2n = 2x = 24 voucher: Ochoa & Salas 14973 (CIP)
2n = 4x = 48 voucher: Ochoa 2159 (CIP)
2n = 3x = 36 voucher: Ochoa 2202 (CIP)
Landrace populations of Solanum tuberosum grow from western Venezuela south to northern Argentina (Andean populations), and then with a gap of distribution in south-central Chile in the islands of the Chonos Archipelago and the adjacent mainland Chile (lowland Chilean populations), as cultivated plants; with the Andean populations growing from 1000-4300 m in elevation, and the lowland Chilean populations at or near sea level. Landrace populations are still maintained out of their natural range, introduced in post-Colombian times, in Mexico and Central America, the Shimla Hills of India, and in the Canary Islands. The modern cultivated potato (also classified as Solanum tuberosum), is cultivated worldwide.
Solanum tuberosum is a member of Solanum sect. Petota Dumort., the tuber-bearing cultivated and wild potatoes. 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).
Cultivated Potatoes Have Been Treated in Various Ways. Cultivated potatoes are taxonomically difficult, and cultivated potatoes have been classified very differently by different taxonomists. Indigenous primitive cultivated (landrace) potatoes are grown throughout mid to high (about 3000-3500 m) elevations in the Andes from western Venezuela to northern Argentina, and then in lowland south-central Chile, concentrated in the Chonos Archipelago. Landrace populations in Mexico and Central America are recent, post-Columbian introductions (Ugent, 1968). The landraces are highly diverse, with a great variety of shapes and skin and tuber colors not often seen in modern varieties. The taxonomic treatment of Solanum tuberosum used here follows Spooner et al. (2007b) that divide cultivated potato species into four species: S. tuberosum, S. ajanhuiri Juz. and Bukasov, S. curtilobum Juz. and Bukasov, and S. juzepczukii Bukasov.
The widely used recent classification of Hawkes (1990) divided cultivated potato into seven species and seven subspecies. Hawkes’ (1990) treatment was not universally accepted. The Russian potato taxonomists Bukasov (1971) and Lechnovich (1971) recognized 21 species. Ochoa (1990, 1999) recognized nine species and 141 infraspecific taxa for the Bolivian cultivated species alone. Bukasov (1971), Lechnovich (1971), Hawkes (1990), and Ochoa (1990) classified potatoes as distinct species under the International Code of Botanical Nomenclature (ICBN) (McNeill et al., 2006). Dodds (1962), in contrast, treated the cultivated species under the International Code of Nomenclature of Cultivated Plants (ICNCP) (Bricknell et al., 2005). Dodds (1962) suggested that there was poor morphological support for most cultivated species, and recognized only S. ×curtilobum, S. ×juzepczukii, and S. tuberosum, with five “groups” recognized in the latter. “Cultivar-groups” (the current terminology) are taxonomic categories used by the ICNCP to associate cultivated plants with traits that are of use to agriculturists. The cultivar-group classification of Dodds (1962) was based on comparative morphology, reproductive biology, cytological and genetic data, and cultural practices. He contended that the morphological characters used by Hawkes (1956a) to separate species exaggerated the consistency of qualitative and quantitative characters.
Ploidy level has been of great importance in the classification and identification of cultivated potatoes. Bukasov (1939) was the first to count chromosomes of the cultivated potatoes and discovered diploids, triploids, tetraploids, and pentaploids and used these data to speculate on their hybrid origins. In historical and current practice, identifications are frequently made only after chromosome counts are determined, and re-identifications made after chromosome counts do not match that expected for the species. The strong reliance on ploidy levels was clearly stated by Hawkes and Hjerting (1989, p. 389): “The chromosome number of 2n = 36 largely helps to identify S. chaucha, but morphological characters can also be used.”
Huamán and Spooner (2002) examined the morphological support for the classification of landrace populations of cultivated potatoes, using representatives of all seven species and most subspecies as outlined in the taxonomic treatment of Hawkes (1990). The results showed some phenetic support for S. ajanhuiri, S. chaucha, S. curtilobum, S. juzepczukii, and S. tuberosum subsp. tuberosum, but little support for the other taxa. Most morphological support is only present by using a suite of characters, all of which are shared with other taxa. These results, combined with their likely hybrid origins and evolutionary dynamics of continuing hybridization led Huamán and Spooner (2002) to recognize all landrace populations of cultivated potatoes as a single species, S. tuberosum, with the eight cultivar-groups: Ajanhuiri Group, Andigena Group, Chaucha Group, Chilotanum Group, Curtilobum Group, Juzepczukii Group, Phureja Group, and Stenotomum Group.
