Hybridization in sub-arctic willow scrub in Scotland

Abstract

This thesis describes the application of molecular markers to distinguish among subarctic willow species in Scotland, and to use this information to assess the extent and dynamics of hybridization and introgression. This final chapter first summarises the key results, then reviews the suitability of the techniques used and suggests how different techniques might give complementary information to that described. An overview is given of how the results in general can be applied to the taxonomy of willows, and this is followed by an assessment of whether hybridization and introgression pose any threat to the taxa and communities studied. Finally, I explore how further research might facilitate willow conservation as well as contributing to knowledge on reproductive barriers and hybridization in plants, and in Salix in particular.SPECIES DELIMITATION When reference populations of each of six species of sub-arctic willow in Scotland (S. arbuscula, S. herbacea, S. lanata, S. lapponum, S. myrsinites, S. reticulata) were examined using nuclear AFLPs, all taxa were readily separable in pairwise comparisons. In contrast, chioroplast DNA revealed trans-specific polymorphism and a lack of taxonomically informative markers. It is difficult to say whether this chioroplast sharing among lineages represents reticulation or ancestral polymorphism. Although the information from the two marker systems is to some degree in conflict, overall, the genetic data are consistent with at least some barriers to gene flow between taxa, and the presence of morphologically and genetically definable entities.HYBRID IDENTIFICATION When samples representing a set of six hybrid combinations (based on morphological data) were examined by AFLPs, four of the six putative morphological hybrid combinations were confirmed via AFLP analysis. Two other putative hybrids possessed AFLP genotypes that fell within the range of variation of the parental taxa; these were the samples in which the morphological evidence was most ambiguous. In general, species pairs identified as hybridizing based on morphological data, showed lower differentiation at AFLP loci than those species pairs among which hybrids are not currently recognised. Although the discrimination of IF1 hybrids and from backcrosses or F2s is difficult based on the AFLP data, the results do suggest that hybridisation is not confined to the production of Fis.EVIDENCE FOR CRYPTIC OR EXTENSIVE INTROGRESSION Evidence for cryptic and/or extensive introgression was lacking for most of the six species pairs studied. Occasional exceptions exist where individual plants harbour the morphology of one species and a genotype typical of another. This pattern was bi-directional in the case of S. lanata and S. lapponuin.More extensive exceptions to the general trend of little or no introgression included S. myrsinites, which shows introgression of markers from the lowland S. inyrsinifolia at several locations. Perhaps the most notable exception involves S. arbuscula and S. lapponurn. Hybrids have been described where these species co-occur. An increase in the number of markers in individuals from certain sympatric sites, and lower differentiation among the species at sympatric sites than in allopatric reference populations was detected and interpreted as evidence of introgression. The frequency of hybridization, and the molecular differentiation among species both vary between four sympatric sites in central Scotland.A more detailed study of S. arbuscula and S. lapponuin at two study sites, Meall nan Gabhar and Meall Ghaordie, revealed that introgression has occurred to the extent that it is difficult to clearly differentiate among the species using neutral molecular markers at these sites. The two species can, however, be clearly differentiated using morphological markers, and chioroplast DNA polymorphism shows a difference in frequency between the species. A higher proportion of morphologically identified hybrids were detected at Meal! Ghaordie where barriers to gene flow appear to be weaker, possibly due to a combination of environmental heterogeneity affecting flowering onset and the flowering of hybrids acting as a bridge for inter-specific gene transfer. At Meall nan Gabhar, only two morphological hybrids were found, yet introgression was more extensive than at Meal! Ghaordie, in the sense that differentiation among species was less pronounced.