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Hybridization in sub-arctic willow scrub in Scotland

Molecular and morphological markers were used to clarify the species and hybrid taxonomy of sub-arctic <i>Salix </i>species in Scotland, and to investigate the extent of hybridization and introgression in sub-arctic willow scrub communities. The utility of molecular and morphological markers for the identification of hybrids between pairs of species was compared. A detailed study of hybridization between <i>S.</i> <i>arbuscula </i>and <i>S. lapponum </i>was conducted at two sites. The potential for differences in phenology to act as a reproductive barrier between the species was estimated. The results were used to determine the importance of hybridization and introgression in the systematics of montane <i>Salix, </i>and to provide guidance for conservation programmes aimed at restoring sub-arctic willow scrub communities. The six morphologically circumscribed montane species considered in this study (<i>S.</i> <i>arbuscula, S. herbacea, S. lanata, S. lapponum, S. myrsinites, S. reticulata) </i>can be distinguished clearly with nuclear AFLP markers, but not with chloroplast DNA markers. A relationship between the occurrence and frequency of hybridization and genetic distance between species pairs was detected. At two locations in central Scotland (Meall nan Gabhar, Meall Ghaordie) <i>S. arbuscula </i>and <i>S. lapponum </i>can be distinguished clearly with morphometric analysis of six foliar characters. They maintain their molecular differentiation but to a lesser degree than in allopatric reference populations. A significant but low correlation (Meall nan Gabhar <i>r</i><sup>2</sup><i> </i>= 0.173; Meall Ghaordie <i>r</i><sup>2</sup> = 0.095) between morphological distance and molecular distance in individuals was detected. Bayesian assignment analysis based on AFLP data was, however, unable to resolve distinct parental species groups at these two sites, suggesting a significant level of genetic exchange between the taxa. Morphological differentiation was site independent, whereas molecular differentiation was site dependent and showed considerable mixing of the species gene pools when considering both sites together. Despite this evidence for genetic exchange, flowering asynchrony was found to constitute a strong barrier to inter-specific gene flow at both sites (RI<sub>phen</sub> = 0.778 at Meall nan Gabhar, and 0.702 at Meall Ghaordie) with <i>S. lapponum </i>flowering before <i>S. arbuscula. </i>A relationship between flowering onset and altitude contributes to spatio-temporal isolation between species. This barrier is slightly weaker at one site (Meall Ghaordie) and more morphological hybrids were detected at this site. This case study represents a situation where rare hybridization has led to the widespread introgression of neutral markers while diagnostic morphological traits remain differentiated. In the final chapter, the information obtained about the extent of hybridization and introgression is used to provide guidance for conservation management strategies for sub-arctic willow scrub in Scotland.
Date January 2006
CreatorsForrest, Alan
PublisherUniversity of Edinburgh
Source SetsEthos UK
Detected LanguageEnglish
TypeElectronic Thesis or Dissertation

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