Evaluation of BC2 progenies derived from 3x-2x and 3x-4x crosses of Lilium hybrids: a GISH analysis

An allotriploid (ALA, 2n=3 x=36) BC(1) plant was obtained by backcrossing a diploid F(1) interspecific hybrid (LA, 2n=2 x=24), derived from a Lilium longiflorum (L genome) and an Asiatic hybrid (A genome), to the latter parent. This allotriploid was backcrossed to a diploid Asiatic hybrid (2n=2 x=24) and to an allotetraploid (LLAA, 2n=4 x=48) LA hybrid. A total of 25 plants of these crosses were examined for ploidy level, and 12 individuals were analyzed for their genome constitution through genomic in situ hybridization (GISH). In most cases the progenies from the triploid-diploid (3 x-2 x) crosses consisted of aneuploids. Further more, there was evidence for the formation of near-haploid (x=12+2) to triploid (3 x=36) gametes in the allotriploid BC(1) plant. The progenies of triploid-tetraploid (3 x-4 x) cross also consisted of mostly aneuploids but in this case the triploid female parent had contributed predominantly near-triploid (2n) gametes for the origin of BC(2) progenies. The different ploidy levels observed between 3 x-2 x and 3 x-4 x crosses are possibly caused by preferential fertilization or survival resulting in a different ratio of chromosome numbers between the embryo and endosperm. Though Lilium has a tetrasporic, eight-nucleate type of embryo sac formation (Fritillaria type), the observed difference between the progeny types in 3 x-2 x and 3 x-4 x crosses is comparable to that of observed in monosporic eight nucleate types (Polygonum type) that predominate in most genera of Angiosperms. An important feature of the genome constitution of the progenies was that the homoeologous recombinant chromosomes were transmitted intact from BC(1) to BC(2) progenies in variable numbers. In addition, there was evidence for the occurrence of new homoeologous recombinations in the triploid BC(1). Of the two euploid BC(2) plants one had originated through the parthenogenetic development of a 2n egg and the other had originated through indeterminate meiotic restitution (IMR).

Karyotype analysis of Lilium longiflorum and Lilium rubellum by chromosome banding and fluorescence in situ hybridisation

Detailed karyotypes of Lilium longiflorum and L. rubellum were constructed on the basis of chromosome arm lengths, C-banding, AgNO3 staining, and PI-DAPI banding, together with fluorescence in situ hybridisation (FISH) with the 5S and 45S rDNA sequences as probes. The C-banding patterns that were obtained with the standard BSG technique revealed only few minor bands on heterologous positions of the L. longiflorum and L. rubellum chromosomes. FISH of the 5S and 45S rDNA probes on L. longiflorum metaphase complements showed overlapping signals at proximal positions of the short arms of chromosomes 4 and 7, a single 5S rDNA signal on the secondary constriction of chromosome 3, and one 45S rDNA signal adjacent to the 5S rDNA signal on the subdistal part of the long arm of chromosome 3. In L. rubellum, we observed co-localisation of the 5S and 45S rDNA sequences on the short arm of chromosomes 2 and 4 and on the long arms of chromosomes 2 and 3, and two adjacent bands on chromosome 12. Silver staining (Ag-NOR) of the nucleoli and NORs in L. longiflorum and L. rubellum yielded a highly variable number of signals in interphase nuclei and only a few faint silver deposits on the NORs of mitotic metaphase chromosomes. In preparations stained with PI and DAPI, we observed both red- and blue-fluorescing bands at different positions on the L. longiflorum and L. rubellum chromosomes. The red-fluorescing or so-called reverse PI-DAPI bands always coincided with rDNA sites, whereas the blue-fluorescing DAPI bands corresponded to C-bands. Based on these techniques, we could identify most of chromosomes of the L. longiflorum and L. rubellum karyotypes.

