Homoeologous chromosome pairing in the distant hybrid Alstroemeria aurea xA-inodora and the genome composition of its backcross derivatives determined by fluorescence in situ hybridization with species-specific probes

A distant hybrid between two diploid species (2n = 2x = 16), Alstroemeria a urea and A. inodora, was investigated for homoeologous chromosome pairing, crossability with A. inodora and chromosome transmission to its BC1 offspri ng. Fluorescence in situ hybridization (FISH) with two species-specific pro bes, 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 BC1 progeny plants. High frequencies of associated chromosomes were obs erved in both genotypes of the Fl hybrid, A1P2-2 and A1P4. In the former, b oth 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 establish ed that hybrid A1P4 contains a reciprocal translocation between the short a rm 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 o ne of the parental species, A. inodora. Two days after pollination, embryo rescue was applied and, eventually, six BC1 progeny plants were obtained. A mong these, two were aneuploids (2n = 2x+1 = 17) and four were triploids (2 n = 3x = 24). The aneuploid plants had originated when the interspecific hy brid 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, re sulting in the four triploid BC1 plants. Of these four plants, three had re ceived 2n pollen grains from the hybrid and one a 2n egg. Using FISH, homoe ologous crossing over between the chromosomes of the two parental species i n the hybrid was clearly detected in all BC1 plants. The relevance of these results for the process of introgression and the origin of n and 2n gamete s are discussed.