Evolutionary consequences of hybridyzation between Guem urbanum and G. rivale
Theoretical considerations predict that hybridization between plants with different breeding systems could potentially be a powerful evolutionary force but very few studies have been carried out to investigate hybridization among outcrossing and selfing species in natural populations. The objective of this thesis was to use G. urbanum and G. rivale, two freely hybridizing species with putatively different breeding systems, and study the processes that take place when the two different mating systems meet. To establish the extent of the morpholofical and genetic marker differences, the breeding system and levels of inbreeding depression in these two species, I examined reference populations from various locations in Great Britain using morphological (nine phenotypic traits) and genetic analyses of a long standing hybrid swarm between G. urbanum and G. rivale, the results revealed that the two parental species differed signigicanly in all nine traits measured and were clearly separated by 616 AFLP markers, of which 8% were species-specific. The breeding system analysis confirmed the highly selfing nature of G. urbanum and the outcrossing nature of G. rivale. No inbreeding depression was discovered in G. urbanum and very low levels in G. rivale. The AFLP analysis (203 loci) of the hybrid swarm showd that four distinct genetic groups can be distinguished (1) a parental G. urbanum life group (2) an F1 like group (3) a Backcross Rivale group and (4) a parental G. rivale like group. No Backcross Urbanum individuals were detected. Morphological data were hightly correlated with genetic data (Spearman Rank Order Correlation = 0.08,p<0.0001) but less informative as morphological data could not separate the Backcross Rivale samples from G. rivale. Analysis of seed progeny from the hybrid swarm showed that there were clear differences in the genotypic composition of the seed porgeny and the plants from which they were collected, revealing additional genetic classes such as Backcross Urbanum individuals which were not represented in the adult plant state. The Backcross Rivale parental group was the only one in which the average offspring position did not differ signigicantly from the mean parental psition in a PCO plot using 203 AFLP markers, indicating that this group might be reporductively isolated from other groups in the hybrid swarm. Taken together the results suggest that hybridization between G. urbanum and G. rivale results in the production of a range of genotypic and phenotypic classes, some of which are selected against. There is evidence for introgression between these two species, and tentative evidence for partial reporduction isolation of some hybrids suggestion that hybridisation between these outcrossing and selfing Geum species may represent a creative evolutionary force.