Flora - Morphology, Distribution, Functional Ecology of Plants
Filling of eco-climatological niches in a polyploid complex – A case study in the plant genus Leucanthemum Mill. (Compositae, Anthemideae) from the Iberian Peninsula
Introduction
Polyploidy or whole genome duplication (WGD) is considered one of the most important mechanisms in plant evolution and speciation (Grant, 1981, Soltis et al., 2009, Stebbins, 1971). While WGD may lead to advantages for the polyploid progeny, like heterosis effects and gene redundancy, there are also a number of disadvantages connected with it, like disruptive effects of nuclear and cell enlargement, problems in meiosis or/and mitosis, regulatory changes in gene expression, and epigenetic instability (Comai, 2005). Despite these disadvantages, the positive effects of polyploidy, like higher stress and disease tolerance, are thought to prevail, which may lead to the often observed higher capability of polyploids to occupy new ecological niches and to a higher efficiency to expand their geographical ranges relative to the diploid progenitors (Hijmans et al., 2007, Spooner et al., 2010, and citations therein). Well-documented examples for this alleged preponderance in geographical respects comes from distribution patterns of polyploid complexes observed in formerly glaciated areas in mountain ranges or at high latitudes, which are dominated by polyploids at the expense of diploids (Brochmann et al., 2004, Löve and Löve, 1949, Tischler, 1935). This was attributed to either the enhanced colonisation abilities of polyploids (e.g., Ehrendorfer, 1980, Stebbins, 1950, Stebbins, 1971, Stebbins, 1985), the increased production of unreduced gametes under environmental stress (Mable, 2004, Otto and Whitton, 2000, Ramsey and Schemske, 1998), or as being a consequence of higher hybridisation frequencies (and allopolyploid formation) in areas under influence of strong climatic oscillations during the Pleistocene and Holocene. Experimental ecological studies have shown that different geographical distributions of diploids and polyploids correlate with the different ecological niches of taxa (e.g., Baack, 2005, Baack and Stanton, 2005, Bayer et al., 1991, Brochmann and Elven, 1992, Petit and Thompson, 1999).
The last years of biogeographical research have witnessed the invention of GIS-based analytic tools that allow to objectify eco-climatological niches of species (Wiens and Donoghue, 2004). The most promising approach is to use distributional data aggregated from numerous point observations together with climatological (and other environmental) information to assess presumptive niche requirements of a taxon (its ‘ecological envelope’; Akin, 1991). The recently developed application of maximum entropy methods (Maxent version 3.3.2; Phillips et al., 2006) provides the necessary algorithms for these niche reconstructions. This method offers many advantages, as it seems to perform better as other methods such as GARP (Genetic Algorithm for Rule-set Prediction; Stockwell and Noble, 1992, Stockwell and Peters, 1999) or Bioclim (Nix, 1986) and it requires only presence data, allowing to make inference on the potential distribution of poorly known species (Elith et al., 2006, Kozak and Wiens, 2006, Phillips et al., 2006). Therefore, the modelling of eco-climatological niches and distribution ranges enables a comparison of the (climatologically mediated) niche-dependent potential distribution of a taxon with its presently realised distribution range (e.g., Serra-Diaz et al., 2012). In the case of polyploid species complexes this may allow us to answer the question whether there are ploidy levels, where the advantages of polyploidy outweigh the disadvantages and lead to a more effective expansion of geographical ranges, and others where the opposite is the case.
The genus Leucanthemum Mill. (Compositae, Anthemideae) forms a large polyploid complex that comprises 41 species (Euro+Med, 2006) and is distributed all over the European continent, with one species (L. ircutianum) even reaching Siberia and some species introduced to many temperate regions of the northern and southern hemisphere (Meusel and Jäger, 1992). Leucanthemum species can be found in as different habitats as calcareous dry grasslands, wet meadows and alpine communities – and even on serpentine derived soils or in brackish water. The highest diversity of the genus is encountered on the Iberian Peninsula, where it is represented by 16 species, nine of them endemic to this geographical region (Vogt, 1991). Due to the fact that these 16 species (classified into 20 taxa) uninterruptedly span the range between the diploid (2n = 2x = 18) and the dodecaploid (2n = 12x = 108) level – with one species even reaching the dokosaploid (2n = 22x = 198) level – and a recent revision with a number of very detailed distribution data is available (Vogt, 1991), the genus provides a well-suited study group for the proposed comparison between realised and potential distribution ranges. Due to the high number of chromosome counts throughout the geographical ranges of each of the taxa discriminated in this revision and the rather narrow cytological-morphological taxon concept applied (Vogt, 1991), the 20 taxa on which the present study is based are considered to lack intra-taxonomic variation of chromosome numbers to a great extent. Additionally, the prerequisite of the representative sampling across the entire area of the Iberian Peninsula, which is necessary to search for correlations between geographical variables and species occurrences, is well met in this genus through the painstaking revisionary work done by Vogt (1991), making this genus an extremely suitable study-group for the testing of possible correlations between ploidy level and distribution patterns.
The purpose of our present study was therefore to assess sizes of realised and potential distribution areas for all 20 taxa of Leucanthemum on the Iberian Peninsula, to describe the filling of potential areas as an index, and to infer whether this filling quotient shows any interpretable correlation with ploidy level.
Section snippets
Data collection
Assessment of realised distributions and eco-climatological modelling of potential distribution ranges of Leucanthemum species on the Iberian Peninsula was based on lists of herbarium specimens given in the revision of the genus for Spain and Portugal by Vogt (1991), excluding the species L. paludosum, L. decipiens, and L. arundanum that are nowadays treated as members of the genera Mauranthemum Vogt & Oberpr. (the two former) and Rhodanthemum (Vogt) B.H. Wilcox et al., respectively (
Estimated sizes of realised distribution ranges
Geo-referencing of herbarium records for the 20 taxa of Leucanthemum under study resulted in a minimum of 5 (Leucanthemum montserratianum) and a maximum of 235 data points (L. pallens) with a median value of 46 data points per taxon (Table 1). Diameters of taxon-specific buffer circles ranged between 1.3 km in the highly polyploid (22x) and narrowly endemic L. lacustre and 23.7 km in the NE Spanish and SW French species L. monspeliense, respectively (mean value: 6.1 km). As a consequence, sizes of
Polyploidy and realised distribution ranges
Our present data do not support the expectation that polyploid Leucanthemum species show significantly larger ranges than the diploid ones. Despite the fact that the most widespread species, L. pallens, is a hexaploid, other hexaploid and higher polyploid taxa exhibit very restricted occurrences. With most of the diploid species being also very narrowly distributed (with the exception of L. vulgare subsp. pujiulae), the tetraploid level appears to be the optimum in respect to absolute size of
Acknowledgements
Financial support of the present contribution came through a research grant to C.O. from the German Research Foundation (DFG) in the project ‘Consequences of polyploidy: Phylogeny, phyloecology, and expression of duplicated genes in Leucanthemum Mill. (Compositae, Anthemideae)’ (OB 155/10-1). We thank two anonymous reviewers for their helpful comments on a previous version of the manuscript.
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Present address: Institut für Spezielle Botanik, Friedrich-Schiller-Universität Jena, Philosophenweg 16, D-07743 Jena, Germany.