Clues to the evolution of genome size and chromosome number in Tabebuia alliance (Bignoniaceae)

Abstract

The 82 genera recognized for Bignoniaceae are organized in eight major tribes, Jacarandeae, Tourrettieae, Tecomeae, Coleeae, Crescentiae, Oroxyleae, Catalpeae and Bignonieae. In addition, a monophyletic group, Crescentiina, including the Tabebuia alliance and the Paleotropical clade is recognized. The Tabebuia alliance, endemic to the Neotropics, includes c. 14 genera of trees and shrubs and is comprised of c. 147 species mainly of the genera Tabebuia and Handroanthus. We measured the nuclear DNA contents of 19 Bignoniaceae species to address genome size and chromosome number evolution of the Tabebuia alliance. Nuclear DNA content was estimated using flow cytometry and chromosome counts were performed using propidium iodide-stained metaphase cells. We also obtained data for three different Bignoniaceae species from the Royal Botanical Garden, Kew Plant DNA C-values database. We obtained the phylogenetic tree of analysed species from published data and used independent phylogenetic contrast to correlate chromosome number and 1C DNA content. An almost threefold range of variation in nuclear DNA content was observed among the 22 Bignoniaceae species that could only partially be explained by the occurrence of polyploidy. We found no significant correlation between chromosome number and DNA content among the analysed species. Our results suggest that both increases and decreases of genome size occurred in the evolution of the analysed Bignoniaceae Juss. species, with a large variation among Tabebuia alliance species.