Novel Variation in Tomato Species Hybrids

Several novel variants that appeared in F2 populations of hybrids between L. esculentum and the self-incompatible species L. peruvianum and L. chilense are described. The deviating phenotypic features of these variants are completely outside the normal range of variation of their respective F2 families and were not known to be present in the parental species. Tests of inheritance, where performed, indicate that one or more recessive genes is responsible for the abnormal condition. Conclusive genetic evidence was obtained only for the variant having old-gold flowers derived from the hybrid of L. esculentum x L. chilense, indicating the action of a single recessive gene, og, of major effect. Appropriate test crosses demonstrated that og is present in heterozygous condition in the chilense parent and that, furthermore, it gains phenotypic expression within that species equal to that in the F2 or backcross with L. chilense. Information concerning the origin of the other novelties is not so conclusive, yet the available facts are in keeping with an explanation like that for og-namely, fixation, on selfing the F1 hybrid, of recessive genes present in heterozygous form in the self-incompatible parental species. Limited experience in inbreeding L. peruvianum and L. chilense reveals a great wealth of recessive genes of major effect that would be expected on the basis of this hypothesis. For reasons presented, hypotheses based on high mutation rates induced by hybridity and on complementary interaction of normal alleles of the parents are considered less likely to explain the origin of the variants. Certain other instances of aberrant segregates in the F2 of species hybrids reported in the literature might also possibly be explained by recessive genes derived from the self-incompatible parents.