Formation of unreduced pollen in Arabidopsis thaliana.

Sexual polyploidization, as a result of 2n gamete formation, is thought to be the main process involved in the origin of polyploid plants (Bretagnolle & Thompson, 1995). These gametes, possessing the somatic rather than the gametophytic chromosome number, can be formed either during microsporogenesis (2n pollen, big pollen, diplandroidy) or megasporogenesis (2n eggs, diplogynoecy). In general, unreduced gametes are diploid; however, triploid and tetraploid gametes have been described on diploid sporophytes and referred to as 3n and 4n gametes, respectively (Veilleux, 1985).