We have examined 67 accessions of Lactuca sativa L. and five related species for variation at restriction fragment length polymorphism loci. Fifty-five clones were used to probe genomic DNA digested separately with three restriction endonucleases; this defined 143 loci. Variation in L. sativa was sampled in detail. Lactuca sativa is closely related to L. serriola but not to any of the other species in this survey: L. saligna, L. virosa, L. perennis, and L. indica. However, Lactuca sativa is not a direct extraction from the sampled populations of L. serriola; other L. serriola populations or some other unknown entities were involved in the evolution of cultivated lettuce. Numerous genetic differences characterized each cultivated type, suggesting a polyphyletic origin for L. sativa.
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Kesseli et al. (1991) studied this question.