Cytogenetic and molecular studies of Dahlia Cav. species and hybrids.

Abstract

The genus Dahlia Cav. (Asteraceae, Heliantheae-Coreopsidinae) contains a range of chromosome numbers, with x=16, 17 and 18. Observations on the chromosomes of species of Dahlia based on x=16 showed that some have 2n=32, others 2n=64,with a third group having both chromosome numbers in the same taxon. Karyotype investigations showed that the chromosomes can be divided into groups of 14 metacentrics plus two different sized submetacentrics per set of l6 chromosomes. Meiotic analysis at metaphase I and pachytene,by synaptonemal complex spreading, showed that the 2n=32 species have exclusive bivalent formation whereas the 2n=64 species have small numbers of univalents plus quadrivalents but that the majority of configurations are bivalents. This study proposes that Dahlia species with 2n=32 are allotetraploids whereas those species and chromosome races with 2n=64 are their autoploid derivatives. The presence of a bivalent-promoting mechanism in the 2n=32 species may account for their meiotic behaviour, as their component genomes appear so similar, and it is suggested that this mechanism is also responsible for the low number of quadrivalents in the 2n=64 taxa.