A Reexamination of Species Boundaries and Hypotheses of Hybridization Concerning Solanum megistacrolobum and S. toralapanum (Solanum sect. Petota, series Megistacroloba): Molecular Data

Abstract

Solanum megistacrolobum and S. toralapanum are two phenetically similar wild potato (Solanum sect. Petota) species, classified in series Megistacroloba, that together are distributed from southern Peru to northwestern Argentina. They have variously been synonymized, recognized as varieties of S. megistacrolobum, or recognized as distinct species. We used 22 singleto low-copy random genomic DNA probes from potato, hybridized to total DNA digested with Dral, EcoRI, EcoRV, and HindIII, to investigate their taxonomic status. We also investigated the hybrid origin of S. raphanifolium and the hybrid origin of S. acaule subsp. aemulans. Our results are concordant with a separate morphological study, showing weak differentiation between S. megistacrolobum and S. toralapanum, but possible only with multivariate methods. These combined morphological and molecular results most closely fit the contemporary treatment of S. megistacrolobum and S. toralapanum at the varietal level. We propose the new combination S. megistacrolobum subsp. toralapanum in order to bring consistency to taxa within sect. Petota, where different authors recognize identical taxa as varieties or subspecies. Our results also show possible hybridization between S. megistacrolobum and S. acaule subsp. aemulans in Argentina, discount the hybridization hypothesis of S. raphanifolium, and show that the species-specific bands of S. albicans relative to S. acaule are shared with many other species in sect. Petota. Recent advances in molecular biology have provided new tools for systematic questions. At the species level or below, restriction fragment length polymorphisms (RFLP's) have provided powerful new data to assess phenetic similarity. Combined with morphological data, RFLP studies are enabling researchers to answer questions not possible with previous methods. Beckman and Soller (1986a, 1986b), Tanksley (1983), and Whitkus et al. (in press) have described many of the useful properties of RFLP's. These include ubiquity, stability, Mendelian codominant inheritance, the presence of multiple alleles for a given locus, and freedom from pleiotropic effects. Additionally, RFLP's are detectable in all tissues at all developmental stages. This, coupled with the relatively long shelf life of DNA, makes RFLP analysis a convenient method for assessing genetic variation and lower-rank systematic questions. This study employs single-to low-copy nuclear DNA probes to generate nuclear restriction fragment length polymorphisms (nRFLP's) to investigate hypotheses of species boundaries and hybridization involving Solanum megistacrolobum Bitter and S. toralapanum Card. and Hawkes. These are closely related and phenetically similar wild potato taxa (Solanum L. sect. Petota Dumort.) distributed from southern Peru to northwestern Argentina. Both are classified in series Megistacroloba, a group of diploid (2n = 24), self-incompatible, and largely interfertile species (Buck 1966; Hawkes 1990; Ochoa 1990; Pandey 1960). There is much intra-and interspecific morphological variability between S. megistacrolobum and S. toralapanum. These two taxa have been taxonomically treated as conspecific (Ochoa 1984), varieties of a single species (Ochoa 1990), or as distinct species (Correll 1962; Gorbatenko 1989; Hawkes 1990). Previous studies have shown that nRFLP's can result from both structural changes (i.e., insertions, deletions, inversions) and site mutations. Data distinguishing these alternatives have come from inferences based on comparisons of numbers of nRFLP patterns and length of areas being probed, or on recurrent patterns of polymorphisms using the same probe with different enzymes (Neuhausen 1992; Wang and Tanksley 1989; Wang et al. 1992). The only way