With the aim of utilizing allotriploid (2n = 3x = 36) lily hybrids (Lilium) in introgression breeding, different types of crosses were made. First, using diploid Asiatic lilies (2n = 2x = 24), reciprocal crosses (3x − 2x and 2x − 3x) were made with allotriploid hybrids (AOA) obtained by backcrosses of F 1 Oriental × Asiatic hybrids (OA) to Asiatic cultivars (A). Secondly, the AOA allotriploids were crossed with allotetraploid (OAOA, 2n = 4x = 48), in 3x − 4x combination. Finally, the AOA allotriploids where crossed to 2n gamete producer F 1 OA hybrids (3x − 2x(2n)). Two types of triploids were used as parents in the different types of crosses, derived from: (a) mitotic polyploidization and (b) sexual polyploidization. Ploidy level of the progeny was determined by R. Barba-Gonzalez Centro de Investigación y Asistencia en Tecnología y, Diseño del Estado de Jalisco, A.C. Biotecnología Vegetal, Av. Normalistas #800, Colinas de la Normal, Guadalajara, Jalisco, C.P. estimating the DNA values through flowcytometry as well as chromosome counting. The aneuploid progeny plants from 3x − 2x and reciprocal crosses had approximate diploid levels and in 3x − 4x crosses and 3x − 2x (2n) the progeny had approximate tetraploid levels. Balanced euploid gametes (x, 2x and 3x) were formed in the AOA genotypes. Recombinant chromosomes were found in the progenies of all crosses, except in the case of 2x − 3x crosses through genomic in situ hybridization (GISH) analyses. Recombinant chromosomes occurred in the F 1 OA hybrid when the triploid AOA hybrid was derived through sexual polyploidization, but not through mitotic polyploidization with two exceptions. Those recombinant chromosomes were transmitted to the progenies in variable frequencies.

In an effort to transfer Botrytis resistance from Oriental lilies to Asiatic hybrids (Lilium, 2n=2x=24) we made a large number of F 1 hybrids between these two distantly related species. Because these species belong to two different taxonomic sections, the F 1 hybrids were totally sterile and could not be directly used in breeding. Therefore, two approaches were used for utilizing the F 1 hybrids. First, the somatic chromosome number of the F 1 s 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 BC 1 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.

With the aim of transferring important horticultural and disease resistance characters that are available in different Lilium species (2n=2x=24) into the cultivars of lilies, an extensive interspecific hybridization program is in progress at the Plant Research International, Wageningen. In this context, we have made thousands of interspecific hybrids, and investigated them for the occurrence of 2n gametes. These sexual polyploid progenies that were produced were cytologically analyzed through genomic in situ hybridization (GISH). Out of the five different sets of interspecific hybrids that produce 2n-gametes, the following four hybrids involved species that belonged to different taxonomic sections: L. longiflorum x Asiatic hybrids; L. longiflorum x L. henryi; Oriental x Asiatic hybrids and L. auratum x L. henryi. One was an intra-sectional species hybrid involving the cultivar 'Enchantment' x L. pumilum. In all the four intersectional hybrids meiosis was highly irregular during meiosis and produced almost exclusively first division restitution (FDR) gametes. On the contrary, the single hybrid, 'Enchantment' x L. pumilum had normal chromosome pairing and produced only second division restitution (SDR) gametes. Although the frequency of 2n-gamete production in all cases was greatly influenced by the environment, it was possible to produce backcross progenies in all cases. Analyses of backcross progenies through GISH proved that intergenomic recombination occurred in the case of FDR gametes that were produced in all the four interspecific hybrids. Furthermore, analyses of BC 2 and BC 3 progenies in some cases proved that recombinant chromosomes were transmitted normally. These investigations have shown that meiotic polyploidization is highly promising for lily breeding.