Backcross progenies, BC1 and BC2, derived from crossing a hexaploid potato (+) tomato fusion hybrid to tetraploid potato, were monitored for the presence of alien tomato chromosomes. For discriminating the tomato chromosomes from those of potato, both mitotic and meiotic chromosome preparations were labelled differentially through total genomic DNA in situ hybridization (GISH). The presence or absence of individual tomato chromosomes in the BC progeny was identified through RFLP analyses using chromosome-specific DNA probes. The results indicated that the BC 1 plant possessed only nine chromosomes of tomato instead of the expected haploid set of 12. Of the nine persistent tomato chromosomes, there were three pairs that formed bivalents and three singles that formed univalents at meiosis. One of the single chromosomes was strikingly longer as a result of some structural mutation and could easily be identified in the hexaploid parent, the BC1 and in some of the BC2 plants. From the analyses of BC2 progeny there were indications of the presence in the BC1 of tomato chromosomes 1, 3 and 6 in duplicate, and chromosomes 8, 9 and 10 in the haploid condition. The number of tomato chromosomes transmitted to BC2 plants varied from one to six. The results of GISH revealed that a few BC2 plants possessed more tomato chromosomes than those deduced from RFLP analysis. This indicated the presence of some tomato chromosomes in duplicate and others in the haploid condition. The potential value of these genotypes for establishing potato lines with additions and substitutions of tomato chromosomes is discussed.

Fluorescence in situ hybridization (FISH) was used to localise two species-specific repetitive DNA sequences, A001-I and D32-13, and two highly conserved 25S and 5S rDNA sequences on the metaphase chromosomes of two species of Alstroemeria. The Chilean species, Alstroemeria aurea (2n = 16), has abundant constitutive heterochromatin, whereas the Brazilian species, Alstroemeria inodora, has hardly any heterochromatin. The A. aurea specific A001-I probe hybridized specifically to the C-band regions on all chromosomes. The FISH patterns on A. inodora chromosomes using species-specific probe D32–13 resembled the C-banding pattern and the A001-I pattern on A. aurea chromosomes. There were notable differences in number and distribution of rDNA sites between the two species. The 25S rDNA probe revealed 16 sites in A. aurea that closely colocalised with A001-I sites and 12 in A. inodora that were predominantly detected in the centromeric regions. FISH karyotypes of the two Alstroemeria species were constructed accordingly, enabling full identification of all individual chromosomes. These FISH karyotypes will be useful for monitoring the chromosomes of both Alstroemeria species in hybrids and backcross derivatives.Key words: Alstroemeria, fluorescence in situ hybridization, FISH, repetitive DNA, ribosomal DNA, karyotype.

A distant hybrid between two diploid species (2n = 2x = 16), Alstroemeria aurea and A. inodora, was investigated for homoeologous chromosome pairing, crossability with A. inodora and chromosome transmission to its BC 1 offspring. Fluorescence in situ hybridization (FISH) with two species-specific probes, A001-I (A. aurea specific) and D32-13 (A. inodora specific), was used to analyse chromosome pairing in the hybrid and the genome constitution of its BC 1 progeny plants. High frequencies of associated chromosomes were observed in both genotypes of the F 1 hybrid, A1P2-2 and A1P4. In the former, both univalents and bivalents were found at metaphase I, whereas the latter plant also showed tri-and quadrivalents. Based on the hybridization sites of DNA probes on the chromosomes of both parental species, it was established that hybrid A1P4 contains a reciprocal translocation between the short arm of chromosome 1 and the long arm of chromosome 8 of A. inodora. Despite regular homoeologous chromosome pairing in 30% of the pollen mother cells, both hybrids were highly sterile. They were backcrossed reciprocally with one of the parental species, A. inodora. Two days after pollination, embryo rescue was applied and, eventually, six BC 1 progeny plants were obtained. Among these, two were aneuploids (2n = 2xǹ1 = 17) and four were triploids (2n = 3x = 24). The aneuploid plants had originated when the interspecific hybrid was used as a female parent, indicating that n eggs were functional in the hybrid. In addition, 2n gametes were also functional in the hybrid, resulting in the four triploid BC 1 plants. Of these four plants, three had received 2n pollen grains from the hybrid and one a 2n egg. Using FISH, homoeologous crossing over between the chromosomes of the two parental species in the hybrid was clearly detected in all BC 1 plants. The relevance of these results for the process of introgression and the origin of n and 2n gametes are discussed.

Chromosome numbers were determined in metaphase complements of root-tip meristems of 107 tomato (+) potato somatic hybrids, obtained from five different combinations of parental genotypes. Of these hybrids 79% were aneuploid, lacking one or two chromosomes in most cases. All four hybrids that were studied at mitotic anaphase of root tips showed laggards and bridges, the three aneuploids in a higher frequency than the single euploid. Hybrid K2H2-1C, which showed the highest percentage of aberrant anaphases, possessed 46 chromosomes. Fluorescence in situ hybridization with total genomic DNA showed that this hybrid contained 23 tomato, 22 potato, and 1 recombinant chromosome consisting of a tomato chromosome arm and a potato chromosome arm. The potato parent of K2H2-1C was aneusomatic in its root tips with a high frequency of monosomic and trisomic cells and a relatively high frequency of cells with one fragment or telosome. Meiotic analyses of three tomato (+) potato somatic hybrids revealed laggards, which occurred most frequently in the triploid hybrids, and bridges, which were frequently present in pollen mother cells (PMCs) at anaphase I of hypotetraploid K2H2-1C. We observed putative trivalents in PMCs at diakinesis and metaphase I of eutriploid A7-82A and quadrivalents in part of the PMCs of hypotetraploid K2H2-1C, suggesting that homoeologous recombination between tomato and potato chromosomes occurred in these hybrids. All three hybrids showed a high percentage of first division restitution, giving rise to unreduced gametes. However, shortly after the tetrad stage all microspores completely degenerated, resulting in exclusively sterile pollen.

Chromosome association and chiasma formation were studied in pollen mother cells at metaphase I of four allotriplod BC1 plants (2n ¼ 3x ¼ 24) obtained from the backcross of the hybrid Alstroemeria aurea  A. inodora with its parent A. inodora. We distinguished the chromosomes of both parental species by genomic in situ hybridization (GISH), whereas the individual chromosomes were identified on the basis of their multicolour FISH banding patterns obtained after a second hybridization with two species-specific satellite repeats as probes. All the four BC1 plants possessed two genomes of A. inodora and one of A. aurea. Variable numbers of recombinant chromosomes, resulting from meiotic recombination in the interspecific hybrid, were present in these plants. The homologous A. inodora chromosomes generally formed bivalents, leaving the homoeologous A. aurea chromosomes unassociated. High frequencies of trivalents were observed for the chromosome sets that contained recombinant chromosomes, even when the recombinant segments were small. Chromosome associations in the trivalents were restricted to homologous segments. The implications of the absence of homoeologous chromosome pairing on gamete constitution and prospects for introgression in Alstroemeria are discussed.