Introgression of Lilium rubellum Baker chromosomes into L. longiflorum Thunb.: a genome painting study of the F1 hybrid, BC1 and BC2 progenies

Interspecific hybrids between Lilium longiflorum (L, 2n = 2x = 24) and Lilium rubellum (R, 2n = 2x = 24) were produced with the aim of transferring desirable horticultural traits from L. rubellum to L. longiflorum. All F1 hybrids (LR, 2n = 2x = 24) and BC1 individuals (LLR, 2n = 3x = 36) were phenotypically uniform for plant height, flowering time, leaf shape and flower colour. The BC1 plants were, in spite of their triploid nature, fertile and could be used as a female parent in backcrossings with autotetraploid L. longiflorum (LLLL, 2n = 4x = 48). Twelve BC2 individuals were obtained and three of them were selected for further chromosome analysis. As L. longiflorum and L. rubellum chromosomes were indistinguishable in the hybrids, genomic in-situ hybridization (GISH) was applied to establish the parentage of the chromosomes of the F1 hybrids and the BC1 and BC2 progenies. GISH confirmed the LLRR constitution of the doubled amphimonoploid (allodiploid), and the LLR constitution of all BC1 plants. The three selected BC2 plants were, as expected, aneuploid, containing three complete sets of L. longiflorum chromosomes and six, seven or eight L. rubellum chromosomes, respectively. However, L/R translocation or recombinant chromosomes could not be demonstrated in the mitotic metaphase complements of the F1, BC1 and BC2 plants. In spite of the high frequencies of homoeologous recombination in the F1 hybrids (LR) pollen was found to be sterile in all cases. At metaphase I of the pollen mother cells of the BC1 plants, genome painting did not reveal any cases of homoeologous pairing and recombination between L and R chromosomes. This lack of exchange between homoeologous chromosome segments indicates complete preferential pairing of the L and R chromosomes in the F1 (amphidiploid) and BC1 plants. It seems that the preferential pairing in the F1 and BC1 hybrids hinder the introgression of the chromosome segments or species-specific genes into the recipient for breeding purposes.

Homoeologous recombination in 2n-gametes producing interspecific hybrids of Lilium (Liliaceae) studied by genomic in situ hybridization (GISH)

Interspecific hybridization of Lilium longiflorum (L) with Asiatic (A) lily hybrids results in so-called LA-hybrids. Some of these hybrids produce 2n-pollen, which were used to perform crosses on Asiatic and Oriental (O) hybrids, resulting in ALA- and OLA-hybrids. Recombination between homoeologous chromosomes (introgression) and the mechanism of 2n-pollen formation in these hybrids were studied using genomic in situ hybridization (GISH). A clear differentiation between the chromosomes of L. longiflorum, Asiatic, and Oriental hybrids was observed in four ALA- and one OLA-hybrid using GISH. Two ALA-hybrids showed 3 and 5 recombinant chromosomes with a total of 5 and 10 crossover sites per hybrid, respectively. These occurred at random positions on the chromosomes. The number and the location of the rDNA-sites were determined using in situ hybridization and provided a tool, the FISH-marker, for identifying the NOR-bearing chromosomes in the lily hybrids. Evidence for the occurrence of the FDR-mechanism (first division restitution) of 2n-pollen formation in the LA-hybrids was obtained on the basis of absence of homologous chromosomes of L. longiflorum in the ALA- and OLA-hybrids.Key words: Lilium longiflorum, introgression, FDR, interspecific hybridization, FISH.

Somatic embryogenesis and plant regeneration in Lilium longiflorum Thunb

Friable callus was obtained from styles and flower pedicels of Lilium longiflorum Snow Queen and the Oriental lily hybrid Star Gazer on Murashige and Skoog (MS) media containing either 2 µM dicamba or 2 µM picloram. Cell suspension cultures were established by suspending the callus of L. longiflorum Snow Queen in liquid medium containing 2 µM dicamba. Through a purification process, a fine fast-growing cell suspension was obtained. This suspension was composed of a homogenous population of small dense cells, which tended to organise into embryo like structures (ELS). In liquid culture with the auxin dicamba, the ELS underwent continuous callus formation. When transferred to solidified hormone-free MS medium, the ELS germinated, forming complete plantlets. Histological investigation showed that in the ELS both shoot and root meristems were distinctly evident. It was concluded that the ELS obtained were in fact somatic embryos.