Contemporaneous and Recent Radiations of the Worldts
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Contemporaneous and recent radiations of the world’s major succulent plant lineages Mónica Arakakia, Pascal-Antoine Christina, Reto Nyffelerb, Anita Lendelb, Urs Egglic, R. Matthew Ogburna, Elizabeth Spriggsa, Michael J. Moored, and Erika J. Edwardsa,1 aDepartment of Ecology and Evolutionary Biology, Brown University, Providence, RI 02912; bInstitut für Systematische Botanik, Universität Zürich, CH-8008 Zürich, Switzerland; cSukkulenten-Sammlung Zürich, Mythenquai 88, CH-8002 Zürich, Switzerland; and dDepartment of Biology, Oberlin College, Oberlin, OH 44074 Edited* by Peter Crane, Yale School of Forestry and Environmental Studies, New Haven, CT, and approved April 12, 2011 (received for review January 13, 2011) The cacti are one of the most celebrated radiations of succulent southwestern United States, the central Andes of Peru and plants. There has been much speculation about their age, but Bolivia, and eastern Brazil (2). Despite their ecological impor- progress in dating cactus origins has been hindered by the lack of tance, the timing of cactus origins and diversification has fossil data for cacti or their close relatives. Using a hybrid phyloge- remained enigmatic. Previous work has emphasized the fact that nomic approach, we estimated that the cactus lineage diverged the cacti are extremely diverse yet almost exclusively New World from its closest relatives ≈35 million years ago (Ma). However, in distribution, suggesting a possible origin between 90 and 65 Ma, major diversification events in cacti were more recent, with most which would allow maximal time for diversification and a spatial species-rich clades originating in the late Miocene, ≈10–5 Ma. Di- separation of Africa and South America (3, 4). Others have sug- versification rates of several cactus lineages rival other estimates of gested a more recent origin, because of limited molecular se- extremely rapid speciation in plants. Major cactus radiations were quence divergence among the major cactus lineages (5, 6). contemporaneous with those of South African ice plants and There are no relevant fossil records for cacti or their closest North American agaves, revealing a simultaneous diversification relatives, which has made it difficult to estimate divergence times of several of the world’s major succulent plant lineages across mul- in the group (e.g., ref. 7). However, researchers have recently tiple continents. This short geological time period also harbored made significant progress in dating the origins of major angio- EVOLUTION the majority of origins of C4 photosynthesis and the global rise sperm lineages (8), and we exploited these advances to infer the of C4 grasslands. A global expansion of arid environments during timing of cactus origin and diversification with a two-step ap- this time could have provided new ecological opportunity for both proach. First, we sequenced whole chloroplast genomes from 12 succulent and C4 plant syndromes. Alternatively, recent work has cacti and relatives (Table S1) and combined these data with fi ≈ – identi ed a substantial decline in atmospheric CO2 15 8 Ma, a larger whole-chloroplast data matrix of 90 seed plants (8) to which would have strongly favored C4 evolution and expansion build a broadly sampled phylogeny of angiosperms. We then of C4-dominated grasslands. Lowered atmospheric CO2 would also used multiple fossil calibration points within a Bayesian frame- substantially exacerbate plant water stress in marginally arid envi- work to estimate divergence times and confidence intervals for ronments, providing preadapted succulent plants with a sharp ad- several key nodes in cacti and relatives (Fig. S1). To look more vantage in a broader set of ecological conditions and promoting specifically at patterns and timing of diversification within the their rapid diversification across the landscape. major cactus lineages, we performed a series of additional dating analyses on a second phylogeny generated from fewer loci but climate change | paleobotany | CAM photosynthesis that included a greatly expanded taxon sampling within the cacti (Fig. 1, Fig. S2, and Table S2). We then identified the timing of lants are generally classified as succulent when they exhibit major radiations in cacti and their relatives by implementing Ppronounced water storage in one or more organs. High a likelihood approach that optimizes the number and placement degrees of succulence are most often associated with a suite of of shifts in diversification rate across a phylogeny (9). other characteristics that together confer survival in water-limited environments. This “succulent syndrome” usually includes a Results shallow root system that permits rapid uptake of unpredictable Our analysis of 102 