Longiflorum and Asiatic lilies of the genus Lilium of the family Liliaceae are two important groups of modern lily cultivars.One of the main trends of lily breeding is to realize introgression between these groups.With cut style pollination and embryo rescue,distant hybrids between the two groups have been obtained.However,the F1 hybrids are highly sterile or some of them could produce a small number of 2n gametes,and their BC1 progenies are usually triploids.Dutch lily breeders have selected many cultivars from these BC1 progenies based on their variation.It is presumably suggested that such variation could be caused by intergenomic recombination and abnormal meiosis during gamete formation in F1 hybrids of Longiflorum × Asiatic(LA) hybrids in Lilium.Therefore,the meiotic process of ten F1 LA hybrids was cytologically investigated using genomic in situ hybridization and traditional cytological methods in the present research.The results showed that:at metaphase I,the homoeologous chromosome pairing among different F1 hybrids ranged from 2.0 to 11.4 bivalents formed by homoeologous chromosomes per pollen mother cell(PMC),and very few multivalents,and even very few bivalents were formed by two chromosomes within one genome rather than homoeologous chromosomes in some PMCs;at anaphase I,all bivalents were disjoined and most univalents were divided.Both the disjoined bivalents(half-bivalents) and the divided univalents(sister chromatids) moved to the opposite poles,and then formed two groups of chromosomes;because the two resulting half-bivalents retained their axes in the cell undisturbed,many crossover types,including single crossovers,three strand double crossovers,four strand double crossovers,four strand triple crossovers,and four strand multiple crossovers between the non-sister chromatids in the tetrads of bivalents,were clearly inferred by analyzing the breakpoints on the disjoined bivalents.The present investigation not only explained the reason for sterility of the F1 LA hybrids and the variation of their BC1 progenies,but also provided a new method to analyze crossover types in other F1 interspecific hybrids as well. Longiflorum and Asiatic lilies of the genus Lilium of the family Liliaceae are two important groups of modern lily cultivars. One of the main trends of lily breeding is to realize introgression among these groups. Witch cut style pollination and embryo rescue, distant hybrids between the two groups have been obtained. Host, the F1 hybrids are highly sterile or some of them could produce a small number of 2n gametes, and their BC1 progenies are usually triploids. Dutch lily breeders have selected many cultivars from these BC1 progenies based on their variation. It is presumably suggested that such variation could be caused by intergenomic recombination and abnormal meiosis during game formation formation in F1 hybrids of Longiflorum × Asiatic (LA) hybrids in Lilium.Therefore, the meiotic process of ten F1 LA hybrids was cytologically investigated using genomic in situ hybridization and traditional cytological methods in the present research. The results showed that: at metaphase I, the homoeologous chr omosome pairing among different F1 hybrids ranged from 2.0 to 11.4 bivalents formed by homoeologous chromosomes per pollen mother cell (PMC), and very few multivalents, and very few fewvalents were formed by two chromosomes within one genome rather than homoeologous chromosomes in some PMCs; at anaphase I, all bivalents were disjoined and most univalents were divided.Both the disjoined bivalents (half-bivalents) and the divided univalents (sister chromatids) moved to the opposite poles, and then formed two groups of chromosomes; because the twotage half -bivalents retained their axes in the cell undisturbed, many crossover types, including single crossovers, three strand double crossovers, four strand double crossovers, four strand triple crossovers, and four strand multiple crossovers between the non-sister chromatids in the tetrads of bivalents, were clearly inferred by analyzing the breakpoints on the disjoined bivalents. the present investigation not only explained the reason for sterilityof the F1 LA hybrids and the variation of their BC1 progenies, but also provided a new method to analyze crossover types in other F1 interspecific hybrids as well.