Mechanisms of speciation in Silene

Abstract: A fundamental question in the field of evolutionary biology is how new species originate. Investigating speciation benefits from an integrated approach, which requires a solid understanding of ecology, reproductive biology, geographical distribution, underlying genetic architecture of reproductive isolation (RI), demographic history and genomic divergence. In this thesis, I studied the evolution of reproductive isolation in the sister species Silene dioica (L.) Clairv. and S. latifolia Poiret. The aims of the thesis are to investigate (1) the individual and cumulative contributions of extrinsic and intrinsic reproductive barriers between S. dioica and S. latifolia, (2) the genetic architecture of traits associated with reproductive barriers (3) the demographic history of lineage-split between the two species, (4) genomic patterns of divergence between the species.I found that multiple extrinsic pre- and postzygotic barriers resulting from ecological differentiation contributed most to total RI, while intrinsic barriers had substantial individual strength but contributed weakly to total RI. QTL mapping revealed evidence for genetic coupling of QTLs controlling traits associated with RI, although QTLs were overall widely distributed. QTLs related to sexually dimorphic traits were located on or near the pseudo-autosomal region of the sex chromosomes. The best-supported demographic model suggests heterogeneous population size and migration rates among genome-wide loci and points to the presence of barrier loci. Genomic divergence (measured using FST and dXY) was commonly accentuated around the middle of linkage groups and near QTLs for traits associated with reproductive barriers.In summary, the results in my thesis indicate that the speciation process is driven by multiple interacting and complex reproductive barriers. The genomic divergence landscape is shaped by interplay of the magnitude of gene flow, the strength and timing of selection, and other  confounding factors such as genomic features.