Alexandre Réjaud is an evolutionary biologist finishing up his Ph.D. at the Université Paul Sabatier in France. He combines distribution and sequencing data to understand the mechanisms leading to amphibians’ diversification. Alexandre shares his recent work on the historical biogeography of Amazonian frogs.
Personal links. ResearchGate
Institute. Laboratoire Evolution et Diversité Biologique, Université Paul Sabatier, Toulouse, France
Academic life stage. Last stage of his PhD (defense planned for January 2021!)
Major research themes. Amazonia, Amphibians, Biodiversity, Biogeography, Neotropics, Phylogenetics
Current study system. During my PhD, I have been studying Amazonian amphibians. There are around 600 described species and probably many more that are undescribed. What I find the most interesting about this system is its incredible diversity, where species with very different ecologies can be found in close proximity to each other. For example, ranging from the small and colourful Ranitomeya that live and breed in Bromeliads to the flat Surinam toads (Pipa) that inhabit aquatic ecosystems and carry their eggs under their skin. My research aims at identifying where and when this Amazonian amphibian diversity was formed. To this end, I compared the historical biogeography of six clades of frogs that diversified throughout the Neogene (~23 Mya) within Amazonia.
Recent paper in JBI. Réjaud, A., Rodrigues, M. T., Crawford, A. J., Castroviejo‐Fisher, S., Jaramillo, A. F., Chaparro, J. C., … & Fouquet, A. (2020). Historical biogeography identifies a possible role of Miocene wetlands in the diversification of the Amazonian rocket frogs (Aromobatidae: Allobates). Journal of Biogeography, 47(11), 2472-2482 Accessible here
Motivation behind this paper. Within the Neotropics, Amazonia has been identified as the main source of diversification, meaning that Amazonian lineages dispersed towards the other Neotropical areas and strongly contributed to increasing their diversity. However, we still know little about biogeography and diversification within this area. This is largely because Amazonia is so vast (more than six million km²) and challenging to access that it is difficult to sample thoroughly. Benefiting from the intensive sampling efforts deployed over the past 30 years and from international collaborations, we are only starting to accumulate datasets that span the entire Amazonia distribution. The frog genus, Allobates is one of the first Amazonian frog genera for which we managed to achieve sufficiently extensive geographical and taxonomical sampling, allowing us to reconstruct its historical biogeography. Because Amazonian amphibians are rapidly declining and have much undescribed diversity, the Allobates frogs provide a useful system to study the generation and maintenance of diversity in the Neotropics.
Allobates femoralis, a species broadly distributed across Amazonia (Picture credit: Antoine Fouquet)
Key methodologies. Species diversity is still unknown in Allobates, with discoveries being made frequently (e.g., 14 new species described in the last decade). This undescribed diversity raises the issue of how to deal with species boundaries in an understudied system. We partly addressed this issue by performing a molecular species delimitation analysis. In total, we gathered 932 georeferenced 16S sequences for the whole genus across Amazonia and other Neotropical regions (Atlantic forest, dry diagonal, lesser Antilles, northern Andes, trans-Andean forests); accounting for 41 out of the 55 described species and we identified 50 operational taxonomic units (OTUs). We sequenced complete mitogenomes for 32 out of 50 OTUs and combined it with available GenBank resources, allowing us to reconstruct a time-calibrated phylogeny accounting for all the OTUs found in Allobates. This extensive sampling allowed us to perform tests about the diversification of this group over the entire Amazon, answering questions such as: Did the Allobates western Amazonian diversity originate mainly in situ or through repeated dispersal events? Was the diversification constant through time or can we detect key periods where diversification was greatest?
Any challenges you and your co-authors faced along the way? As briefly explained in the above section, the main challenge when studying tropical diversity is the lack of information on species boundaries and geographic distribution, impacting a more basic metric such as the number of species within a complex. Relying only on our current perception of diversity greatly increases the risk of not sampling a species, which can affect biogeographic inferences. We partially solved this issue by using molecular species delimitation. We suspected that the risks of not sampling species were particularly high within Allobates. Indeed, most species with large distributions were split into several OTUs, with up to four for Allobates olfersioides and A. tapajos. Overall, OTUs were often restricted to one of the three major Amazonian areas we used in the study: Western Amazonia, the Brazilian Shield and the Guiana Shield. Surprisingly, A. femoralis, which was the only species with a pan-Amazonian distribution (i.e., occurring in the three Amazonian areas simultaneously), was delimited as a single OTU.
Major results. We identified Western Amazonia as the principal source of diversification in Allobates. This part of Amazonia currently harbors the most Allobates species, which mostly originated in situ as a result of an intense diversification phase that occurred between 14 and 10 million years ago (Mya). Although it is difficult to determine the mechanisms of this diversification with confidence, this period is concomitant with the Pebas mega-wetland system in Western Amazonia, followed by an expansion of terra firme habitats (i.e., unflooded forest). Furthermore, distribution patterns of closely related species suggest that modern Amazonian river courses shaped species ranges and possibly promoted allopatric speciation over the past 10 million years. So far, most Amazonian biogeography case studies focused on clades that diversified during more recent periods (Plio-Pleistocene), and our study is among the first to investigate early Neogene processes.
Picture of a Bothrops taeniatus spotted during fieldwork around Tarapoto, Peru.
Next steps in this research. Next, we intend to investigate broader and more general patterns of diversification in Amazonia using a comparative biogeography approach by combining the analysis of several species complexes. This approach will determine how Amazonian diversification and dispersal rates varied across groups and through time, emphasizing the effect of ecological adaptiveness. The processes of Amazonian diversification are complex, and this ecological variation should be accounted for to elaborate refined biogeographical hypotheses. However, ecology is rarely considered in comparative studies because of the lack of reliable data at the species level in the tropics, which is not surprising when the species are not even described.
If you could study any organisms on Earth, what would it be? Tough question and many answers come to my mind! If I had to pick one, I would choose the evolution of reproductive modes in the European salamanders (Caudata: Salamandridae). In this family, while most species are semiaquatic oviparous, some salamanders have switched to ovoviviparity or viviparity, which reduces water dependance for reproduction. It gets even crazier in the fire salamander (Salamandra salamandra), in which there are viviparous and ovoviviparous populations!
Anything else to add? I had my first and only Amazonian field trip experience a few months after beginning my PhD. The herping trip lasted about two weeks in central Peru (San Martin province). For those who don’t know what a herping session consists of, it is basically wandering around during a few hours past nightfall with a headlamp, sometimes with no one in sight − which can be disturbing for your first time in such an unknown environment. I won’t lie, I was not confident initially (especially when I slipped and fell several meters on top of a cordillera), but what a life-changing experience! We were lucky enough to spot rare and cool species such as an Atelopus pulcher or a few Bothrops taeniatus (see picture). However, going down the Cordillera Escalera to the Amazonian lowlands, the displeasing sight of endless palm tree fields reminded us how fragile and how fast these fascinating ecosystems are vanishing.