Repetitive DNA Dynamics and Polyploidization in the Genus Nicotiana (Solanaceae)

Large variations in genome size are observed in angiosperms as a result of whole-genome duplications and the balance between amplification and deletion of repetitive DNA, together explaining the observed variation in plant genome size. In the genus Nicotiana, there are 42 cytogenetically diploid species that have been classified into eight sections. There are also six allopolyploid Nicotiana sections that have evolved from species in different diploid sections. The phylogenetic relationships among these Nicotiana species, along with recurrent polyploidization events, permits the divergence of repetitive content in both diploid and allopolyploid genomes to be compared through evolutionary time. In this chapter, we review genome size variation in Nicotiana that reveals both genome upsizing and genome downsizing in different polyploid species. We discuss the divergence of specific repetitive elements, including tandemly repeated satellite DNAs, retroelements, and intergenic spacers as well as the sub-repeats contained in 35S rDNA. The lag-phase hypothesis, which describes post-polyploid radiations, is posed as a potentially important mechanism of evolution in Nicotiana section Suaveolentes, the largest polyploid section that consists of over half the current species diversity.