© 2006 Society of Systematic Biologists
Dating Dispersal and Radiation in the Gymnosperm Gnetum (Gnetales)—Clock Calibration When Outgroup Relationships Are Uncertain
1 University of Missouri-St. Louis, Department of Biology 8001 Natural Bridge Road, St. Louis, Missouri, 63121, USA The Missouri Botanical Garden St. Louis, Missouri, 63166, USA E-mail: wonhs{at}snu.ac.kr (H.W.) renner{at}umsl.edu (S.R.R.)
Edited by Peter Linder: Associate Editor
 |
Abstract |
|---|
Most implementations of molecular clocks require resolved topologies. However, one of the Bayesian relaxed clock approaches accepts input topologies that include polytomies. We explored the effects of resolved and polytomous input topologies in a rate-heterogeneous sequence data set for Gnetum, a member of the seed plant lineage Gnetales. Gnetum has 10 species in South America, 1 in tropical West Africa, and 20 to 25 in tropical Asia, and explanations for the ages of these disjunctions involve long-distance dispersal and/or the breakup of Gondwana. To resolve relationships within Gnetum, we sequenced most of its species for six loci from the chloroplast (rbcL, matK, and the trnT-trnF region), the nucleus (rITS/5.8S and the LEAFY gene second intron), and the mitochondrion (nad1 gene second intron). Because Gnetum has no fossil record, we relied on fossils from other Gnetales and from the seed plant lineages conifers, Ginkgo, cycads, and angiosperms to constrain a molecular clock and obtain absolute times for within-Gnetum divergence events. Relationships among Gnetales and the other seed plant lineages are still unresolved, and we therefore used differently resolved topologies, including one that contained a basal polytomy among gymnosperms. For a small set of Gnetales exemplars (n = 13) in which rbcL and matK satisfied the clock assumption, we also obtained time estimates from a strict clock, calibrated with one outgroup fossil. The changing hierarchical relationships among seed plants (and accordingly changing placements of distant fossils) resulted in small changes of within-Gnetum estimates because topologically closest constraints overrode more distant constraints. Regardless of the seed plant topology assumed, relaxed clock estimates suggest that the extant clades of Gnetum began diverging from each other during the Upper Oligocene. Strict clock estimates imply a mid-Miocene divergence. These estimates, together with the phylogeny for Gnetum from the six combined data sets, imply that the single African species of Gnetum is not a remnant of a once Gondwanan distribution. Miocene and Pliocene range expansions are inferred for the Asian subclades of Gnetum, which stem from an ancestor that arrived from Africa. These findings fit with seed dispersal by water in several species of Gnetum, morphological similarities among apparently young species, and incomplete concerted evolution in the nuclear ITS region.
Keywords: Bayesian relaxed clock; biogeography; Ephedra; Gnetum; long-distance water dispersal; polytomies in molecular clock dating
Received June 23, 2005; Revised September 2, 2005; Accepted March 13, 2006
2 Present Address: Seoul National University, College of Natural Sciences, School of Biological Sciences, Seoul, Korea 151-742
3 Present Address: Department of Biology, Ludwig Maximilians University, D-80638, Munich, Germany E-mail: renner{at}lrz.uni-muenchen.de
CiteULike 
Connotea 
Del.icio.us What's this?
This article has been cited by other articles:
|
|
 |
|
  J. J. Wiens Paleontology, Genomics, and Combined-Data Phylogenetics: Can Molecular Data Improve Phylogeny Estimation for Fossil Taxa? Syst Biol, May 19, 2009; (2009) syp012v1. [Abstract] [Full Text] [PDF]
|
|
|
|
|
|
S. S. Renner, G. W. Grimm, G. M. Schneeweiss, T. F. Stuessy, and R. E. Ricklefs Rooting and Dating Maples (Acer) with an Uncorrelated-Rates Molecular Clock: Implications for North American/Asian Disjunctions Syst Biol, October 1, 2008; 57(5): 795 - 808. [Abstract] [Full Text] [PDF]
|
|
|
|
 |
|
 R. Wilson, J. Heinrichs, J. Hentschel, S.R. Gradstein, and H. Schneider Steady diversification of derived liverworts under Tertiary climatic fluctuations Biol Lett, October 22, 2007; 3(5): 566 - 569. [Abstract] [Full Text] [PDF]
|
|
|
|
|
|
P. S. Manos, P. S. Soltis, D. E. Soltis, S. R. Manchester, S.-H. Oh, C. D. Bell, D. L. Dilcher, and D. E. Stone Phylogeny of Extant and Fossil Juglandaceae Inferred from the Integration of Molecular and Morphological Data Sets Syst Biol, June 1, 2007; 56(3): 412 - 430. [Abstract] [Full Text] [PDF]
|
|