Hybridization in sub-arctic willow scrub in Scotland

Molecular and morphological markers were used to clarify the species and hybrid taxonomy of subarctic Salix species in Scotland, and to investigate the extent of hybridization and introgression in subarctic 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 S. arbuscula and S. lapponwn 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 Salix, 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 (S. arbuscula, S. herbacea, S. lanata, S. lapponum, S. myrsinites, S. reticulata) 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. Hybridization described using morphological characters was confirmed for four out of six bi-parental combinations using AFLPs, and the distribution of markers characteristic of parental species described in hybrid accessions. No molecular evidence for hybridization was detected for the two remaining putative hybrid combinations. This agrees with morphological assessments that cast doubt on the identity of these putative hybrids. Little evidence for hybridization and introgression en masse was detected where willow species grow in close sympatry. The main exception to this trend involved S. arbuscula and S. lapponum which, despite distinctive morphological characteristics, appear to hybridize and introgress where they occur together. Variation in the extent of introgression was detected in different populations of these species. In contrast, populations of S. lanata and S. reticulata which are not currently accepted as producing hybrids showed no evidence of hybridization or introgression in sympatry. Rare examples were detected of accessions identified in the field as one species possessing a genotype typical of another species. At two locations in central Scotland (Meal] nan Gabhar, Meall Ghaordie) S. arbuscula and S. lapponum 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 r2 = 0.173; Meall Ghaordie r2 = 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 hfl = 0.778 at Meall nan Gabhar, and 0.702 at Meall Ghaordie) with S. lapponum flowering before S. arbuscula. A relationship between flowering onset and altitude contributes to spatio-temporal isolation between species. This barrier is slightly weaker at one site (Meal] 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. Criteria are set out for choosing populations that are appropriate for use as source material in augmentation programmes. Consideration is given as to whether hybrid genotypes should be incorporated into existing taxon-based conservation management strategies. The importance of conserving the natural process of hybridization is discussed. Chapter