Introgression of Solanum chacoense (Solanum sect. Petota): Upland Populations Reexamined

Solanum chacoense is a highly variable wild potato species widely distributed from central Bolivia to central Argentina, from sea level to over 3,000 m in the Andes. Upland populations of S. chacoense were thought to have arisen from introgression of lowland populations with S. microdontum. Our study uses morphological data, molecular data from singleto low-copy nuclear DNA restriction fragment length polymorphisms (RFLP's), and random amplified polymorphic DNA (RAPD's) to reinvestigate this hypothesis. The data do not support introgression but show relative equal divergence of upland and lowland populations from S. microdontum. These results suggest that other hybridization hypotheses need to be reexamined in sect. Petota. However, the similarity of many taxa in the group and alternative explanations of morphological or molecular intermediacy will complicate conclusions of hybridization. Solanum L. sect. Petota Dumort. , the potato and its wild relatives, includes 232 species (Hawkes 1990). An alternative classification (Child 1990), supported by morphological and chloroplast DNA (cpDNA) data (Spooner et al. 1993) removes the eight non-tuberous species to sect. Etuberosum (Bukasov and Kameraz) A. Child, sect. Juglandifolium (Rydb.) A. Child, and sect. Lycopersicum (Mill.) Wettst. The remaining 224 tuberous species occur from the southwestern United States to south central Chile, with the greatest diversity of species in Peru and Bolivia. Species occur from sea level to over 4,500 m, but most occur between 2,000 and 3,500 m. Most members of sect. Petota are diploids (2n=24), but a few are triploid, tetraploid, pentaploid, or hexaploid (Correll 1962; Hawkes 1990). Section Petota is taxonomically complex. There is much disagreement regarding species boundaries, affiliation of species to series, rank of infraspecific taxa, and hypotheses of hybridization (Spooner and van den Berg 1992a). Diploid and polyploid hybrid speciation and introgressive hybridization are hypothesized to be common and important evolutionary mechanisms in sect. Petota (Ugent 1970; Hawkes 1978, 1990). Approximately 12% (26) of the 224 species in sect. Petota have been hypothesized to have arisen by hybrid speciation (listed in Spooner and van den Berg 1992b). In addition, local hybridization not leading to widespread introgression or speciation is believed to be common. Hawkes and Hjerting (1969, 1989) provided extensive lists of such putative occasional hybrids. Some species in sect. Petota have strong interspecific reproductive isolating mechanisms. One mechanism is governed by a process involving ratios of maternal/paternal effective ploidy in the endosperm, and evidenced by endosperm breakdown (Endosperm Balance Number, EBN; Hanneman 1994). In addition, some interspecific crosses within EBN levels are inhibited by stylar barriers (Abdalla and Hermsen 1972; Fritz and Hanneman 1989). However, most diploid species, especially those in South America, can artificially be hybridized to form viable interspecific progeny, although some may show reduced fertility in subsequent generations (Hawkes 1990). Often, different species occur sympatrically, providing the opportunity for natural hybridization (Hawkes 1978, 1990). Solanum chacoense Bitter was the first wild potato species investigated for introgressive hybridization in sect. Petota (Hawkes 1962). The latest comprehensive taxonomic revision (Hawkes 1990) and a subsequent review of evolution in sect. Petota (Hawkes 1994) continue to cite S. chacoense as one of the better-documented examples of introgressive hybridization in the group. A recent review of introgression in plants, however, cites S. chacoense as an example of an introgression hypothesis where alternative explanations were not adequately pursued (Rieseberg and Wendel 1993). Solanum chacoense grows from central Bolivia south to central Argentina, Brazil, and Paraguay, and from the Atlantic coast to the Andes, up to 3,700 m. It covers an area 1,700 km north-south by 1,500 km east-west (Hawkes 1962), and is one of the most widely distributed wild potato species.