Genomic change and gene silencing in polyploids.

Combining the genomes of two species through hybridization and chromosome doubling can create a new allopolyploid species virtually overnight. Although common in Nature, such genetic mergers might not be easy. Recent studies, mostly in plants, suggest that polyploidization can induce a flurry of genetic and epigenetic events that include DNA sequence elimination and gene silencing. Without warning, in through the front door of your little house comes the family of a distant relative, toting all their belongings and unleashing their unruly dogs into your tidy dwelling. And they've come to stay. Surprise! The premise for a new television comedy? Possibly; but in this case, an analogy for those fertilization events in Nature that bring together related, but not identical, nuclear genomes to create an interspecies hybrid. The success and permanence of these sudden genetic mergers is hardly guaranteed. Unlike hybrids formed by crossing two members of the same species, interspecific diploid hybrids are typically evolutionary dead-ends (e.g. mules, the sterile hybrids of horses and donkeys). Differences in chromosome number or organization tend to disrupt chromosome pairing and assortment during meiosis, yielding defective gametes and hence sterility. Doubling the chromosomes of each parent, either before or after the hybridization event, can overcome the problem by providing each chromosome with a precise pairing partner, thus allowing fertility and persistence of the hybrid (Fig. 1a). The formation of such polyploid hybrids, known as allopolyploids, is a powerful evolutionary process in the creation of new species, especially in plants, having produced such familiar crops as cotton and bread wheat 1–5. Despite their success in Nature, the initial formation of an allopolyploid must be a shock. The resulting crowded genome, like a dwelling made cramped by relatives who will not leave, presents a number of challenges, not the least of which is the potential for conflict because of different instructions being given for the same tasks. Also, transposable elements, the troublesome hounds of the genome, are often unleashed in new hybrids (for reasons that are not yet clear) and can roam their new environment causing damage. And, swapping partners when it's time to pair up can only bring trouble and strife (Fig. 1b). Evidence is emerging that new allopolyploids might deal with these challenges by silencing some of the redundant 'chatter' and by finding ways to reduce the incidence of chromosomal infidelity. A record of success Polyploidy can result from doubling a single species' genome …