Quick guide on balanced lethal systems

Balanced lethal systems pose an evolutionary paradox: they should not evolve because they cut reproductive output in half, yet they have done so time and again. The aim of my ERC Starting Grant project is to solve this evolutionary mystery. I give a bit of background on balanced lethal systems, summarize the main hypotheses on their origin, and outline how a combination of genomics and evolutionary modelling may provide the key to finally understanding them, in a new ‘quick guide’ published in Current Biology. Follow this link for temporary free access.

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The balanced lethal system in Triturus, known as chromosome 1 syndrome, in action. Heterozygous offspring experience normal embryonic development, while homozygous offspring experience developmental arrest and die, halfway through normal embryogenesis. Hence, chromosome 1 syndrome is responsible for half of eggs laid never hatching. (Thanks to Michael Fahrbach for use of his pictures.)

Reference: Wielstra, B. (2020). Balanced lethal systems. Current Biology 30(13): R742-R743.

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Misleading mitochondrial DNA in a Balkan crested newt

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Male T. macedonicus. Picture by Michael Fahrbach.

Historical biogeographical scenarios based on DNA data have in the past mostly relied on a single genetic marker: mitochondrial DNA. As it became easier and cheaper to consult more and more nuclear DNA markers, it has become apparent that mitochondrial DNA can be misleading.

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Distribution of distinct mitochondrial (left) and nuclear DNA groups in T. macedonicus.

In a paper published in the Biological Journal of the Linnean Society we show this for the Macedonian crested newt (Triturus macedonicus). These guys show the deepest intraspecific mitochondrial DNA divergence of any crested newt species. However, the distinct geographical mitochondrial DNA groups observed in T. macedonicus are not matched at all in the nuclear genome. The moral of the story: take mitochondrial DNA with a grain of salt.

Reference: Wielstra, B., Arntzen, J.W. (2020). Extensive cytonuclear discordance in a crested newt from the Balkan Peninsula glacial refugium. Biological Journal of the Linnean Society 130(3): 578-585.

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Is historical hybrid zone movement underappreciated?

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Two crested newt studies, previously highlighted here and here, support historical hybrid zone movement – movement on the scale of hundreds of kilometers and spanning multiple millennia. Before these crested newt examples, little to no empirical evidence for historical hybrid zone movement was available. Does this mean that crested newt hybrid zones are particularly dynamic? Or has the prevalence of historical hybrid zone movement in other systems simply been overlooked? In a perspective piece in Journal of Biogeography I argue the latter: historical hybrid zone movement is likely to be more common than currently appreciated.

Reference: Wielstra, B. (2019). Historical hybrid zone movement: more pervasive than appreciated? Journal of Biogeography 46(7): 1300-1305.

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This project has received funding from the European Union’s Horizon 2020 research and innovation programme under the Marie Skłodowska-Curie grant agreement No. 655487.
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Two PhD positions available in the Wielstra lab

Please note that the deadline to apply for these positions has now passed. Two more postdoc positions will be announced in the context of my ERC Starting Grant so keep an eye on this website.

LogosIn the context of my ERC Starting Grant project I seek to hire two PhD students to begin in fall 2019. Please see the advertisement here.

40 developing_embryoA lucky Triturus embryo that has avoided the arrested development associated with ‘chromosome 1 syndrome’. This picture is by Michael Fahrbach and taken from his recent book (Fahrbach & Gerlach (2018) The genus Triturus).

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Cracking cryptic banded newt species

Genetic studies in the family Salamandridae have regularly revealed cryptic species – genetically distinct species that previously went unrecognized due to their morphological similarity. The most recent salamandrid example is provided by the banded newts (genus Ommatotriton). While the split between the southern banded newt (O. vittatus) versus ‘O. ophryticus’ is generally accepted, this is not the case for the further split of the latter taxon into the Caucasian banded newt (O. ophryticus sensu stricto) and the Anatolian banded newt (O. nesterovi). From morphology O. vittatus is known to be distinct, but no differences are known to separate the other two banded newt species.

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Pictures of the three banded newt species, courtesy of Michael Fahrbach, Sergé Bogaerts, and the late Bayram Göçmen and Max Sparreboom. The vernacular names are newly proposed in this study.

