Abstract
Background: Whole-genome duplications (WGD) have dominated the evolutionary history of plants. One consequence of WGD is a dramatic restructuring of the genome as it undergoes diploidization, a process under which rearrangements of various sizes scramble the genetic material, leading to a repacking of the genome. Here, we investigate the WGD history in the columbine genus Aquilegia, a basal eudicot, and trace the origins of its chromosomes, in particular the unique “nucleolus” chromosome 4 [1].
Results: Our results support the notion that basal eudicots share an ancient tetraploidy with core eudicots. Comparison to the grape genome reveals that the columbine genome has diverged relatively little from the ancestral tetraploid. As few as seven fusions and a single fission can explain the present-day karyotype of columbines. Unlike other columbine chromosomes of similar size, all of which are products of ancient fusion events, chromosome 4 appears to have reached its size due to an expansion of repetitive DNA, partly due to numerous segmental duplications.
Conclusions: The columbine genome provides an important insight into the karyotype evolution of basal eudicots, and also presents an unusual example of a chromosome that appears to decay almost as if it were a sex chromosome. More genomes from basal eudicots will be needed to understand eudicot karyotype evolution and the origins of this chromosome.