Genome size and karyotype evolution in Tradescantia section Cymbispatha (Commelinaceae)

Abstract

Karyotype morphology and nuclear DNA content are compared in species of Tradescantia section Cymbispatha from North and South America. Comparison of DNA amounts confirms the polyploid ancestry previously suggested for several cytotypes and shows that basic genome size has been unaffected by the Robertsonian fusions constituting a major evolutionary change in this group. The 2C-values of Mexican tetraploids are double those of their Central American diploid relative Tradescantia gracillima. The 2C-value of the South American tetraploid T. cymbispatha is comparatively lower as a result of a reduction in the size of some chromosomes. This reduction is most likely to have been achieved by Robertsonian fusions in a hybrid whose parental chromosome sets differed in size. The results provide evidence for selection of ancestral diploids with low C-value as mechanism for reduction of genome size in polyploids.Key words: Tradescantia, genome size, polyploidy, Robertsonian fusion, DNA reduction.