Issues of interspecific hybridization between Solanum lycopersicum L. and Solanum sisymbriifolium Lam

Interspecific hybridization plays a crucial role in tomato (Solanum lycopersicum L.) breeding for introducing beneficial traits from wild relatives, such as resistance to biotic and abiotic stress. Sticky nightshade (Solanum sisymbriifolium Lam.) is a promising source for the introgression of desirable traits. Reports on hybridization between S. lycopersicum and S. sisymbriifolium remain contradictory, differently describing the progeny as doubled haploids or interspecific hybrids. In the present study, we clarify the nature of the progeny derived from hybridization between S. lycopersicum and S. sisymbriifolium by analyzing the early stages of ovule development and characterizing the morphological and cytogenetic features of the resulting plants. Interspecific hybridization between S. lycopersicum and S. sisymbriifolium encounters postzygotic reproductive barriers. Only a small proportion of developing ovules in male-sterile tomato lines, whether self-pollinated or pollinated with S. sisymbriifolium followed the normal developmental pathway. In most cases, either ovule development was arrested, or parthenocarpic seed-like bodies formed instead of normal seeds. Embryo rescue enabled the recovery of 12 plants resulting from interspecific hybridization of S. lycopersicum and S. sisymbriifolium. Morphologically, the plants closely resembled the S. lycopersicum parent, although they displayed several traits distinctive from those of the parental tomato lines. Notably, yellow-orange mature fruits developed in progeny from the cross of green-fruited S. lycopersicum and red-fruited S. sisymbriifolium. Analysis of chromosome numbers in root meristems revealed mixoploidy (2n = 16-26), and meiotic analysis during microsporogenesis showed multiple aberrations in meiosis. Thus, comprehensive embryological, morphological, and cytogenetic analyses provide evidence for the hybrid origin of the obtained plants. This confirms the possibility of overcoming postzygotic barriers between these species and opens avenues for the utilization of S. sisymbriifolium in tomato breeding programs.