Genetic structure of Emphasis Type="Italic"Saxif

Authors: Tania J Walisch Diethart Matthies Sylvie Hermant Guy Colling

Publish Date: 2014/05/29

Volume: 301, Issue: 1, Pages: 251-263

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Abstract

We used RAPD markers to study the population genetic structure and diversity of Saxifraga rosacea subsp sponhemica a rare Central European endemic rock plant with a highly disjunct distribution Because of strong isolation current gene flow between populations is very low or absent However an isolation by distance pattern of genetic differentiation suggested historical gene flow during the last glaciation when suitable habitats for S sponhemica were much more abundant In most populations considerable genetic variability has been preserved due to the longevity of S sponhemica Our results suggest that longlived plant species can maintain historic genetic patterns despite small size and strong isolation of populations Several RAPD loci were identified to be nonneutral and their frequencies correlated with climatic gradients indicating natural selection Adaptive genetic variation could be important for adaptation to environmental changes like ongoing climate change The taxon does not appear to be genetically threatened in the short term but populations are threatened by habitat destruction The establishment of new populations in suitable habitats with seeds from the same region may be a suitable conservation measure avoiding potential maladaptation due to local adaptationSpecies have undergone important range contractions and expansions during the glacial and interglacial periods of the Pleistocene Hewitt 1996 During the glaciations the Central European lowlands were covered by steppetundra vegetation suitable for coldadapted plant species which were then widely distributed t Mannetje 2007 In the postglacial warming period these species migrated to the cold previously inhospitable alpine or arctic regions but some remnant populations survived in lowland habitats with suitable conditions A disjunct distribution in combination with a habitat type that had already existed during the glaciation is often considered to be an indicator for the glacial relict status of populations Walter and Straka 1970 Cliffs are a typical habitat type that has existed and remained stable since glaciations because it was hardly affected by forest recolonisation in the postglacial period or by human activity during the Holocene Because cliffs are naturally rare and fragmented in lowland Europe glacial relicts occurring on cliffs are suitable model species to study the effects of longterm fragmentation Tang et al 2010The effects of habitat fragmentation on the genetics of plant populations have been a topic of many recent studies reviewed by Young et al 1996 Leimu et al 2006 Honnay and Jacquemyn 2007 Many rare species have been found to harbour less genetic diversity than more widespread species compilation by Hamrick and Godt 1990 Cole 2003 Nybom 2004 due to the loss of alleles through random genetic drift eg Young et al 1996 Frankham and Wilcken 2006 Yuan et al 2012 Furthermore reduced gene flow among isolated populations in fragmented habitats has led to strong genetic differentiation between populations of many rare species eg Fischer and Matthies 1998 Šmídová et al 2011 Wagner et al 2011 Loss of genetic variation and genetic differentiation is expected to increase with time since fragmentation Coates 1988 Gitzendanner and Soltis 2000 Zawko et al 2001 Thus ice age relict populations that have been fragmented for a long time are expected to show strong genetic differentiation and low genetic diversity Strong genetic differentiation has been reported for isolated alpine relict populations such as Saxifraga cernua Bauert et al 1998 Erinus alpinus Stehlik et al 2002 and for the lowland remnant populations of Saxifraga paniculata Reisch et al 2003 However not all studies have found low genetic diversity in ice age relicts Lutz et al 2000 Reisch et al 2003 presumably due to the longevity of the species buffering random genetic drift Genetic variation has profound implications for species conservation Schaal et al 1991 Ellstrand and Elam 1993 Ouborg et al 2006 and assessing genetic variation within and between populations is essential for efficient conservation measures for rare species Loss of genetic variability and increased inbreeding in small populations Young et al 1996 Frankham et al 2002 may result in reduced fitness of offspring Ellstrand and Elam 1993 Keller and Waller 2002 In the long term reduced genetic variation may lower the evolutionary potential of a species in the face of changing environmental conditions such as ongoing climate changeGenetic variation is often assessed by studying the variability of neutral markers However adaptive loci that are responding to environmental variation may provide more relevant information on the potential of populations for rapid adaptation Hoffmann and Willi 2008 Manel et al 2012 Recently developed genome scan methods allow detecting candidate loci under selection on the assumption that natural selection is a locusspecific force which increases the frequencies of locally beneficial alleles in a population Strasburg et al 2012 The distribution of these candidate loci among populations may then be compared with the distribution of environmental factors such as temperature or precipitation that affect adaptive genetic variationWe studied the genetic population structure and diversity of the endangered longlived plant Saxifraga rosacea Moench subsp sponhemica CC Gmel DA Webb an endemic of Central Europe Because of its disjunct distribution and habitat type screes and cliffs the species is considered to be an ice age relict Thorn 1960 Walter and Straka 1970 We used RAPDmarkers to address the following questions 1 How is genetic variation distributed among regions populations and individuals Does the genetic distance between populations increase with geographic distance 2 Are populations of S rosacea subsp sponhemica characterised by low genetic diversity and does genetic diversity increase with population size 3 Are there loci putatively under selection and is their frequency related to climatic variablesSaxifraga rosacea subsp sponhemica hereafter called by its synonym S sponhemica CC Gmel is an evergreen perennial that grows either in compact cushions formed by short and suberect shoots or as loose mats formed by procumbent and rather long shoots Tutin et al 1968 Cushion size is highly variable 1–100 cm and the number of rosettes per plant ranges from 1 to over 600 Individual rosettes are semelparous but the genets are iteroparous S sponhemica is able to spread sexually via seeds and vegetatively via rosettes pers observation Hemp 1996 Demographic data indicate that genets of S sponhemica can live for several decades Decanter pers commThe flowers of S sponhemica are strongly protandrous Webb and Gornall 1989 but flowers ripen at different times within the same genet which allows geitonogamous pollination Common pollinators are Diptera Muscidae and Syrphidae and Apidae Webb and Gornall 1989 we also observed some Coleoptera species as flower visitors S sponhemica has a mixed mating system with a selfing rate of about 468  Walisch unpubl S sponhemica generally occurs on north to east facing rock faces scree slopes and stone walls with no or little direct sunlight Hemp 1996 pers observation which are fragmented habitats in lowland Europe A few populations occur also on walls next to natural rock populationsDistribution grey areas of Saxifraga sponhemica modified from Jalas and Suominen 1976 The sampling regions are marked as black dots on the map In Luxembourg LU 13 populations were sampled in Germany DE four in Belgium BE five in France FR two and six in two regions of the Czech Republic four in České středohoří CZSt and two in Český kras CZKr

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