For overcoming F1-sterility in interspecific hybrids, mitotic and meiotic polyploidisation is applied in lily and can result in fertile allopolyploids. The mechanism of viable pollen production of mitotic and meiotic polyploidisation is quite different. Mitotic polyploids are obtained by artificial chromosome doubling and results in normal pairing of homologous chromosome set, which enables the formation of 2x gametes during the meiosis. Meiotic polyploidisation, on the other hand, leads in

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... cases of restitutional chromosome division, to the formation of unreduced gametes (2n gametes). In contrast to mitotic doubling, homoeologous recombination can occur in these gametes. Genomic in situ hybridization (GISH) was used to discriminate parental chromosomes in the F1 hybrids and backcross progenies to detect homoeologous recombination. Mitotic polyploidisation showed no homoeologous recombinations between the parental genomes whereas in meiotic polyploids it was detected in a high frequency. The use of 2n gametes is therefore the most promising approach for the introgression of desirable characters in using interspecific hybrids in breeding. In both cases, the frequency of viable gametes appeared to be low and limited to a few F1-hybrids. A new and promising method is developed in which 2ngametes can be induced by the application of laughing gas (N 2 O). This method proved to be successful in lily and the occurrence of homoeologous recombination, a characteristic of meiotic polyploidisation, was detected. In tulip where the production of mitotic polyploids requires at least five years, it will speed up breeding with interspecific hybrids enormously.