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An artificial crossing was made between four Viola xwittrockiana (2n = 48) lines and one Viola cornuta inbred (2n = 26) line as well as among V. Xwittrockiana lines to investigate the cytologic characteristics, fertility, and heteroses of hybrids. The sizes of flowers and stomata and the leaf areas of the V. Xwittrockiana plants were larger than those of the V. cornuta plants. The karyologic stability of V. cornuta was greater than that of V. Xwittrockiana, and ≈20% of individuals in the latter deviated from the given 2n = 48. Capsule setting after interspecies hand pollination ranged from 28% to 94%, and F1 seed germination ranged from 16% to 88%, revealing no obvious pre- or postzygotic selection mechanisms. The chromosome numbers in V. Xwittrockiana x V. cornuta F1 seedlings varied (2n = 34-38), with about half the cells exhibiting the intermediate (2n = 36) value. These interspecific hybrids exhibited positive heterosis in flower size (range, 11% to 66%) and negative heterosis in plant height (range, -12% to -57%). Both of these traits are desirable in pansy breeding programs. [ABSTRACT FROM AUTHOR]

Background and Aims Since the advent of molecular phylogenetics, numerous attempts have been made to infer the evolutionary trajectories of chromosome numbers on DNA phylogenies. Ideally, such inferences should be evaluated against cytogenetic data. Towards this goal, we carried out phylogenetic modelling of chromosome number change and fluorescence in situ hybridization (FISH) in a medium sized genus of Araceae to elucidate if data from chromosomal markers would support maximum likelihood-inferred changes in chromosome numbers among close relatives. Typhonium, the focal genus, includes species with 2n = 65 and 2n = 8, the lowest known count in the family. Methods A phylogeny from nuclear and plastid sequences (96 taxa, 4252 nucleotides) and counts for all included species (15 of them first reported here) were used to model chromosome number evolution, assuming discrete events, such as polyploidization and descending or ascending dysploidy, occurring at different rates. FISH with three probes (5S rDNA, 45S rDNA and Arabidopsis-like telomeres) was performed on ten species with 2n = 8 to 2n = 24. Key Results The best-fitting models assume numerous past chromosome number reductions. Of the species analysed with FISH, the two with the lowest chromosome numbers contained interstitial telomeric signals (Its), which together with the phylogeny and modelling indicates decreasing dysploidy as an explanation for the low numbers. A model-inferred polyploidization in another species is matched by an increase in rDNA sites. Conclusions The combination of a densely sampled phylogeny, ancestral state modelling and FISH revealed that the species with n = 4 is highly derived, with the FISH data pointing to a Robertsonian fusion-like chromosome rearrangement in the ancestor of this species. [ABSTRACT FROM PUBLISHER]