Overcoming sterility and unilateral incompatibility of Solanum lycopersicum x S-sitiens hybrids

The wild tomato relative Solanum sitiens is a rare endemic plant of the Atacama Desert of Chile. This species exhibits unique traits-tolerance to drought, salinity and low temperatures, resistances to certain pathogens, and modified fruit ripening-that could be of interest to tomato breeders. However, strong reproductive barriers, including hybrid inviability, sterility, and unilateral incompatibility (UI), prevent or impede introgression with cultivated tomato. Using embryo culture, we obtained a large number of F-1 S. lycopersicum x S. sitiens hybrids, and evaluated their fertility and compatibility as male or female parents. Diploid hybrids were functionally male-sterile and strongly rejected pollen of S. lycopersicum by UI. Chromosome doubling produced fertile allotetraploids, from which allotriploids (two genomes of S. lycopersicum, one of S. sitiens) were derived by crossing to diploid tomato. Rejection of tomato pollen occurred lower in styles of allotriploid than diploid hybrids. The pollen compatibility factors ui1.1 and ui6.1 were not sufficient for compatibility on S. sitiens allotriploid hybrids, unlike comparable hybrids with S. lycopersicoides. Pollen from F-1 S. lycopersicum x S. pennellii was used to overcome UI on pistils of diploid S. lycopersicum x S. sitiens hybrids. This cross yielded progeny that were fertile and compatible in direct backcrosses to S. lycopersicum. These results establish a feasible method for transferring the S. sitiens genome into cultivated tomato.