Based on a superficial inspection of photographs, it seemed to me that two ‘band’ characters might be useful for species identification. Now, based on a survey of almost 700 museum specimens, we show in a paper in Salamandra that in O. nesterovi the lateral white band generally I) continues from the front limbs up to the eye and II) is interrupted by large specks on the tail, while this is generally not the case in O. ophryticus (see the figure above). In the third member of the genus, O. vittatus, both character states for each ‘band’ character occur at similar frequency, but the lateral white band is considerably broader compared to the other two species. Hence, all three banded newt species can be identified from morphology.

Reference: Üzüm, N., Avcı, A., Olgun, K., Bülbül, U., Fahrbach, M., Litvinchuk, S.N., Wielstra, B. (2019). Cracking cryptic species: external characters to distinguish two recently recognized banded newt species (Ommatotriton ophryticus and O. nesterovi). Salamandra 55(2): 131-134.

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This project has received funding from the European Union’s Horizon 2020 research and innovation programme under the Marie Skłodowska-Curie grant agreement No. 655487.
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Quick guide on crested and marbled newts

Triturus is great, showing intricate ritualized mating behaviour where males congregate at leks and perform elaborate dances to wow the females, representing an adaptive radiation reflecting gradients of aquaticness, providing strong support for the hypothesis of historical hybrid zone movement, and representing the best-known example of an evolutionary enigma known as a balanced lethal system – as I explain in a new ‘quick guide’ published in Current Biology.

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Thanks to Michael Fahrbach and Paolo Mazzei for use of their pictures in the compilation accompanying the quick guide.

Reference: Wielstra, B. (2019). Triturus newts. Current Biology 29(4): R110-R111.

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This project has received funding from the European Union’s Horizon 2020 research and innovation programme under the Marie Skłodowska-Curie grant agreement No. 655487.
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Finally! A resolved crested newt phylogeny

The main motivation to conduct the outgoing phase of my Marie Skłodowska-Curie fellowship at the University of California, Los Angeles was to design a ‘sequence capture by target enrichment’ pipeline for Triturus. At UCLA, Evan McCartney-Melstad and Brad Shaffer possess the necessary experience as they use this technique for their system of hybridizing tiger salamanders (genus Ambystoma). Adapting the pipeline to Triturus went superbly and the first paper based on this dataset has now been published in Molecular Phylogenetics and Evolution.

36 A. californiense curled poseCalifornia tiger salamander (Ambystoma californiense) by the Shaffer lab

Triturus is an example of an adaptive radiation: the more slender a species’ body build, the longer its annual aquatic period. This suggests a causal relationship between body build evolution and ecological specialization. While adaptive radiations provide some of the best examples of evolution in action (think Darwin finches and cichlid fishes), the relationships between the species involved are notoriously difficult to decipher. This problem also applies to Triturus: despite several attempts, the phylogeny was hitherto unresolved, hampering our ability to retrace the eco-morphological evolution in Triturus.

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The new Triturus tree (the newt pictures are by Michael Fahrbach)

With our ‘sequence capture by target enrichment’ pipeline we can sequence about 6000 DNA markers for the genus Triturus. To try and resolve the Triturus phylogeny we obtained a dataset including representatives of all marbled and crested newt species. Three distinct tree-building methods each recovered the same Triturus tree. Now we were able to place Triturus’ eco-morphological divergence into an evolutionary context: the red arrows on the Triturus tree above show inferred additions of trunk vertebrae, resulting in more slender and aquatic bodies. Our results show that the genus gradually evolved more elongated and aquatic newts. Nice!

Reference: Wielstra, B., McCartney-Melstad, E., Arntzen, J.W., Butlin, R.K., Shaffer, H.B. (2019). Phylogenomics of the adaptive radiation of Triturus newts supports gradual ecological niche expansion towards an incrementally aquatic lifestyle. Molecular Phylogenetics and Evolution 133: 120-127.

