Hybrid zones are a main focal point of my work on newts. The genomes of distinct populations are brought together in the genetically admixed offspring that are produced in a hybrid zone. This means that any evolved incompatibility between the genomes, hampering cooperation inside a single organism, are exposed in the hybrid zone. Hence, hybrid zones provide crucial insight into the genetic barriers that underlie the origin of species. In a new ‘quick guide’ published in Current Biology I introduce hybrid zones. Follow this link for temporary free access.
A hybrid zone between the two crested newt species: Triturus ivanbureschi in the west (yellow) and T. anatolicus in the east (blue). The graph depicts the proportion of alleles diagnostic for the eastern species from east to west. A value of 0 corresponds to the western species and a value of 1 to the eastern species. Intermediate frequencies are observed in the hybrid zone (in green on the map). (As usual, thanks to Michael Fahrbach for use of his pictures.)
A banded newt to brighten your day (Michael Fahrbach)
Gemma Palomar and Wieław Babik lead a study, just out in Genome Biology and Evolution, that focuses on the evolution of the immune system in salamanders. Antigen processing genes and major histocompatibility complex class I molecules are considered to co-evolve in jawed vertebrates. We show that, at least in salamanders, the data do not fit all the predictions of this hypothesis.
Reference: Palomar, G., Dudek, K., Wielstra, B., Jockusch, E.L., Vinkler, M., Arntzen, J.W., Ficetola, G.F., Matsunami, M., Waldman, B., Těšický, M., Zieliński, P., Babik, W. (2021). Molecular evolution of antigen processing genes casts doubts on their coevolution with MHC class I genes in salamanders. Genome Biology and Evolution 13(2): evaa259.
The southern crested newt (Triturus karelinii) has a striking distribution pattern. It is endemic to the Pontocaspian region and its range comprises three disconnected sections: a Crimean, a Caucasian and a Caspian one. A previous mtDNA phylogeographical survey suggested that the Caucasian range section was colonized from the Caspian one and that the Crimean range section was subsequently colonized from the Caucasian one.
The three disjunct range sections of Triturus karelinii.
In a paper published in Amphibia-Reptilia we look into this proposed colonization history in a bit more detail. Nuclear DNA shows little genetic differentiation between the three range sections and species distribution modelling suggests that they only recently became isolated. While the Crimean range section was indeed only recently colonized, the Caspian and Caucasian ones have been inhabited long-term. Our findings support extensive gene flow between the currently isolated range sections and little genetic diversity across the southern crested newt range.
Reference: Wielstra, B., Arntzen, J.W. (2021). Genetic homogeneity in a Pontocaspian crested newt suggests recent isolation of its three allopatric range sections. Amphibia-Reptilia 42(2) 179-187.
A male pygmy marbled newt (the bold one on the left) and a male marbled newt (looking rather submissive). Pictures by Michael Fahrbach.
The hybrid zone between the marbled newt (Triturus marmoratus) and the pygmy marbled newt (T. pygmaeus) is probably moving northwards. This movement was initially predicted based on enclaves of the northern species (T. marmoratus), positioned well inside the range of the southern species (T. pygmaeus). In species with limited dispersal capabilities, such as newts, enclaves most likely represent a species holding on locally, while a competitor is gradually overtaking it. If the two species involved also hybridize, then enclaves are a good marker of hybrid zone movement.
The hybrid zone between the pygmy marbled newt (light) and marbled newt (dark) is hypothesized to move northwards, based on marbled newt distribution relics left in the wake of the hybrid zone.
The ranges of crested newts were heavily affected by the cold-warm cycles of the Pleistocene ice age. The Italian crested newt (Triturus carnifex) is particularly interesting in this regards: it occurs in both the Balkan and Italian Peninsulas: two of the main ‘glacial refugia’ of Europe, where species managed to survive the glacial cycles (cold spells) and from which recolonization of temperate Europe was possible during interglacial cycles (warm spells). The Balkan and Italian T. carnifex populations were already known to be genetically quite distinct in their mitochondrial DNA (but that is just a single gene) and based on many markers obtained with my sequence capture protocol (but that included just a few samples).
Using a thorough sampling and my Ion Torrent protocol, we show in a new paper that the Balkan and Italian populations meet at a narrow hybrid zone in northeast Italy, suggesting that they truly have become quite distinct as they were recurrently isolated during glacials. We also confirm deep genetic divergence within the Italian Peninsula, in line with a refugia-within-refugia scenario. An earlier mitochondrial DNA study was pretty much on the mark in terms of the inferred evolutionary history of the Italian crested newt, but our new nuclear DNA results do provide a clearer picture. We show that a quite distinct mtDNA lineage occurs right in the middle of the Balkan-Italian hybrid zone. We hypothesize this might be a ‘ghost lineage’ (a distinct mitochondrial DNA lineage that does not correspond to a noticeable nuclear DNA group) that managed to survive in a local newt population that was repeatedly genetically swamped when the Balkan and Italian populations regained secondary contact. Also, we show that mitochondrial DNA from the northern Italian population has introgressed (flowed) into the southern one. We attribute this to population replacement with hybridization, with south outcompeting north.
Reference: Wielstra, B., Salvi, D., Canestrelli, D. (2021). Genetic divergence across glacial refugia despite interglacial gene flow in a crested newt. Evolutionary Biology 48(1): 17-26.
Loïs Rancilhac heads a nice salamander study that is just out in Molecular Phylogenetics and Evolution. Click here for temporary free access. Lots of collaborators contributed a ton of genetic data for almost all the genera that make up Salamandridae – the salamander family that includes our beloved newts. This vast dataset allowed Loïs to recover a highly supported phylogenetic tree, with some cool improvements over previous attempts. I just love that the flamboyant banded and alpine newts are each other’s closest relatives, they truly are a perfect match! However, for me the most important insight is that the smooth newts and the crested/marbled newts are recovered as sister lineages. The new Salamandridae tree will allow my lab to conduct more targeted comparative genomics in our quest to infer the evolution of the balanced lethal system in Triturus. Reference: Rancilhac, L., Irisarri, I., Angelini, C., Arntzen, J.W., Babik, W., Bossuyt, F., M., Künzel, S., Lüddecke, T., Pasmans, F., Sanchez, E., Weisrock, D., Veith, M., Wielstra, B., Steinfartz, S., Hofreiter, M., Philippe, H., Vences, M. (2021). Phylotranscriptomic evidence for pervasive ancient hybridization among Old World salamanders. Molecular Phylogenetics and Evolution 155: 106967.
My first MSc student Willem Meilink has been awarded an NWO Promotiebeurs voor leraren. This will allow him to pursue his PhD in my lab, while continuing his job as a high school teacher. We know that in the balanced lethal system in Triturus two distinct forms of chromosome 1 are required for survival. Willem will use a variety of genetic techniques to test if the irrevocable loss of 50% of reproductive output, concerning the unfortunate offspring that inherit the same chromosome form from both their parents, can be explained by crucial genes simply being absent on one of the two chromosome forms.
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.
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.)
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.
Distribution of distinct mitochondrial (left) and nuclear DNA groups in T. macedonicus.