2n = 3x = 36 voucher: Van den Berg et al. 603 (PTIS) (Hijmans, et al. 2007)
2n = 2x = 24 voucher: Okada 5341 (BAL) (Hijmans, et al. 2007)
Solanum chacoense is widely distributed from southern Peru (Dept. Puno) to central Argentina (Depts. La Pampa and Buenos Aires), in a wide diversity of habitats, in full sun or in dense shade, in dry or moist areas, among bushes, in scrub or thorn forests or savannas, near the seashore, in moist subtropical forests, on rocky slopes, in cultivated fields, banana plantations, or roadsides, 0-3700 m in elevation. It has also escaped from cultivation outside its native range in the USA and Europe.
Solanum chacoense is a member of Solanum sect. Petota Dumort., the tuber-bearing cultivated and wild potatoes. Within sect. Petota, Solanum chacoense is a member of a very diverse clade related to the cultivated potato. On a higher taxonomic level, it is a member of the informally-named Potato Clade, a group of perhaps 200-300 species that also includes the tomato and its wild relatives (Bohs, 2005).
Solanum chacoense is one of the most variable and widely-distributed wild potato species. It is most similar morphologically to S. kurtzianum and S. malmeanum. From S. kurtzianum it is distinguished by its non-ciliate leaf margins, its generally greater height of up to 2 m, and the generally obtuse angle of insertion of the first distal pair of lateral leaflets. From S. malmeanum, it can be differentiated by its larger and erect habit, by the acute apex of the terminal leaflet and generally by a larger peduncle. Solanum chacoense also is similar to S. berthaultii but can be distinguished from that species by the absence of type B (long-stalked) glandular trichomes, by its less pubescent leaves, and its consistently white to creamy-white corollas.
Hawkes and Hjerting (1969), Morton (1976), and Hawkes (1990) considered S. muelleri as a subspecies of S. chacoense and distinguished this subspecies by broader lateral leaflets (3 to 4 times as long as broad). Matesevach and Barboza (2005) synonymized subspecies muelleri under S. chacoense but Correll (1962) and Mentz and Oliveira (2004) consider it at the species level as based upon the following characters: stem unwinged, the absence of a rosette of basal leaves, larger lateral leaflets, no presence of interjected leaflets and if present only 0.2 cm, and irregular acumens. Matesevach and Barboza (2005) synonymized it under S. chacoense but considered material only from Argentina.
We examined specimens of S. chacoense along its entire range and found it to be a highly variable species, although easily recognizable on the bases of leaf and flower morphology. Considering the characters listed by Mentz and Oliveira (2004) based upon specimens from Brazil, we found that the absence of wings, larger lateral leaflets, lack of interjected leaflets or small interjected leaflets, and irregular acumens occur throughout its entire range.
Solanum calvescens has been differentiated from other tuber bearing species on the basis of very small acumens, anthers not clearly delimited from the filaments, flowers white or purple, and triploid ploidy (Correll, 1962; Hawkes and Hjerting, 1969). Both authors stressed the particular stamens of S. calvescens, similar to S. maglia, not being basally cordate. While the stamens of plants identified as S. calvescens are somewhat atypical, they are not like S. maglia that possess “barrel” shaped stamens without a clear distinction between stamens and filaments. The specific characters observed in the stamens S. calvescens points partly towards the characteristic form of a male sterile plant: small, distorted and pale.
Along its wide distribution, S. chacoense has been found growing with S. kurtzianum, S. microdontum, S. tarijense, and S. spegazzinii. Hybrids have been reported between several of these species. Solanum chacoense was the first tuber-bearing species investigated for introgressive hybridization. Hawkes (1962) considered that hybridization with S. microdontum in northwest Argentina might be responsible for its extended distribution at higher altitudes. Juned et al. (1988) studied populations from Paraguay and Argentina using morphological and isozyme markers and found that introgression could not explain the range of variation detected in Córdoba province. They proposed that regional diversity and clinal variation according to altitude were responsible for the detected variability. Miller and Spooner (1996) investigated the hybrid nature of upland populations of S. chacoense using morphological and molecular data (RAPDs and RFLPs). They distinguished upland from lowland populations on the basis of morphological data; this was partially supported by RAPDs but not by RFLP data. Introgressive hybridization with other species does not appear to be responsible for the variability observed in S. chacoense.
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