HOMOEOLOGOUS PAIRING AND RECOMBINATION IN BACKCROSS DERIVATIVES OF TOMATO SOMATIC HYBRIDS [LYCOPERSICON ESCULENTUM(-PERUVIANUM]() LYCOPERSICON)

Genomic in situ hybridization (GISH) was used to examine genome intera ctions in two allohexaploid (2n=6x=72) Lycopersicon esculentum (+) L. peruvianum somatic hybrids and their seed progenies originated from su bsequent backcrosses to L. esculentum. The ability of GISH to distingu ish between chromatin derived from two closely related species, L. esc ulentum and L. peruvianum (both 2n=2x=24), allowed the precise chromos omal constitution of somatic hybrids and their backcross progenies to be unequivocally established. This enabled the interaction of species genomes to be observed at meiosis, providing clear evidence of strictl y regular homoeologous pairing and the high degree of homoeologous rec ombination in allodiploid plants (2n=2x=24) of the BC1 generation. In segmental allodiploids of the BC2 and BC3 generations, the recombinant chromosomes continued to pair with a homoeologous partner (in the abs ence of a homologous one), and therefore could be stably incorporated into gametes. Chiasmata were found almost exclusively in more distal, rather subterminal, chromosome segments. A considerable proportion of meiotic recombination was detected in subterminal heterochromatic regi ons, often involving distal euchromatin, located in close proximity. G ISH also supplied information on the extent of the overall sequence ho mology between the genomes of L. esculentum and L. peruvianum, indicat ing that despite their different breeding systems, these species may n ot be differentiated to a high degree genetically. The present study h as demonstrated that somatic hybridization between two such closely re lated, but sexually incompatible or difficult to cross species, provid es a way of transferring genes, via homoloeogous crossing-over and rec ombination, across the incompatibility barriers. Indeed, such hybrids may offer the preferred route for gene transfer, which subsequently re sults in more stable gene introgression than other methods.