Mechanisms for partial reproductive isolation in a Bombina hybrid zone in Romania

Differences between taxa which have developed in allopatry can contribute to reproductive isolation in the case of secondary contact. Hybrid zones are ideal study systems in which to investigate the role of preand postzygotic mechanisms for the reduction or inhibition of gene flow. This thesis describes a hybrid zone between the fire-bellied toads Bombina bombina and B. variegata in Romania. The spatial arrangement of populations in this hybrid zone resembles a broad mosaic, with B bombina restricted to scattered big ponds and B. variegata-like hybrids occupying the surrounding less permanent water bodies. This structure is in striking contrast with the steep clinal transitions found in hybrid zones in Croatia, Poland and the Ukraine. A detailed comparison between the transects in Romania and Croatia revealed that the underlying distribution of habitat is the most likely factor determining the structure of a Bombina hybrid zone. Furthermore, habitat preference is stronger in Romania than in Croatia. Despite habitat preference, B. bombina adults occasionally migrate out of ponds and reproduce in intermediate habitat, thus causing introgression at neutral markers in the B. variegata-like populations there. In Vines et al. (in press), we used the genetic structure to quantify this migration and then assessed how much selection is required to counteract the breakdown of adaptive differences. The necessary level of selection is plausible but neutral divergence is probably collapsing. Breeding site preference in adults and natural selection in embryos and tadpoles may be important forces against immigrant B. bombina alleles in B. variegata-like populations. I found a consistent shift in breeding habitat preference towards B. variegata in intermediate habitat. I also quantified natural selection in tadpoles as this should constitute a similarly important but postzygotic mechanism for partial reproductive isolation. There was significant intrinsic selection against B. bombina alleles in B. variegata-like families. This fits the prediction that selection should be against immigrant B. bombina alleles rather than heterozygotes. There was no direct evidence for extrinsic selection in tadpoles, although it is strongly suggested by breeding habitat preference in adults. This issue is worth further investigation. I also investigated tadpoles after selection at the phenotypic level. B. variegatalike tadpoles grow and develop faster than B. bombina-like ones in intermediate habitat, which affords them an adaptive advantage in the face of desiccation. Considering phenotypic plasticity, B. bombina-like tadpoles show the same high level and continuous range as B. variegata. This finding is probably related to the high rate of introgression in the Romanian hybrid zone. I showed that habitat preference and selection are important mechanisms for the maintenance of reproductive isolation in this Bombina mosaic hybrid zone and may play an important role for reproductive isolation in incipient species.