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AOBPreview originally published online on December 18, 2007
Annals of Botany 2008 101(3):403-419; doi:10.1093/aob/mcm305
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© The Author 2007. Published by Oxford University Press on behalf of the Annals of Botany Company. All rights reserved. For Permissions, please email: journals.permissions@oxfordjournals.org

ITS Evolution in Platanus (Platanaceae): Homoeologues, Pseudogenes and Ancient Hybridization

Guido W. Grimm1 and Thomas Denk2,*

1 Institute of Geosciences, Eberhard-Karls-University, Sigwart Strasse 10, 72076 Tübingen, Germany
2 Department of Palaeobotany, Natural History Museum, Box 50007, 10405 Stockholm, Sweden

* For correspondence. E-mail thomas.denk{at}nrm.se

Received: 20 August 2007    Returned for revision: 1 October 2007    Accepted: 6 November 2007    Published electronically: 18 December 2007

Background and Aims: Platanaceae is an old family of angiosperms extending back to the Early Cretaceous but consisting of a single extant genus, Platanus. Species of Platanus have long been known to hybridize, and the London plane, Platanus x hispanica, is a well-known example of a hybrid species that formed in historical times. In addition, morphological studies have suggested past interspecific or interlineage hybridization and reticulation as possibly important factors in the evolution of the genus. This study aims at unravelling the complex evolutionary information contained in internal transcribed spacer (ITS) sequences.

Methods: The ITS of the 35S nuclear ribosomal DNA are biparentally inherited, multi-copy markers with a high potential for resolving intrageneric relationships especially when ancient hybridization (reticulation) is involved. Phylogenetic trees, splits graphs and motif analysis are used to extract phylogenetic information from 223 cloned ITS sequences, representing ten species and varieties of Platanus. Non-pseudogenous and pseudogenous sequence motives are assessed to explain how different evolutionary modes contribute to possibly conflicting character state patterns in the ITS.

Key Results: It was found that putative non-functional ITS copies (‘pseudogenes’) form distinct groups in phylograms and splits graphs, and that pseudogenous lineages reflect ancient hybridization events conserved in the ITS. Specifically, pseudogenous clones of an ‘Atlantic’ North American clade share sequence motives with non-pseudogenous clones of the western (‘Pacific’) North American P. racemosa species aggregate. In addition, evidence was found for recent lateral gene flow as a possible factor in the evolution of the central Mexican P. rzedowskii.

Conclusions: Broad ITS data sets that cover intra- and interindividual variability reveal past and ongoing speciation processes in Platanus. Evolutionary pathways can be visualized with splits graphs, but not with bifurcating trees.

Key words: Ancient hybridization, bipartition networks, evolutionary patterns, internal transcribed spacer, intraindividual variability, motif analysis, phylogenetic networks, Platanus, pseudogenes


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