The Impact of Polyploidy on Grass Genome Evolution

Polyploidy is an evolutionary process whereby two or more genomes are brought together into the same nucleus, usually by hybridization followed by chro- mosome doubling. As a result, the new polyploid is genetically isolated from its diploid progenitor(s) and a new species is formed. The importance of polyploidy was recognized early in the 20th century and for the past decades many studies have ad- dressed the different categories of polyploids, their mode of formation, their cytogenetic behavior, the ecological implications, the impact of polyploidy on various population genetics aspects such as heterozy- gosity, mating mode, buffering of mutations, etc. (Stebbins, 1950, 1971). In the past few years, molec- ular and computational tools have provided new ways to probe the history of genomes, leading to the discovery that polyploidy is even more widespread than previously thought. The once controversial proposal that evolution moves forward through whole genome duplication (Ohno, 1970) is gaining recognition thanks to se- quence analysis that is more sensitive than the tradi- tional methods (chromosome counting, analysis of meiotic chromosome pairing, and marker-based map- ping) that have been used to assess polyploidy. Accu- mulating evidence shows that genome duplications occurred in the lineage of all vertebrates (Wolfe, 2001,