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Karyotype evolution can be reconstructed by the characterization of chromosome morphology, based on the position of centromeres. Different karyotypes often reflect speciation events within phylogenies as they can establish crossing barriers between species. Hence, evolution of karyotypes often is congruent with splits and differentiation of clades within phylogenies of angiosperm genera. Here we study karyotype evolution in the big cosmopolitan genus Ranunculus and in related genera to test the hypothesis that karyotypes are congruent with major clades. We investigated karyotypes on mitoses of 36 species, evaluated literature records for additional 87 species, and reconstructed ancestral states by mapping karyotypes onto a published molecular phylogenetic tree. Altogether ten karyotypes can be discriminated as character states, eight based on the base number x = 8, and two on the base number x = 7. The ancestral type within Ranunculus is characterized by four metacentric and four submetacentric/subtelocentric chromosomes, and dominates in five major basal clades of Ranunculus (R. subg. Auricomus) and in four of the related genera (Coptidium, Halerpestes, Kumlienia, and Trautvetteria). Another four karyotypes are ancestral and predominant in two major clades within Ranunculus (R. subg. Ranunculus), while three further ones occur only on terminal branches of the buttercup phylogeny. Among related genera, Ficaria and Ceratocephala also show derived karyotypes. Karyotype morphology thus supports generic and infrageneric classifications based on molecular and morphological data. Karyotype evolution drives in general to increasing number of chromosomes with asymmetric arms, and to a reduction from x = 8 to x = 7 chromosomes. A review of interspecific homoploid hybridization in sympatric species, and of crossing experiments suggests enhanced crossability of species with the same karyotype and strong crossing barriers between those with different karyotypes. We conclude that karyotype evolution is a major driver of speciation and differentiation of clades within Ranunculus. ISSN:1433-8319 ISSN:1618-0437

Bauert M. R., Kälin M., Edwards P. J. and Baltisberger M. 2007. Genetic structure and phylogeography of alpine relict populations of Ranunculus pygmaeus and Saxifraga cernua. Bot. Helv. 117: 181-196. Ranunculus pygmaeus and Saxifraga cernua are arctic-alpine species with similar disjunct distributions: both occur as small, isolated relict populations in the Alps, while they are more widespread in the Arctic. To improve our understanding of their glacial and postglacial history, we investigated their genetic diversity within and among populations collected in the Alps and in the Arctic using 80 RAPD primers. We found only two genotypes of R. pygmaeus, one in the Alps and one in the Arctic. The absence of genetic diversity within each region is probably the consequence of postglacial colonization from a single source, followed by inbreeding in very small populations. In S. cernua, we found six genotypes among 11 populations in the Alps but no genetic variation within alpine populations. This limited genetic variation probably results from an extreme reduction and fragmentation of populations during successive glacial periods. In contrast, there was a high level of genetic variation both among and within all arctic populations of S. cernua. We suggest that this arose at least partly through the immigration of plants from multiple sources after the last glaciation. The higher genetic diversity of S. cernua compared to R. pygmaeus might also be related to their contrasting breeding systems: R. pygmaeus is an inbreeding diploid which propagates only by seeds, while S. cernua is a clonal polyploid which propagates mainly by vegetative means. Clonal growth, by prolonging the life span of a genotype, might contribute to the maintenance of genetic diversity under conditions which are difficult for sexual reproduction and seedling establishment