Leaf anatomy of two reciprocally non-monophyletic mountain plants (Heliosperma spp.): does heritable adaptation to divergent growing sites accompany the onset of speciation?

Protoplasma, Oct 2016

Evolution is driven by natural selection, favouring individuals adapted in phenotypic traits to the environmental conditions at their growing site. To shed light on ecological and (epi-) genetically based differentiation between Heliosperma pusillum and Heliosperma veselskyi, two reciprocally non-monophyletic, but morphologically and ecologically divergent species from the south-eastern Alps, we studied various leaf anatomical traits and investigated chloroplast ultrastructure in leaves of the two species grown either in their natural habitat or in a common garden. The alpine H. pusillum occurs in open, wet rock habitats, whereas its close relative H. veselskyi is restricted to dry, shady habitats below overhanging rocks in the montane belt. H. pusillum exhibited higher thickness of leaves and palisade layers as adjustments and/or adaptations to higher irradiance and a higher stomatal area index reflecting better water availability. Traits were adjusted plastically, but differed between species grown in a common garden, suggesting that the differentiation between the two species is not solely based on phenotypic plasticity but also has a genetic basis. Our study thus supports the hypothesis that differentiation between the highly interfertile species is likely driven by natural selection.

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Leaf anatomy of two reciprocally non-monophyletic mountain plants (Heliosperma spp.): does heritable adaptation to divergent growing sites accompany the onset of speciation?

Leaf anatomy of two reciprocally non-monophyletic mountain plants (Heliosperma spp.): does heritable adaptation to divergent growing sites accompany the onset of speciation? Clara Bertel 0 Peter Schönswetter 0 Božo Frajman 0 Andreas Holzinger 0 Gilbert Neuner 0 Handling Editor: Peter Nick 0 0 Institute of Botany, University of Innsbruck , Innsbruck , Austria Evolution is driven by natural selection, favouring individuals adapted in phenotypic traits to the environmental conditions at their growing site. To shed light on ecological and (epi-) genetically based differentiation between Heliosperma pusillum and Heliosperma veselskyi, two reciprocally non-monophyletic, but morphologically and ecologically divergent species from the south-eastern Alps, we studied various leaf anatomical traits and investigated chloroplast ultrastructure in leaves of the two species grown either in their natural habitat or in a common garden. The alpine H. pusillum occurs in open, wet rock habitats, whereas its close relative H. veselskyi is restricted to dry, shady habitats below overhanging rocks in the montane belt. H. pusillum exhibited higher thickness of leaves and palisade layers as adjustments and/or adaptations to higher irradiance and a higher stomatal area index reflecting better water availability. Traits were adjusted plastically, but differed between species grown in a common garden, suggesting that the differentiation between the two species is not solely based on phenotypic plasticity but also has a genetic basis. Our study thus supports the hypothesis that differentiation between the highly interfertile species is likely driven by natural selection. Adaptation; Alpine plants; Environmentally induced speciation; Leaf anatomy; Leaf ultrastructure - Adaptation to environmental conditions can become manifest in various phenotypic traits, as natural selection favours individuals being able to successfully survive and reproduce in a particular environment (Coyne and Orr 2004). Habitat heterogeneity, which is particularly pronounced in mountain areas (Scherrer and Körner 2011), may trigger the formation of ecotypes within species. Ecotypes originate from differentiation of populations that are adapted in various phenotypic traits to a specific microenvironment (Hufford and Mazer 2003; Lowry 2012). Ecotypic differentiation is most likely driven by a combination of heritable and non-heritable traits (Pfennig et al. 2010; Bonduriansky et al. 2012). Despite the absence of intrinsic reproductive barriers, ecological isolation may over time lead to the formation of new species (Lowry 2012). An example of a recent ecotypic differentiation is provided by Heliosperma pusillum and Heliosperma veselskyi (Caryophyllaceae, Fig. 1). The former has a broad distribution throughout the southern and central European mountain ranges and occurs in humid, partly sun-exposed rock crevices and screes in the upper montane to alpine zone (1700–2300 m a. s. l.; authors’ personal observations). In contrast, H. veselskyi is restricted to a few scattered populations below cliff overhangs in the lower montane belt of the south-eastern Alps and northernmost Balkan Peninsula (500–1300 m a. s. l.; Neumayer 1923; Frajman and Oxelman 2007). Its growing sites are usually characterised by low irradiance and limited water availability (Bertel et al. 2016). Due to morphological divergence, both entities have been described at the species rank and are still treated as independent species (e.g. Fischer 2008; Frajman and Oxelman 2007; Poldini 2002). H. pusillum is glabrous or has sparsely hairy leaves, whereas H. veselskyi is characterised by a dense indumentum of long multicellular glandular hairs (Janka 1858; Neumayer 1923; Frajman and Oxelman 2007; Fig. 1 H. pusillum (a) grows on alpine scree sites, whereas H. veselskyi (b) occurs below cliff overhangs in the montane belt. H. veselskyi differs morphologically from its high elevation relative by broader leaves, which are covered by a thick indumentum of multicellular, glandular hairs. Despite their ecological and morphological divergence, both species are genetically inseparable (photos modified from Bertel et al. 2016; (a), R. Flatscher; (b), M. Sonnleitner) Fischer 2008). Chloroplast and nuclear low copy DNA sequence data (Frajman and Oxelman 2007; Frajman et al. 2009) as well as highly resolving restriction associated DNA markers sampled across the nuclear genome (Trucchi et al., unpublished) suggest that the two species are phylogenetically not distinct and that H. veselskyi is inextricably nested within H. pusillum. H. veselskyi thus rather represents a habitat specific ecotype, whose disjunct populations have evolved postglacially from geographically close populations of H. pusillum. For the sake of simplicity, we treat H. pusillum and H. veselskyi as species throughout the text in spite of the lack of consistent genetic divergence and their highly debatable taxonomic value. Significant diffe (...truncated)


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Clara Bertel, Peter Schönswetter, Božo Frajman, Andreas Holzinger, Gilbert Neuner. Leaf anatomy of two reciprocally non-monophyletic mountain plants (Heliosperma spp.): does heritable adaptation to divergent growing sites accompany the onset of speciation?, Protoplasma, 2017, pp. 1411-1420, Volume 254, Issue 3, DOI: 10.1007/s00709-016-1032-5