2n = 4x = 48 voucher: Ochoa & Salas 14945 (CIP) (Hijmans, et al. 2007)
Solanum acaule is widespread and common in upland habitats from northern Peru (Dept. Cajamarca), south through Bolivia to northern Argentina (Prov. San Juan), and with one record in northern Chile (Antofagasta Region), on dry rocky hillsides, high puna, among herbs, spiny shrubs and low woods, along streamsides, dry river beds and alluvial cones, (2000) 2400-4700 m in elevation.
Solanum acaule is a member of Solanum sect. Petota Dumort., the tuber-bearing cultivated and wild potatoes. Within sect. Petota, Solanum acaule 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).
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Correll (1962), Hawkes (1990), and Ochoa (1999) placed S. acaule (tetraploid) in series Acaulia Juz., and related it to S. albicans (Ochoa) Ochoa, a hexaploid species from Ecuador and northern Peru. Many studies have made it clear, however, that both S. acaule and S. albicans are also closely related to S. demissum Lindl., a species from Mexico and Guatemala that Correll (1962) and Hawkes (1990), placed in the separate series Demissa Bukasov. The relationships of S. acaule, S. albicans, and S. demissum were shown by Bonierbale et al. (1990) and Debener et al. (1990) with single- to low-copy nuclear restriction fragment polymorphisms, by Spooner et al. (1995) and Kardolus (1999) with morphological data, by Kardolus (1998) and Jacobs et al. (2008) with Amplified Fragment Length Polymorphism (AFLP) data, and by Nakagawa and Hosaka (2002) with plastid and nuclear RFLPs. Based on these data Spooner et al. (2004) extracted S. demissum from its other North and Central American hexaploid relatives in series Demissa Bukasov, and classified S. acaule, S. albicans, and S. demissum in an informal Acaulia Group.
The taxonomy of S. acaule and S. albicans has been a state of flux, with changing ideas on the status of S. albicans, and with varying treatments of infraspecific categories within S. acaule at the subspecies, varietal, and form ranks. We do not partition S. acaule into infraspecific categories as do Correll (1962), Hawkes (1990), and Ochoa (1999).
Hosaka and Spooner (1992) used 94 probe-enzyme combinations of nuclear RFLPs to examine 105 accessions of all three subspecies (sensu Hawkes, 1990) of S. acaule (subsp. acaule, aemulans, punae) and of S. albicans throughout their distribution area, from northern Peru to northern Argentina. They were able to support the recognition of S. albicans and all subspecies of S. acaule except subsp. acaule and subsp. punae that were combined into one. In the present treatment we recognize S. acaule, S. albicans, and S. ×aemulans at the species level with no infraspecific categories, and treat subsp. punae as a synonym of S. acaule.
Kardolus (1999) used phenetic analysis of morphological data to distinguish most accessions of subsp. acaule and subsp. punae, and he ascribed the mixture of the two in the study of Hosaka and Spooner (1992) to be due to misidentifications. McGregor et al. (2002) likewise supported both subsp. acaule and subsp. punae with AFLP data, but the molecular distinction between the two was slight, and again, reidentifications were needed to make their AFLP and morphological data fit each other. Taxonomists have distinguished these two taxa by overlapping character states of habit and pubescence, with subsp. punae very rosette-like and with long-spreading hairs on the stem, petiole, and leaf, and subsp. acaule with less flat rosettes and shorter appressed hairs. We find these characters to be so variable as to be impossible to use.
More difficult to treat is S. acaule subsp. palmirense. Kardolus (1998) described this as the first known hexaploid cytotype of S. acaule, based on a single collection from Chimborazo Province, Ecuador (Spooner et al. 5070). At that time, this was the only known Ecuadorian collection of either S. acaule or S. albicans, but our treatment found another collection of S. albicans from the same province (Ochoa 13395). Kardolus (1998) classified subsp. palmirense in S. acaule, rather than in S. albicans, based on AFLP affinities of the former, despite the fact that its ploidy and morphology was a near match for S. albicans. A subsequent AFLP study by McGregor et al. (2002) also grouped subsp. palmirense with S. acaule. Solanum albicans is considered to be a hybrid of S. acaule and another species (Hawkes, 1990), and its AFLP distinction from other populations of S. albicans may be due to another hybrid origin. While Kardolus’s (1998) placement of subsp. palmirense into S. acaule may be correct, we consider this issue unsolved from lack of sufficient comparative material. We synonymize this subspecies into S. albicans because of its morphological and ploidy affinities with that species.
Solanum acaule is in the pedigree of the hybrid cultivated potato species S. juzepczukii Bukasov (a triploid hybrid of diploid clones of S. tuberosum L. and S. acaule) and S. curtilobum Juz. and Bukasov (a pentaploid hybrid of tetraploid clones of S. tuberosum and S. juzepczukii) (Hawkes, 1962; Schmiediche et al., 1980, 1982). It is also a parent in the hybrid wild species S. ×indunii K. A. Okada and A. M. Clausen (S. acaule × S. boliviense Dunal) (Okada and Clausen, 1982), and S. ×viirsooi K. A. Okada and A. M. Clausen (S. acaule × S. infundibuliforme Phil.) (Okada and Clausen, 1985).
Kardolus and Groendijk-Wilders (1998) studied the inflorescence architecture of members of series Acaulia, series Demissa, and series Megistacroloba. They described five types of inflorescence architecture, including a type not recognized previously of a monochasium with an "extra" flower at its base in series Acaulia and S. demissum. None of these were species-specific and even varied on individual plants, but showed species-specific tendencies towards one type.