Chapter 7 Historical Biogeography of Trigonostemon and Dimorphocalyx (Euphorbiaceae)
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Cover Page The handle http://hdl.handle.net/1887/78385 holds various files of this Leiden University dissertation. Author: Yu, R. Title: A monograph of the plant genus Trigonostemon Blume Issue Date: 2019-09-18 Chapter 7 Historical biogeography of Trigonostemon and Dimorphocalyx (Euphorbiaceae) R.-Y. Yu1, P.C. van Welzen1,2 1 Naturalis Biodiversity Center, P.O. Box 9517, 2300 RA Leiden, The Netherlands; corresponding author e-mail: [email protected]. 2 Institute of Biology Leiden, Leiden University, P.O. Box 9505, 2300 RA Leiden, The Netherlands. Accepted for publication in Botanical Journal of the Linnean Society. Abstract Trigonostemon and Dimorphocalyx are two morphologically similar genera in tropical Asia. We estimated their divergence times through a Bayesian clock analysis and reconstructed the historical biogeography using a likelihood analysis under the Dispersal-Extinction- Cladogenesis (DEC) model and a Statistical Dispersal-Vicariance analysis (S-DIVA). We have found that the two genera differ in their historical biogeography: Trigonostemon originated on SE Asian mainland, but one section dispersed to the Malay Peninsula and Borneo, where rapid speciation events occurred during the Pleistocene, whereas Dimorphocalyx originated and extended to its current distribution from Borneo. The dispersal routes of both genera are well supported by the tectonic history and are comparable to the conclusions in previous case studies. Long-distance dispersals across Wallace’s line are of particular interest in biogeography. Our data support the hypothesis that the Philippines is the most often used stepping stone to cross Wallace’s line. Furthermore, we consider that the frequent change of sea levels during the Pleistocene propelled the diversification of sect. Trigonostemon in Borneo and the Malay Peninsula. Additional key words dated phylogeny Dispersal-Extinction-Cladogenesis Pleistocene Quaternary speciation Statistical Dispersal-Vicariance analysis Tritaxis Wallace’s line 222 A monograph of the plant genus Trigonostemon Blume ― Chapter 7 INTRODUCTION Southeast Asia ranges from the Alpine-Himalayan mountain belt south-eastwards to a massive collection of islands across the equator between 95°E and 140°E (Hall 2009), which is commonly known as the Malay Archipelago (Wallace 1869) or Malesia (Zollinger 1857, van Steenis 1948b, 1950, Raes & van Welzen 2009). The SE Asian mainland (Sundaland continental core) and W Malesia (Sunda Shelf) were already attached in the Mesozoic, while the much younger E Malesia was formed by continental fragments that mainly rifted from the Australian part of the splitting Gondwana since the Eocene (Hall 2009). Malesia represents a rich biodiversity. This area harbours a total of approximately 42,000 plant species (Roos 1993), 70% of which is endemic (van Welzen 2005). Plant dispersals in this area have facilitated the floristic exchange between the continents of Asia and Australia (e.g., Sirichamorn et al. 2014, Crayn et al. 2015, Buerki et al. 2016, Hauenschild et al. 2018). The Malesian islands form a more or less linear pathway of stepping stones for plant dispersals (van Welzen et al. 2005). However, significant water barriers (e.g., Makassar Strait and Banda Sea) exist in Wallacea, the central part of Malesia (most of Java, Lesser Sunda Islands, Sulawesi, Philippines, Moluccas; van Welzen et al. 2011), and the stepping stones (e.g., Moluccas) only emerged above sea level since 10 Ma (Hall 2009). At present, the climate in Malesia is everwet in the west (Sunda Shelf: Malay Peninsula, Sumatra, Borneo, SW Java) and the east (Sahul Shelf: New Guinea); while in most parts of Wallacea, there is a yearly dry monsoon. Because climate (especially the yearly precipitation and temperature) is a very important regulator of species occurrences (Boucher-Lalonde et al. 2012, Araújo et al. 2013), it may have overruled the historical dispersal pathways. Relatively few studies attempted to reveal the exact migration route for plants across Wallace’s line. The most often used route between east and west is via the Philippines (Nauheimer et al. 2012, Denduangboripant et al. 2001; or vice versa, e.g., Thomas et al. 2012, Jønsson et al. 2010 [bird]), but other routes are also found (e.g., via Borneo and Sulawesi, Grudinski et al. 2014, Thomas et al. 2012, Evans et al. 2003 [frog]; or between Java and the Lesser Sunda Islands (e.g., proposed route by Su & Saunders 2009, present in Figure 7; Chantarasuwan et al. 2016). General migration patterns can only be discovered when a sufficient number of case studies are performed. In this paper, we add an example of the historical biogeography of Trigonostemon Blume and Dimorphocalyx Thwaites, by investigating their geographical origin and dispersal routes, particularly those across Wallace’s line, and comparing these to the previous studies. Trigonostemon and Dimorphocalyx are two closely related genera in the Euphorbiaceae, comprising about 60 (Yu & van Welzen 2018) and 13 species (van