Tomato-like nightshades: Affinities, autoecology, and breeders’ opportunities

Abstract

The group of nightshades I would like to discuss consists of four species: Solahum juglandifolium Dun., S. lycopersicoides Dun., S. ochranthum Dun., and S. rickii Corr. (Solanaceae). Belonging to section Petota, subsection Potatoe, series Juglandifolia, they differ from other species in Potatoe and resemble Lycopersicon in respect to absence of tubers and presence of yellow corolla. Other tomato-like features that distinguish them from the great majority of Solanum species are articulated pedicels and pinnately segmented leaves. In the aggregate, the association of all of these traits in the four species renders them unique amongst the myriad Solarium species. They nevertheless differ from Lycopersicon in several important features to be considered below and are reproductively isolated from them by formidable reproductive barriers, which have been circumvented only in crosses with S. lycopersicoides, and even in this instance, only as a consequence of experimental manipulations. All of these species are diploids (2n = 24). My interest in this group was stimulated in the 1940s by a collection of S. lycopersicoides provided by Professor T. H. Goodspeed, collected by R. D. Metcalf in 1942 (his No. 30382) near Candarave, Dept. Tacna, Peril, under the auspices of the third University of California Botanical Garden Expedition to the Andes. Of the seed collected, only a few germinated at the UC-Berkeley Botanical Garden, but we were able to maintain a clone therefrom and utilize it in experiments to which I shall refer later. Since that time it has been my good fortune to visit populations of all four species in their native habitats, to make many collections of them, and to culture them for seed increase and various studies. This group is important for several reasons. Breeders constantly probe new sources of germplasm for purposes of plant improvement. The cultivated tomato (L. esculentum L.) is one of a large group of crop species whose genetic variability is so greatly reduced that they lack many genes needed for breeding purposes. In the case of the tomato, this depletion evidently resulted from events that ensued prior to, during, and subsequent to domestication (Rick 1976). The generally accepted immediate ancestor of the tomato is var. cerasiforme of the cultivated species--a largely self-pollinated taxon that evidently migrated from its ancestral area in the north central Andes to its area of domestication in Mesoamerica. Founder events during this migration (doubtlessly often in small populations) would have tended to diminish genetic variability existing in the original source(s), a situation verified by comparisons of genetic content of monogenically determined isozymes (Rick et al. 1974; Rick and Fobes 1975). The domestication process itself in a self-pollinator would have tended to further reduce reserves as a result of selection for desired types. Finally, transport to Europe, further mi