ECR Feature: Rowan Schley on the diversification of pantropical trees

Rowan J. Schley is a postdoc at the University of Exeter and the University of Edinburgh. He is particularly interested in using genomic approaches to study diversity in tropical ecosystems. Rowan shares his recent work on the diversification of the pantropical tree genus, Pterocarpus, and the relative roles of biome-switching and long-distance dispersal.

Rowan with Wallace’s Flying Frog (Rhacophorus nigropalmatus).

Personal links. ResearchGate | Twitter | Website | ORCID | GoogleScholar

Institute. University of Exeter & University of Edinburgh

Academic life stage. Postdoc

Major research themes. I am particularly interested in asking questions about speciation, hybridisation, diversification, biogeography and genome evolution to understand the superlative diversity of the tropics.

Current study system. I work on tropical trees, which as a whole are relatively understudied despite their incredible diversity. As an example, one hectare of the Ecuadorian Amazon may contain more tree species than the entirety of Europe (>600 species). It is critically important to understand how this diversity was assembled, both to further our understanding of how species diversify and because tree species are the basis of many tropical ecosystems. In particular, the genus, Pterocarpus (Fabaceae/Leguminosae), is an excellent study system for understanding tropical tree diversification and biogeography because it is ecologically diverse, exhibits multiple dispersal phenotypes and is found across the tropics.

Recent paper in JBI. Schley R. J., Qin, M., Vatanparast, M., Malakasi, P., de la Estrella, M., Lewis G. P., Klitgård, B. (2022) ‘Pantropical diversification of padauk trees and relatives was influenced by biome-switching and long-distance dispersal’. Journal of Biogeography.

(left) Pterocarpus rohrii sensu lato; (right) Pterocarpus angolensis © Gwilym P. Lewis

Main motivation. Many plant groups are found pantropically, and there has been much debate about how such distributions arise. The prevailing narrative was that of vicariance, where the splitting of continental landmasses led to isolation of populations on different continents, and therefore, different evolutionary trajectories. However, time-calibrated molecular phylogenies have shifted this paradigm for many groups, and now, dispersal is believed to have led to the pantropical distributions of many tropical tree species. This is because the estimated divergences between many plant groups post-date continental splitting, and because long-distance dispersal is well known in plants. The macroevolutionary consequences of dispersal can be influenced by phenotypes which promote dispersal at ecological scales, as shown by work on the Podocarpaceae (Klaus & Matzke, 2020) and Annonaceae (Onstein et al., 2019). This work showed that dispersal traits influenced the distribution, diversity and ecology of these plant groups, inspiring us to ask similar questions in Pterocarpus.

Key methodologies. To test how seed dispersal phenotypes influenced Pterocarpus’ biogeographical history we first inferred a ‘dated’ phylogenetic tree. This helped us understand the evolutionary relationships between Pterocarpus species in a temporal context. We built this tree based on DNA sequence datacollected from RBG Kew’s herbarium, and used fossils to calibrate the tree to understand when speciation events happened within Pterocarpus. We then performed biogeographical analyses to test whether the dispersal phenotypes of Pterocarpus species influenced the evolutionary history and geographical distribution of the group. For example, dispersal phenotypes which facilitate long-distance dispersal by water (e.g., floating fruits) may lead to very broad species distributions, whereas other phenotypes may restrict species distributions. We tested the influence of these traits on geographical range evolution as well as on adaptation to different biomes, and we assessed how many shifts between biomes occurred across Pterocarpus’ evolutionary history.

Challenges. A major challenge was to find a method which allowed us to ‘date’ our phylogenetic tree from the vast amount of next-generation sequencing data we collected. This was pretty hard because most of the methods which we can use to date phylogenetic trees were designed to use much smaller datasets (e.g., only one or a few genes), but we used more than 300 genes in our phylogenetic analyses. Because of this, we decided to use a method which was designed for dating phylogenetic trees built with entire genomes (i.e., even more data than we used in this study) called ‘MCMCtree’ (Yang et al., 2007). We were also particularly fortunate in the case of Pterocarpus, because there has been much genetic work done on closely related groups, and so we could leverage existing, smaller datasets to do a corroborative analysis using older methods, and ensure that our MCMCtree analyses were accurate.

Major results. We found that there were two evolutionary lineages within Pterocarpus – one of which diversified in the Neotropics, and the other in the Palaeotropics. These groups diverged during the Miocene (~12 Ma), and most of the species diversification in Pterocarpus occurred during this epoch. Interestingly, we found that seed dispersal phenotypes had little significant effect on contemporary distributions, but that dispersal did have a significant effect. This suggests that random, rare ‘sweepstakes dispersal’ influenced Pterocarpus’ distribution. It was also apparent that biome-switching mostly occurred into rainforests and savannas. These environments are prone to disturbance and so experience high turnover of plant species, facilitating colonisation from other biomes. Biome switching was also likely promoted by Miocene climate change, such as the aridification of Africa. Overall, our results suggest that rare long-distance dispersal, coupled with climate change and speciation in different biomes, likely explain the wide distributions of many pantropical tree genera.

Next steps. I’d love to investigate the ‘weirdest’ Pterocarpus of all – P. dubius. This looks very different to other Pterocarpus species, its phylogenetic position is different when inferred with plastid vs nuclear genes, and it was previously circumscribed in a separate genus as Etaballia dubia. It would be great to unpick these peculiarities using plastid genomes and by investigating phylogenetic incongruence, because it seems that this odd placement may result from chloroplast capture following ancient hybridisation. In addition, Klitgård et al. will be circumscribing new taxa within the P. rohrii species complex and publishing IUCN red list assessments for threatened Pterocarpus species.

Rio Tiputini in Yasuní National park, Ecuador.

If you could study any organism on Earth, what would it be? To study the extraordinary diversity of any tropical taxon is a dream come true. I have a particular love for working on tropical trees and understanding the evolution of diversity through that lens. That said, as a naturalist I am interested in a broad range of questions across the tree of life, and I have also been lucky enough to work on orchids and cichlid fishes. I would love the opportunity to further understand speciation in diverse coral reef taxa (e.g., Acanthurid fishes, Scarid fishes, Scleractinian corals), phenotypically diverse and strange plant groups (e.g., mangroves and Nepenthes pitcher plants) or in an island radiation like Scalesia. That’s the great thing about the tropics – there is so much to study, not to mention so much to conserve!

Anything else you would like to share? Pterocarpus species are valuable timber trees under immense pressure from logging. They are known as ‘rosewoods’ and ‘bloodwoods’, among many other common names. These names refer to the sought-after red colour of their wood, and to the red sap that is exuded when their trunks are cut. There is a short BBC film called ‘Trees that bleed’ that documents the poaching of rosewoodtrees in West Africa – it is really worth a watch!

Published by jbiogeography

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