Polyploidy: genome obesity and its consequences.

Polyploidy is a major evolutionary feature of many plants and some animals (Grant, 1981;Otto & Whitton, 2000). Allopolyploids (e.g. wheat, cotton, and canola) were formed by combination of two or more distinct genomes, whereas autopolyploids (e.g. potato, sugarcane, and banana) resulted from duplication of a single genome. Both allopolyploids and autopolyploids are prevalent in nature (Tate et al., 2004). Recent research has shown that polyploid genomes may undergo rapid changes in genome structure and function via genetic and epigenetic changes (Fig. 1) (Levy & Feldman, 2002;Osborn et al., 2003;Chen, 2007). The former include chromosomal rearrangements (e.g. translocation, deletion, and transposition) and DNA sequence elimination and mutations, whereas epigenetic modifications (chromatin and RNAmediated pathways) give rise to gene expression changes that are not associated with changes in DNA sequence. Over time, polyploids may become ‘diploidized’ so that they behave like diploids cytogenetically and genetically. Comparative and genome sequence analyses indicate that many plant species, including maize, rice, poplar, and Arabidopsis, are recent or ancient diploidized (paleo-) polyploids.