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This project has received funding from the European Union’s Horizon 2020 research and innovation programme under the Marie Skłodowska-Curie grant agreement No. 655487.
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The Wielstra lab at Leiden University and Naturalis

LogosIt’s official, on 1 February I will start my ERC Starting Grant project as a tenure track Assistant Professor at the Institute of Biology at Leiden University in the Netherlands. Meanwhile I will be Honorary Researcher at Naturalis Biodiversity Center. In the context of my project I will hire two PhD students and two postdocs. I expect to start advertising positons halfway 2019, so stay tuned!

ERC - Fahrbach arrest
An unfortunate Triturus embryo, experiencing the arrested development that is associated with the balanced lethal system that is shared by all Triturus species, known as ‘chromosome 1 syndrome’. This picture is by Michael Fahrbach and taken from his recent book (Fahrbach & Gerlach (2018) The genus Triturus).

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The complicated conservation issue of genetic pollution

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Male T. carnifex. Picture by Michael Fahrbach.

Invasive species can threaten native biota by means of competition, predation and infection. A less-known risk is genetic pollution: the (partial) replacement of local genotypes via hybridization. A particular challenge of quantifying invasive hybridization is that the closely related species involved tend to be morphologically similar. As a consequence, conservation action would depend on large scale genotyping. Addressing genetic pollution is a notoriously contentious issue with complications arising at the stage of obtaining and interpreting information.

In the context of my Marie Skłodowska-Curie fellowship I proposed a secondment at the Invasive Alien Species Team. I collaborated with the Dutch NGO RAVON (Reptile, Amphibian, and Fish Conservation Netherlands) as well. We developed a molecular toolkit that allows that allows efficient screening for genetic pollution in a Dutch case of invasive hybridization in crested newts and that could easily be adapted to other cases of invasive hybridization.

Furthermore, we summarize the state of the field and provide guidelines for establishing much-needed policy for the management of genetic pollution. There are clear parallels between the crested newt case and the work of Brad Shaffer, my outgoing host at UCLA, on tiger salamanders in California. Here the native Californian tiger salamander is threatened by invasive hybridization with the introduced barred tiger salamanders. Brad’s experience helped shape the report we wrote, which I hope can help address the complicated conservation issue of genetic pollution.

Reference: Wielstra, B., Arntzen, J.W., Butlin, R.K., van Delft, J.J.C.W., Vrieling, K., Shaffer, H.B. (2018). Molecular toolkit and guidelines for the management of genetic pollution. Reptile, Amphibian and Fish Conservation Netherlands (RAVON), Nijmegen.

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I initiated this work as a Newton International Fellow. This project has received funding from the European Union’s Horizon 2020 research and innovation programme under the Marie Skłodowska-Curie grant agreement No. 655487.
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ERC Starting Grant for the Wielstra lab

I am pleased to announce that I have been awarded an ERC Starting Grant. In this five year research program I will untangle the evolution of balanced lethal systems. Natural selection is supposed to keep lethal alleles (dysfunctional copies of crucial genes) in check. Yet, in a balanced lethal system the frequency of lethal alleles is inflated. Because two forms of a chromosome (let’s call them A and B) carry distinct lethal alleles that are reciprocally compensated for by functional genes on the alternate chromosome form, both chromosome forms – and in effect their linked lethal alleles – are required for survival (see figure below). The inability of natural selection to purge balanced lethal systems appears to defy evolutionary theory.

BLS FINAL

The most illustrious balanced lethal system is observed in Triturus newts and is known as chromosome 1 syndrome. All adult Triturus newts invariably possess two forms of chromosome 1, known as 1A and 1B. Yet, according to the rules of Mendelian inheritance, half of the offspring produced is homozygous (possessing two copies of chromosome 1A or 1B). These homozygotes die halfway embryological development – half of Triturus embryos never hatch! All we currently know about chromosome 1 syndrome derives from classical karyological studies, mainly by Herbert Macgregor and colleagues. Taking advantage of modern genetic techniques, my research team will determine the genomic basis of chromosome 1 syndrome and we will propose a new model on how these bizarre balanced lethal systems could evolve. Keep an eye out on the Wielstra lab* website for all the awesome research that will derive from my ERC Starting Grant.

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*Yes, I am allowed to say Wielstra lab now.
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