chloroplast genomes produced a topology and precipitation; a thick, waxy cuticle that prevents excessive water set of age estimates for major angiosperm nodes that are highly loss; and Crassulacean acid metabolism (CAM), an alternative congruent with those of previous studies (8, 10) (Fig. S1). Age photosynthetic pathway that allows plants to uptake atmospheric estimates for particular nodes were extremely robust to removing CO at night when water loss is minimized (1). Although some 30 various combinations of fossil constraints (Table S3). Our phy- 2 ≈ plant lineages have been classified as succulent, only a small logenomic analyses suggest that the cacti are 35 million years old subset of those are species-rich and ecologically important ele- (Ma), which is much younger than many previous assumptions ments of arid and semiarid ecosystems worldwide. These lineages (3, 4) but consistent with speculation based on limited divergence include the ice plants (Aizoaceae, ≈2,000 spp), the spurges (Eu- of molecular sequences (5, 6). phorbia, ≈2,100 spp., ≈650 of which are succulent), the stonecrops (Crassulaceae, ≈1,400 spp.), the aloes (Aloe, ≈400 spp.), the Agave ≈ Author contributions: M.A. and E.J.E. designed research; M.A., P.-A.C., R.N., A.L., U.E., agaves ( , 200 spp.), the stapeliads and asclepiads (Apoc- R.M.O., E.S., and M.J.M. performed research; M.A., P.-A.C., and E.J.E. analyzed data; ynaceae-Asclepiadoideae, ≈3,700 spp., ≈1,150 of which are suc- and M.A. and E.J.E. wrote the paper. ≈ culent) and especially the cacti (Cactaceae, 1,850 spp.) (2). The authors declare no conflict of interest. The cacti represent the most spectacular New World radiation *This Direct Submission article had a prearranged editor. of succulent plants. Most cacti exhibit a highly specialized life Data deposition: The sequences reported in this paper have been deposited in the Gen- form, with extremely succulent, photosynthetic stems and leaves Bank database (accession nos. HQ620718–HQ621695). fi that have been modi ed into spines (3). The lineage has a broad 1To whom correspondence should be addressed. E-mail: [email protected]. distribution, but is most prominent in semiarid and arid regions, This article contains supporting information online at www.pnas.org/lookup/suppl/doi:10. with several main centers of diversity in arid Mexico and the 1073/pnas.1100628108/-/DCSupplemental. www.pnas.org/cgi/doi/10.1073/pnas.1100628108 PNAS Early Edition | 1of6 Downloaded by guest on September 27, 2021 8 CACTACEAE 7 6 2 3 5 4 DIDIEREACEAE S.S. 1 CORE RUSCHIOIDEAE 50 40 30 20 10 Ma Fig. 1. Time-calibrated phylogeny of the cacti and their relatives. Colored branches indicate shifts in diversification: Blue branches represent lineages with significantly lower net diversification than the background rate; green, orange, and pink branches indicate higher diversification and/or species turnover (see Table 1 for parameter estimates and clade names). Gray boxes indicate ecologically important succulent clades: Cactaceae (New World); Malagasy Didier- eaceae (Madagascar); core Ruschiodeae (Aizoaceae, Southern Africa). Furthermore, divergence time estimates from our densely diversification rate (Fig. 1, number 2, and Table 1). However, the sampled phylogeny of cacti and close relatives indicates that most dramatic species radiations in the cacti occurred many mil- many of the important species radiations within this group are lions of years after the evolution of a fully succulent syndrome and actually quite recent (Fig. 1, Table 1, and Table S4). The para- were not associated with any obvious anatomical or physiological phyletic Pereskia comprises the first two diverging, species-poor innovations. We identified five additional shifts in diversification cactus lineages and are woody trees and shrubs with slightly suc- rate in the cacti, the majority occurring within the last 8 Ma (Table culent leaves. The succulent cactus “life-form” emerged in a step- 1). With the exception of the genus Opuntia (the prickly pears), like fashion during early cactus evolution, and certain elements, these shifts occurred at nodes nested within or just outside named such as moderate tissue water storage, conservative water use, and taxonomic groups (2). Our results suggest that the cactus floras of variants of CAM photosynthesis, are found in Pereskia and other the three main centers of cactus diversity and endemism (Mexico, members of the Portulacineae (11, 12). Pronounced morpholog- central Andes, and Brazil) are extremely young, and more or less ical succulence, as exhibited by the core cacti (13), did not evolve contemporary. For example, the North American barrel and co- until ≈25 Ma and was associated with a significant increase in lumnar cacti both experienced upward shifts in speciation rate Table 1. Significant shifts in diversification