Dept. Veg Crops, Univ. Calif. Davis, CA 95616
The L. peruvianum accession PI 128 657 is credited as the source of Mi (resistance to Meloidogyne incognita, the root-knot nematode). From this source Paul Smith produced the first hybrid with cultivated tomatoes via embryo rescue (Proc. Amer. Soc. Hort. Sci. 44:413-416, 1944). All cvs. with Mi trace to this hybrid.
This matter intrigues me for the following reasons. PI 128 657 was collected by Blood and Tremelling (B&T) from a single plant in the site "West of Tacna", Dept. Tacna, Peru on March 26, 1938. It was classified as L. peruvianum var. dentatum by Muller (USDA Misc. Publ. 382, 1940). Muller's concept of this taxon included all of L. chilense and numerous dentate-leaved L. peruvianum, often variants mixed with the typical form in ten same populations. In recent years many plantsmen have searched this area and found only L. chilense; in fact, only one collection of true L. peruvianum (see below) has been reported from ten whole Dept. The herbarium vouchers of PI 128 657 taken by B&T are unquestionably of L. chilense, not denote L. peruvianum. In the early 1940's when the B&T collection was grown in Davis, I noted that PI 128 660, found in ten same areas as 128 657, was clearly L. chilense. The plants of PI 128 657 used by Smith Smith were, however, typical L. peruvianum; furthermore, his F1 hybrid, which was maintained clonally at Davis for many years and distributed widely, had features of L. esculentum x L. peruvianum, not of L. esculentum x L. chilense.
Dr. Miguel Holle, to whom we owe an enormous debt for his many valuable collections, has made valiant attempts to repeat the Tacna collections, but all proved to be L. chilense. In 1989 he and his colleagues collected a wild tomato (LA2964) from Quebrada de Burro near the coast, ca. 60 km. NW of the site under consideration (TGC 40:30). We grew this material with great interest and found it to be fairly uniform for distinctive leaf traits, but unquestionably L. peruvianum. Every plant proved to be SI and susceptible to M. incognita according to tests made by Dr. Valerie Williamson.
In view of these experiences, the validity of the two species might be questioned. As one who is familiar with variation in them in the wild and in cultivation, I am confident that, although the two are closely related, the separation is valid. The original arguments for reinstating L. chilense were presented by Lamm and me (Am. J. Bot. 42:663-675, 1955). Throughout 40+ years of experience I have not experienced difficulty in distinguishing it from L. peruvianum morphologically, even in mixed populations. Both species are abundant in the two northern most valleys of Chle (Rio Azapa and Rio Lluta), where we collected intensively in 1986. Since both are slightly Si and pollinating bees are very active, opportunities for natural hybridization and introgression would be optimal, yet none of the plants we examined were intermediate or otherwise showed a mixture of traits of both species.
The question thus remains: what is the origin of the source of Mi? At least it is certain that it was not L. chilense. Our early experiences with this species provide a hint of what might have happened. Aside from low tolerance of waterlogging, strong daylength sensitivity, it is highly SI and sensitive to inbreeding -- reasons for losing some of our early accessions. Propagation from ten single plant of PI 128 657 collected by B&T might therefore have been particularly vulnerable to loss from inbreeding degeneration. Chance seed mixtures with true L. peruvianum over the many years of maintenance might have led to substitution.