Polyploidy and Evolution in Wild and CultivatedDahliaSpecies

Observations on the chromosomes of nine species ofDahliaCav.(Asteraceae, Heliantheae—Coreopsidinae) show that somehave 2n=32, others 2n=64, with a third group having both chromosomenumbers in the same taxon. Karyotype investigations showed thatthe chromosomes can be divided into groups of 14 metacentricsplus two submetacentrics per set of 16 chromosomes.In situhybridizationusing an rRNA gene probe indicated that the 2n=32 species haveeight hybridization sites whilst the 2n=64 species have 16 sites.Silver nitrate staining of these regions showed that not allof these nucleolar organizers are active. Meiotic analysis atmetaphase I and pachytene, by synaptonemal complex spreading,shows that the 2n=32 species have exclusive bivalent formationwhereas the 2n=64 species have small numbers of univalents plusquadrivalents in addition to bivalents. This study proposesthatDahliaspecies with 2n=32 are allotetraploids whereas thosespecies and chromosome races with 2n=64 are their autopolyploidderivatives. We suggest that a bivalent-promoting mechanismin the 2n=32 species may account for their meiotic behaviouras their component genomes appear so similar, and that thismechanism is also responsible for the low number of quadrivalentsin the 2n=64 taxa.Copyright 1998 Annals of Botany Comapny Chromosome pairing,Dahlia, in situhybridization, karyotype analysis, polyploidy, synaptonemal complex analysis