Polyploidy, the multiplication of whole chromosome sets above the diploid condition, is a dominant feature of flowering plants. Up to 70% of all angiosperm species have experienced a genome multiplication at sometime in their evolutionary past, and the number of chromosome copies ranges from three (2n = 3*) to eighty (2n = 80*) (Master-son 1994; Otto and Whitton 2000). In addition, many spe-
Brian C. Husband. Department of Botany, University of Guelph, Guelph, ON N1G 2W1, Canada. e-mail address: [email protected] Correct citation: Husband, B.C. 2004. Polyploidy and plant adaptation: a framework for future research. In Plant Adaptation: Molecular Genetics and Ecology. Proceedings of an International Workshop held December 11-13, 2002, in Vancouver, British Columbia, Canada. Edited by Q.C.B. Cronk, J. Whitton, R.H. Ree, and I.E.P. Taylor. NRC Research Press, Ottawa, Ontario. pp. 119-126.
cies exhibit variation in ploidy within and among populations (Lewis 1980). Given its prevalence in the plant kingdom and its strong association with plant physiological, morphological and ecological characteristics (Stebbins 1950; Levin 1983, 2002), polyploidy is central to any discussion on the genetics and ecology of adaptation.
Polyploid ecology has historically been an active area of research. Past interest in the biosystematics of polyploid complexes led many researchers to describe the geographic ranges of polyploids and their diploid progenitors (Stebbins 1950; Ehrendorfer 1980). This work focused attention towards the kinds of polyploids (auto vs. allo) that exist and the ecological, physiological and morphological correlates of such genome multiplications. Starting in the 1980's, the research focus shifted to the population biology of polyploidy, which has concentrated on their pathways of formation (Thompson and Lumaret 1992; Soltis and Soltis 1993, 1999; Ramsey and Schemske 1998) and consequences for establishment and reproductive isolation (Lumaret and Barrientos 1990; Petit et al. 1999; Husband and Schemske 2000; Husband et al. 2002). My own work has explored these issues using species that are variable in ploidy, and where the poly-ploids are derived from genomes of a single diploid species (i.e., autopolyploid) (Husband and Schemske 1998; Burton and Husband 1999; Husband 2000; Husband and Schemske 2000; Husband et al. 2002). Surprisingly, however, little is known of the extent to which polyploidy can contribute to adaptation along ecological gradients or the population processes by which this occurs.
My approach here is to develop a conceptual framework for future research, rather than to review existing data. My primary questions, then, are, 'How can research on poly-ploidy provide insights into the process of adaptation along ecological gradients', and 'How will research in molecular biology and genomics facilitate this?' I will argue that research into genome multiplication can be useful for addressing three basic questions in plant adaptation: (1) What are the genetic and ecological limits to adaptive divergence?, (2) Can large major shifts in phenotype due to chromosomal variation contribute to adaptation, beyond typical genic responses to selection?, and (3) When will stochastic processes play a role in adaptive evolution? I will elaborate on each of these points and identify some of the challenges that lie ahead in resolving them. Where possible, I will include selected empirical research from the literature and from my own research on fireweed, Chamerion angustifolium (Ona-graceae), to illustrate relevant points. This perennial herbaceous plant has a broad geographic distribution and inhabits open and disturbed environments across a wide range of lati tudes and altitudes in the northern hemisphere. Most pertinent, C. angustifolium is variable in chromosome number. In North America, it consists of diploid (2n = 2x =36) and autopolyploid (2n = 3x, 4x, 5x) individuals (Mosquin 1967; Mosquin and Small 1971; Husband and Schemske 1998; H.A. Sabara and B.C. Husband, unpublished) that can coexist in sympatry.
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