Mitotic and meiotic polyploidization in lily hybrids for transferring Botrytis resistance

In an effort to transfer Botrytis resistance from Oriental lilies to Asiatic hybrids (Lilium, 2n=2x=24) we made a large number of F1 hybrids between these two distantly related species. Because these species belong to two different taxonomic sections, the F1 hybrids were totally sterile and could not be directly used in breeding. Therefore, two approaches were used for utilizing the F1 hybrids. First, the somatic chromosome number of the F1s was doubled by treating with oryzalin that resulted in allotetraploids (mitotic doubling). These allotetraploids were used for crossing with the parents. Second, 2n gametes were used directly for crossing with the parents for producing sexual polyploids (meiotic doubling). The two types of BC1 progenies were monitored for resistance against Botrytis elliptica through a ¿leaf tip test¿. Disease severity was evaluated on a nominal scale, ranging from 1 resistant (no lesions) to 6 (a high degree of necrosis with mycelium or even spores). In both populations the resistance varied from very susceptible to highly resistant. However, the occurrence of transgression of resistance, meaning that the degree of resistance in some seedlings exceeded that of the parent, was higher in meiotically doubled polyploids as compared to those derived from mitotic doubling. This was explained from the fact that the typical allotetraploids produce uniformly a single genotype of 2x gametes containing both parental genomes because of autosyndetic pairing so that there is no scope for genetic variation. On the contrary, in the case of meiotic polyploids intergenomic recombination occurs between the alien chromosomes that could lead to considerable amount of genetic variation. This phenomenon might be an explanation for the observed transgression of Botrytis resistance in the meiotic polyploid progenies.