Welzen & van Oostrum 2015), respectively. They are both classified in tribe Codiaeae (Pax) Hutch. of the subfamily Crontonoideae (Webster 2014) and are morphologically similar in the connate stamens and the presence of petals in both staminate and pistillate flowers. Both genera have more or less the same SE Asian distribution, ranging from India to S China, throughout continental SE Asia and Malesia to NE Australia, and for Trigonostemon, even to the W Pacific (Govaerts et al. 2000, van Welzen & van Oostrum 2015, Yu & van Welzen 2018). The main differences between the two genera lie in the sexual system and the number and arrangement of the stamens: Trigonostemon species are monoecious shrubs or trees with only one whorl of 3 or 5 connate stamens, whereas most Dimorphocalyx species are dioecious trees and often have 10–15 stamens arranged in two or three whorls, of which only the inner whorls are partly connate. Dimorphocalyx (including Tritaxis Baill., the two genera will be merged as one under the name Tritaxis, see Yu et al. 2019b) was once treated as a Historical biogeography of Trigonostemon and Dimorphocalyx (Euphorbiaceae) 223 section of Trigonostemon sensu lato (Müller Argoviensis 1865, 1866, Beddome 1873), but our recent molecular phylogenetic study (Yu et al. 2019b) demonstrated that Trigonostemon and Dimorphocalyx were two separate monophyletic groups within the C2 clade of the Crotonoideae in the backbone phylogeny of the Euphorbiaceae (Wurdack et al. 2005). Therefore, they are treated as separate genera as in their original generic circumscriptions (Blume 1825; Thwaites 1861). West Malesia is the distribution centre for Trigonostemon. Within the genus, sect. Trigonostemon contains most species (about 50% species of the genus), and these species are likely the result of a series of recent speciation events (which is reflected in the short branches in the phylogram; Yu et al. 2019b). Particular factors in the tectonic and climatological history of this region may have promoted these events. Therefore, another aim of this paper is to provide an explanation for the rapid diversification in sect.Trigonostemon from a historical biogeographic angle. MATERIAL AND METHODS Divergence time estimation The same specimens and molecular markers as in Yu et al. (2019b) were used to estimate the divergence times of Trigonostemon and Dimorphocalyx. A total of 41 species (out of about 60) and 6 varieties of Trigonostemon and 11 species (out of about 13) of Dimorphocalyx were included. The sampling covers the whole distribution of the two genera, except Trigonostemon sect. Pycnanthera Benth. (T. diplopetalus Thwaites and T. nemoralis Thwaites from S India and Sri Lanka) is missing. The molecular dating analysis was performed on matrix 1 of Yu et al. (2019b) using Bayesian Inference via BEAST v.1.10.1 (Suchard et al. 2018). Five molecular markers were used: the nuclear ITS and the chloroplast trnK intron, trnT-L, trnL-F and rbcL. In the data matrix, sequences of all these five markers obtained forTrigonostemon and Dimorphocalyx are aligned with the dataset of the Euphorbiaceae (based on trnL-F and rbcL) of Wurdack et al. (2005). The substitution model of the nuclear and chloroplast markers was selected based on the lowest Akaike Information Criterion scores detected by Modeltest-NG v.0.1.5, which showed the same model for the nuclear and chloroplast markers. The input file was created by BEAUti v.1.10.1 (part of the BEAST package) with the following settings: the dataset contained five partitions (each partition for a single molecular marker); the substitution rates were calculated under the General Time Reversal (GTR) model with a discrete Gamma distribution (+Γ, 4 categories) of evolutionary rates among the sites and a certain number of invariable sites (+I); the divergence times were estimated using the uncorrelated relaxed clock model (Drummond et al. 2006) with a lognormal distribution of rates; the Yule process was selected as tree prior (Yule 1925; Gernhard 2008), and a random starting tree was used; a total of 1.25×108 generations of Markov Monte Carlo chains were performed and trees were sampled every 1×104 generations. The results were examined for Effective Sampling Size (ESS > 200) using Tracer v.1.7.1 (Rambaut et al. 2018). The first 20% of the sampled trees was discarded as burn-in, and the Maximum Clade Credibility (MCC) tree was found using TreeAnnotator v.1.7.5 (part of the BEAST package). Three calibration points were used to estimate the divergence times. As no distribution type of the fossil ages was known, the calibration priors were coded as uniform distributions (Ho 2007) 224 A monograph of the plant genus Trigonostemon Blume ― Chapter 7 with an upper and lower boundary (presented between brackets). Because no fossil records directly identified asTrigonostemon or Dimorphocalyx could be found, we used two secondary calibrations and a fossil record of Hippomaneoidea warmanensis Crepet & Daghlian instead: 1. The crown node of Euphorbiaceae s.s. (Figure 41; the node joining Acalyphoideae s.s. and Euphorbioideae) was assigned a mean age of 89.9 Ma (97.4–81.2 Ma).