Other studies questioned the reality of even these artificial cultivar-groups. For example, the S. tuberosum L. Phureja Group was recognized either as a Cultivar Group or species (S. phureja) based on short-day adaptation, low tuber dormancy, and its diploid (2n = 2x = 24) nature. It was believed to consist of a variety of landraces widely grown in the Andes from western Venezuela to central Bolivia, and to have excellent culinary properties and other traits for developing modern varieties. Ghislain et al. (2006) examined the entire germplasm collection of the Phureja Group at the International Potato Center (CIP) with nuclear simple sequence repeats (nSSR, or microsatellite) to complement a prior RAPD study. The initial goal was to explore the use of these markers to form a core collection of cultivar-groups of potatoes. The nSSR data showed a very unexpected result in that it uncovered 25 unexpected triploid and tetraploid accessions. Chromosome counts of the 102 accessions confirmed these nSSR results and highlighted seven more triploids or tetraploids. Thus, these nSSR markers were good indicators of ploidy for diploid potatoes in 92% of the cases. Because the Phureja Group was defined partly on its diploid nature, and because the nSSR study showed over 30% of the CIP collection to be polyploid, they questioned the validity not only of the Phureja Group but of all cultivar-groups of potato.
Spooner et al. (2007b) considerably expanded the nSSR study of Ghislain et al. (2006) through an extensive study of 742 landraces of all cultivated species (or cultivar-groups), and eight closely related wild species progenitors, with 50 nSSRs and the 241-bp plastid deletion marker generally distinguishing Andean from Chilean potato landraces. The data highlighted a tendency to separate three groups: 1) putative diploids, 2) putative tetraploids, and 3) the hybrid cultivated species S. ajanhuiri (diploid), S. juzepczukii (triploid), and S. curtilobum (pentaploid), but there are many exceptions to grouping by ploidy. Strong statistical support for this tree occurred only for S. ajanhuiri, S. curtilobum, and S. juzepczukii. In combination with recent morphological analyses of Huamán and Spooner (2002), and an examination of the identification history of these collections at CIP, they proposed a reclassification of the cultivated potatoes into four species (1) S. tuberosum, with two cultivar-groups (Andigenum Group of upland Andean genotypes containing diploids, triploids and tetraploids, and the Chilotanum Group of lowland tetraploid Chilean landraces), (2) S. ajanhuiri (diploid), (3) S. juzepczukii (triploid), and (4) S. curtilobum (pentaploid).
Cultivated Potato in Chile. Landrace populations of cultivated potato in the Americas today occur 1) in the high Andes from western Venezuela to northern Argentina, 2) in lowland south-central Chile, in and about the Chonos Archipelago 3) in Mexico and Central America; but the Mexican and Central American populations were introduced in post-Colombian times. If the potato originated in the high Andes, what is the origin and mode of its distribution to lowland Chile?
The earliest putative potato remains of sect. Petota is from Chile, as a preserved potato skin, and is dated as 13,000 years old, long before the probable origin of agriculture (Ugent et al., 1987). These authors identify these remains as S. maglia (an extant species) but from erroneous or unreliable data (Spooner et al., 1991). The first historical account of potatoes in Chile is that of DeCortes Hojea (1557), who observed potatoes cultivated in 1557 by native peoples from Isla Ascension (at the northern end of the Guaitecas Archipelago, also known as Isla Melinka).
The origins of Chilean potato landraces (Solanum tuberosum) are speculative. Juzepczuk and Bukasov (1929) proposed that they originated from indigenous tetraploid wild species S. fonckii Phil. ex Reich, S. leptostigma Juz., and S. molinae Juz. Hawkes (1990) treated these three taxa as simply indigenous populations of S. tuberosum. Juzepczuk and Bukasov (1929) suggested that S. palustre Schltdl. (then treated as S. brevidens Phil.) may have been a progenitor. Hawkes (1990) proposed that the Chilean populations arose from the Andean tetraploid populations of S. tuberosum after transport to Chile. Based on comparative analysis of starch grains from the 13,000-year-old remains and extant wild species, