Patterns of geographic variation in Silene section Elisanthe (Caryophyllaceae): hybridization and migrational history
Abstract: Large-scale patterns of genetic variation in chloroplast (cp) and nuclear DNA in the widespread European herb species Silene latifolia and S. dioica were investigated using cpPCR-RFLPs, non-coding cpDNA sequences, AFLPs and sequences of the internal transcribed spacers (ITS) of nuclear ribosomal DNA (rDNA). The results of the analyses of cpDNA variation using PCR-RFLPs and non-coding sequence data suggest that both S. latifolia and S. dioica colonized C and N Europe during the current postglacial period from several source/refugial regions in southern Europe, and that populations of both species spreading from the same source/refugial region had the same cpDNA. Repeated cycles of hybridization and chloroplast introgression in shared refugia and/or during the early stages of range expansion could have resulted in the homogenization of chloroplast genomes of populations of the two species that were in the same refugial region. The cpPCR-RFLP data suggest that S. dioica may also have survived the LGM in a refuge in south-eastern Europe or Russia and migrated into Fennoscandia from the north via Finland following the retreat of the Weichselian ice sheet. In contrast to results from the analysis of cpDNA, analyses of AFLP and nuclear ribosomal ITS sequences indicate differentiation between the nuclear genomes of S. latifolia and S. dioica. Both AFLPs and ITS sequences reveal limited geographic structure in C and N European populations of S. latifolia. This lack of geographic structure is most likely due to high levels of inter-population gene flow that has led to the mixing and homogenization of the nuclear genomes. European S. latifolia can be divided into western and eastern races on the basis of seed morphology. Experimental crosses did not provide any indication of reproductive barriers between the two races ? progenies from inter-racial crosses did not have lower fitness than progenies from intra-racial crosses. Experimental crosses indicated that inbreeding depression may be occurring in S. latifolia populations ? which is consistent with the weedy habitat of S. latifolia in C and N Europe ? and suggest that there may be significant levels of genetic differentiation within S. latifolia. Silene latifolia and S. dioica belong to Silene section Elisanthe, which also includes three endemic species: S. diclinis, S. marizii and S. heuffelii. Relationships between the species in section Elisanthe were investigated using AFLPs, non-coding cpDNA sequences and ITS sequences. Analysis of cpDNA sequences grouped individuals from the endemic species with the corresponding geographic group of S. latifolia and S. dioica individuals. This geographic grouping could be due to chloroplast introgression or recent, local evolution of the endemic species. AFLPs and ITS sequences gave conflicting perspectives on the relationships of the endemic species to S. latifolia and S. dioica. Silene noctiflora, which was previously included in section Elisanthe, was distinct from the other section Elisanthe species in the section in analyses of AFLPs, non-coding cpDNA sequences and ITS sequences.
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