Understanding how species form, and the factors that contribute to reproductive isolation has been a long-standing goal of evolutionary biology. Cryptic radiations can provide insight into these questions. Dufresnes and colleagues investigate these questions in a cryptic radiation of Eurasian spadefoot toads (Genus Pelobates). They find a correlation between the amount of time spent in allopatry and the level of reproductive isolation between lineages experiencing secondary contact. Get a behind-the-scenes look at the research below with first author Christophe Dufresnes.
What led to your interest in this topic / what was the motivation for this study?
This study was part of my post-doc efforts to compare geographic and genomic patterns of introgression across hybrid zones from several amphibian radiations, in order to understand the pace and the genetic mechanisms of allopatric speciation. For Pelobates, we originally intended to focus only on the P. fuscus/vespertinus hybrid zone in Ukraine/Russia, but inadvertently discovered that our outgroup taxon (the traditionally-recognized P. cf. syriacus) consisted of a cryptic diversification involving several phylogeographic transitions. Instead of one contact, it turned out we could study as many as six within a single radiation. We thus seized that rare opportunity to understand the relationship between genetic divergence and reproductive isolation under natural settings, which had only been attempted in a handful of systems so far.
What difficulties did you run into along the way?
Not much actually, except perhaps time constrains and taxonomic issues. Because we were willing to describe the new Pelobates species/subspecies discovered and use the appropriate names in our Mol. Ecol. article for clarity, we had to synchronize the peer-reviewing and publication of an accompanying paper (ZooKeys 859: 131–158). This was successful thanks to the support of the two journals and both papers were released just a few hours apart. The scientific stages per se went remarkably smoothly. Colleagues from many countries were enthusiastic to send us samples, the RAD-seq wet lab and bioinformatics performed admirably, and data analysis was straightforward. At the end, it took only about a year and a half from project initiation to publication.
What is the biggest or most surprising finding from this study?
We were astonished by the strong divergence (>5My) between the Asian and European populations of P. cf. syriacus, and their lack of interbreeding despite parapatric (and perhaps even sympatric?) distributions near the Bosphorus. While they clearly represent two different species (coined P. balcanicus and P. syriacus), no phenotypic differences have been reported (despite several morphometric surveys), so this was not suspected. But more globally, our big finding is the very neat link between genetic divergence and admixture across phylogeographic transitions. It was beyond expectations since hybridization at contact zones often depends on local factors (dispersal constraints, etc.), which blurs the link. Such a clear relationship supports the Darwinian view of a gradual and dynamic speciation continuum, and extends it to cryptic radiations of eco-morphologically similar species.
Moving forward, what are the next steps for this research?
We have several major follow-ups ongoing. Our Pelobates speciation genomic framework is now being implemented into multi-system comparative analyses aiming to understand how the genetic architecture of reproductive isolation evolves as speciation progresses, by re-analyzing transitions at the locus scale. In parallel, we are characterizing the homomorphic sex chromosomes of Pelobates to gauge their importance in hybrid incompatibilities. Following this research, our co-author Ilias Strachinis has now just started a PhD on Pelobates from the Balkan Peninsula to study the diversity and distribution of the new lineages unraveled. It should provide significant insights for their biogeography and conservation, notably of the mysterious Peloponnese endemic P. balcanicus chloeae.
What would your message be for students about to start their first research projects in this topic?
Even in supposedly well-studied biogeographic regions like the Western Palearctic, do not take the established phylogenies, species delimitations and taxonomies for granted! Significant diversifications may have been overlooked, especially since previous work relied mostly on mitochondrial and poorly-informative nuclear markers, which can be deceptive to disentangle among closely-related lineages. Nowadays, RAD-seq provides affordable, powerful and straightforward resources to address many questions with a combination of population genetic and phylogenetic analyses, so it appears a tool of choice to study the phylogeography of speciation in many taxonomic groups.
What have you learned about science over the course of this project?
That unexpected results are worth exploiting and may lead to fascinating scientific discoveries. An unthinkable amount of biodiversity still lies unnoticed right under our noses. Moreover, our study was only possible (especially in such an efficient timeframe) thanks to a great collective effort bringing together renowned teams of herpetological researchers. From my personal perspective, this human aspect emphasizes how science is best appreciated collaboratively rather than through competitive emulation, and I look forward to reiterate the experience.
Describe the significance of this research for the general scientific community in one sentence.
In cryptic diversifications, whether the continuous nature of speciation leads to discrete, reproductively-isolated entities is mostly dependent on the time they spent in allopatry.
Describe the significance of this research for your scientific community in one sentence.
Our study provides empirical evidence within a single radiation that speciation is a dynamic and reversible process where phylogeographic lineages can merge together upon secondary contact, unless a threshold of evolutionary divergence is reached (>3My in amphibians), in which case they can quickly build up reproductive isolation and become incipient species.
Read the full article here: Dufresnes C, Strachinis I, Suriadna N, et al. Phylogeography of a cryptic speciation continuum in Eurasian spadefoot toads (Pelobates). Mol Ecol. 2019;28:3257–3270. https://doi.org/10.1111/mec.15133