Mariana Vasconcellos is a postdoc at the Universidade Federal do Rio de Janeiro. She is interested in the environmental correlates of diversification and climatic adaptation, with a large focus on amphibians and reptiles. Mariana shares her recent work on unravelling the drivers of diversification in South America’s pajama frog group, revealing some surprising phylogenetic relationships and the effects of past climates.
Mariana Vasconcellos in her natural habitat. Photo credit: Mario Van Gastel.
Institution. Universidade Federal do Rio de Janeiro
Academic life stage. Postdoc
Major research themes. Diversification and climate adaptation of Neotropical species and populations to environmental changes. My research mainly focuses on amphibians and reptiles, but occasionally plants as well.
Current study systems. How have populations and species responded to environmental changes in the past, and based on that, how will they likely respond to ongoing climate change? These questions are at the core of my research. To answer them, I first turned to the Cerrado savanna of central Brazil, where I studied the pajama treefrogs, an interesting group of which most species are endemic to this savanna. They got their name because of the stripes along their bodies, which make them look like they are wearing striped pajamas. Our goal for this study was to understand the factors that explain the occurrence of this species group in very different habitats, such as grasslands, savannas, and rainforest regions. I am now addressing signatures of environmental changes in other herps and plants in the Brazilian Atlantic Forest as well.
The Cerrado landscape with gallery forests meandering in the background, following small rivers, where one pajama treefrog species is found.
Recent JBI paper. Vasconcellos, M. M., Colli, G. R., Cannatella, D. C. (2020). Paleotemperatures and recurrent habitat shifts drive diversification of treefrogs across distinct biodiversity hotspots in sub-Amazonian South America” Journal of Biogeography. 48:305-320 https://doi.org/10.1111/jbi.13997
Motivation behind this paper. Pajama treefrog species occur in very contrasting habitats of open/dry or forest/humid ecoregions across South America, but the group is most diverse in the Cerrado savanna. This is quite curious, as treefrogs are mostly associated with humid habitats and are usually more diverse in forest regions. When we started our research, their phylogenetic relationships and diversification patterns were relatively unknown. Therefore, we thought that a good place to start would be to resolve the phylogenetic relationships in this group to reconstruct their biogeographic history and understand when and how they diversified in the Cerrado savanna. We noticed a very unusual biogeographic pattern of closely related species or clades distributed in regions of contrasting vegetation and climate. Based on phylogenetic niche conservatism, it is expected that closely related species inhabit similar habitats, so we decided to test the effect of eco-similarity across areas in the dispersal of these frogs over time. We also evaluated the contribution of past environmental changes, such as paleotemperatures, in their temporal pattern of diversification.
A male pajama treefrog (Boana buriti) calling to attract females. Photo credit: Gabriel Horta.
Key methodologies. To better understand how the species have dispersed and diversified across different regions, we reconstructed ancestral areas along the phylogeny using the dispersal-extinction-cladogenesis (DEC) model. We evaluated six alternative scenarios of dispersal across the Neotropics using a different dispersal rate matrix for each of our six hypotheses. For example, we tested scenarios for the ‘center-of-origin’ in either forest or open areas, and we also tested whether dispersal was more frequent across ecologically similar areas vs. simply adjacent areas (regardless of ecological similarity). In addition, using species diversification models, we identified the factors that contributed most to the variation in speciation and extinction rates over time, taking into account the effect of past climate, time and species diversity (=ecological limits) upon them.
Major results. The evolution of the gladiator treefrogs, including the pajama treefrogs, is the product of repeated dispersal events between open/dry and forest/humid regions across South America. We did not find support for a single region acting as a ‘center-of-origin’ for the group. Instead, we inferred recurrent range shifts across adjacent dissimilar regions. We also uncovered a strong influence of past climates in their diversification, with speciation rates being temperature-dependent and showing higher rates of speciation during warmer climates. This highlights the very dynamic history of some Neotropical organisms, which might be triggered by environmental changes. These changes have promoted frequent habitat shifts among contrasting adjacent habitats and impacted the rate at which new species arise, possibly contributing to the great species diversity found in the Neotropics.
Unexpected challenges and outcomes. The first main challenge of our research was to sample all the pajama treefrog species. Since most of these species have very restricted distributions, we made extensive field trips across central and southeast Brazil to collect their DNA. These frogs mainly occupy highlands and plateaus with beautiful landscapes and astonishing waterfalls, so the search for specimens was very enjoyable indeed. But after sequencing our samples, our first results turned out to be very surprising: the pajama treefrogs are in fact not a single clade, but instead, three independent clades nested in the larger Boana pulchella group of gladiator treefrogs. This unexpected result meant we had to shift our investigation to a larger group comprising a much larger geographic area across South America (including the Andes, Araucaria Forest, Pampas grassland, in addition to the Cerrado and Atlantic Forest). This change also allowed us to explore an even more complex biogeographic history for the group and to focus on the temporal diversification patterns for the larger Boana pulchella group.
A gladiator treefrog (Boana ericae), part of the larger Boana pulchella group. Photo credit: Guilherme Santoro.
Next steps. The next step in this research is to better understand species limits and the speciation process in this group, of which we have barely scratched the surface, by compiling a more complete current phylogeny for the group. We focused most of our sampling efforts on filling gaps of species sampling. Therefore, our limited population sampling for many species prevented us from confidently addressing species limits in clades where the taxonomy has historically been complicated. With the help of new collaborators, I am now increasing our sampling of the main pajama treefrog clade to include more populations and individuals in a phylogeographic study. Our new goal is to investigate the factors that promote the frequent movement of species between the Cerrado savanna and the Brazilian Atlantic Forest, with a repetitive pattern of species interchange over time.
If you could study any organism on Earth, what would it be? Definitely frogs! They are extremely diverse in tropical regions, and they come in all kinds of sizes, shapes, and colors. Their great diversity of reproductive modes and behavioral mating strategies make them especially interesting organisms to study, not to mention the many habitat specializations for arboreal, fossorial, terrestrial, and aquatic environments that evolved multiple times across different continents. They are great study organisms to understand diversity patterns in the tropics, and to address many interesting biogeographic questions. But to me, one of the best features of frogs from a researcher perspective is how easy they are to observe and collect. If you visit any preserved area in the tropics at night, provided there is surface water (running or still), you can observe several species, within arm’s reach, without the need for complicated gadgets or skills to observe and collect them.
(left) Close-up of the preferred microhabitat for a pajama treefrog species. (right) Waterfalls were frequently visited places to collect treefrogs.
Anything else to add? The current featured research in the Journal of Biogeography is the result of the first chapter of my dissertation before my research started shifting to population genomics. This manuscript was first rejected by another journal (which was indeed not a good fit), and I did not find the time to address the criticisms I received while working on my following chapters and papers before graduation. With the start of a new postdoc position, diving into a new study system with different questions, this paper was sadly neglected for quite some time until I decided to take the time needed to develop it into a new submission. I addressed the criticisms we received, reanalyzed part of the results, and developed a more interesting framework to test the diversification pattern. I had to do quite a lot of re-writing and editing as well. But it was worth it! During the peer review process, the manuscript got even stronger, with a more solid conclusion. I recognize that many of us have papers that just need a final push to be submitted. I hope this can serve as an inspiration to those lacking motivation to work on that dusty, abandoned manuscript again.