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Title: Barriers to gene flow : a Bombina (fire-bellied toad) hybrid zone and multilocus cline theory
Author: Kruuk, Loeske E. B.
Awarding Body: University of Edinburgh
Current Institution: University of Edinburgh
Date of Award: 1997
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1. In a stable hybrid zone between two taxa, natural selection creates a barrier to gene flow which counterbalances the homogenising effects of dispersal and interbreeding. Studies of hybrid zones can contribute to our understanding of the nature of reproductive isolation between two populations, and hence of speciation mechanisms. For example, reproductive isolation may be generated by natural selection acting against alleles in the wrong environment or against alleles in the wrong genetic background. In this thesis, I consider these issues with (i) a study of a hybrid zone between the fire-bellied toads Bombina bombina and B. variegata and (ii) development of analytical and simulation models of geographical variation maintained by dines at multiple loci. 2. In a transect across the Bombina hybrid zone in Croatia, the pattern of change in three phenotypic traits (leg length, belly pattern and egg size) corresponds to the stepped dines observed in previous studies of diagnostic allozyme loci. There is close concordance between the mean values of the traits and between estimates of linkage disequilibrium calculated from associations between alternative phenotypic and genetic variables. Clines in allozyme frequency and spot score are wider in males than in females, suggesting differential dispersal patterns. 3. There is direct evidence for hybrid dysfunction in the field: in samples collected from Bombina breeding sites, embryonic mortality, larval mortality and frequency of larval developmental abnormalities all increase significantly towards the centre of the hybrid zone. However, a cohort analysis of adult toads within a central region shows no evidence of differential mortality with respect to genotype. 4. Bombina bombina and B. variegata typically use different breeding habitats, with B. variegata showing strong avoidance of the semi-permanent ponds favoured by B. bombina. The abundance of aquatic predators is shown to differ between the alternative breeding habitats preferred by either species. Behavioural experiments show that the feeding strategy of B. bombina larvae reduces their risk of predation, relative to that of B. variegata larvae, thus demonstrating an adaptive advantage to the adults habitat preference. As the availability of either habitat type changes across the zone, these adaptations imply that differential adaptation across an environmental gradient is also creating a barrier to gene flow between the taxa. 5. An analytical model of multilocus dines maintained by differential adaptation to alternative environments is developed, considering in particular the effect of selection on neutral markers. The dynamics are qualitatively similar to those of previous models of heterozygote disadvantage. Computer simulations are used to test the analytical predictions, and the restrictions imposed by assumptions of weak selection. These show that while dine shape can accurately estimate parameters such as fitness in hybrid populations, others, such as the number of genes under selection, may be less robust. 6. The effect of a habitat preference in a hybrid zone such as Bombina is explored with a simulation model. Pooling across habitats, overall statistical associations increase with preference strength, illustrating the effect of the habitat preference in maintaining the integrity of the parental genomes. However, the resulting magnitude of linkage disequilibrium and deviations from Hardy-Weinberg proportion within habitats vary nonmonotonically with the strength of the preference. The resulting shape of the zone will be largely dependent on the underlying distribution of habitat availability. 7. The Bombina hybrid zone is therefore maintained by both endogenous and exogenous selection. Although theoretical models show that the dynamics of either regime are similar, the results imply that divergence during allopatry has been driven by adaptation to alternative environments, rather than solely non-adaptive factors. The resulting reproductive isolation has therefore been, at least partially, determined by ecological factors, and environmentally-mediated factors such as a habitat preference will generate further barriers to gene flow between two populations.
Supervisor: Not available Sponsor: Not available
Qualification Name: Thesis (Ph.D.) Qualification Level: Doctoral
EThOS ID:  DOI: Not available