted goose).

The red-breasted goose, a hybrid species or not? (from: http://www.birdphoto.nl/)

 

Two Types?

When do you consider a species to have a hybrid origin? In 2014, Molly Schumer and her colleagues provided three criteria that should be satisfied: (1) genetic or morphological evidence for hybridization, (2) reproductive isolation of the hybrid lineage from its parental species, and (3) evidence that reproductive isolation is a direct consequence of past hybridization. Some authors argued that the third criterion is too strict and “focusing exclusively on [reproductive isolation] may shift the interest away from other crucial elements in HHS, that is, the ecological dimensions of the process and the production of novel diversity.”

As a solution to this debate, I propose to discriminate between two types of hybrid species: type I where reproductive isolation is a direct consequence of hybridization and type II where reproductive isolation is the by‐product of other processes, such as geographical isolation. I applied this classification scheme to the proposed hybrid bird species. “Big Bird” can be considered a type I hybrid species, while the Italian sparrow and the golden‐crowned manakin are type II hybrid species. For the other species, the evidence is still inconclusive.

The Italian sparrow: a type II hybrid species (from: http://www.wikipedia.com/).

 

Exploring the speciation continuum

The overview of hybrid bird species revealed hybrid lineages of different ages, ranging from a few generations (“Bird Bird”) over thousands of years (e.g., Italian sparrow and golden‐crowned manakin) to millions of years old (red‐breasted goose). This spectrum of divergence times allows for the comparison of hybrid genome stabilization and adaptation over time, while taking into account species‐specific processes. The exploration of this hybrid speciation continuum will definitely lead to some important insights.

The hybrid speciation continuum in birds (from Ottenburghs 2018, Ecology and Evolution)

 

On the origin of this paper

The story behind this review is quite intriguing. Let’s start from the beginning: a few years ago I wanted to publish one of PhD chapters as an extensive review. This chapter (which you can read here) provided an overview of avian hybrid zones and patterns of introgression. Unfortunately, the manuscript was rejected at Biological Reviews. One of the reviewers commented on my section about hybrid speciation in birds. He or she stated that this process was so rare in nature that it did not deserve a separate section in the paper. Being a stubborn PhD-student, I decided to turn the section into a separate paper.

While writing this hybrid speciation paper, I came across a discussion in Heredity on the classification of hybrid species (you can find the papers here and here). I wanted to provide my small contribution to this debate and proposed the classification described above.

The manuscript was send to Journal of Evolutionary Biology, where – despite largely positive reviews – it was rejected. The main reason for the rejection was the focus on birds. One reviewer wanted me to apply the classification scheme more broadly. Luckily, it was cascaded down to Ecology and Evolution where is was accepted with minor revisions.

 

A final note of frustration

I do not like to complain, but I want to share the final part of  the publication history. If you check the publication, you will see that it was accepted on 29 August 2018. It appeared online on 05 December 2018. Why did it take so long? The reason is the payment of the Open Access costs. My previous research group at Wageningen University agreed to pay the costs because the paper is part of my PhD project. So, I send the invoice to the financial department of Wageningen University where it got caught up in an administrative hassle. Apparently, Wageningen University needs a very specific format of invoice before they can process it. After several months, I got so fed up with this incompetent indecisiveness and decided to pay the costs myself. It literally took me less than 5 minutes. How difficult can it be to pay a bill?! Anyway, the paper is published. Let’s focus on the positive side.

 

References

Ottenburghs (2018) Exploring the hybrid speciation continuum in birds. Ecology and Evolution. Early Online