Allopolyploid origin and genome differentiation of the parasitic species (Convolvulaceae) revealed by genomic in situ hybridization.

Interspecific hybridization and genome duplication to form allopolyploids are major evolutionary events in angiosperms. In the parasitic genus (Convolvulaceae), molecular data suggested the existence of species of hybrid origin. One of them, , has been proposed as a hybrid between and , both included in sect. . To test this hypothesis, a cytogenetic analysis was performed with CMA/DAPI staining and fluorescent in situ hybridization using 5S and 35S rDNA and genomic probes. Chromosomes of were small with a well-defined centromeric region, whereas had larger, densely stained chromosomes, and less CMA heterochromatic bands. had 2 = 60 chromosomes, about 30 of them similar to those of and the remaining to . GISH analysis confirmed the presence of both subgenomes in the allotetraploid . However, the number of rDNA sites and the haploid karyotype length in were not additive. The diploid parentals had already diverged in their chromosomes structure, whereas the reduction in the number of rDNA sites more probably occurred after hybridization. As phylogenetic data suggested a recent divergence of the progenitors, these species should have a high rate of karyotype evolution.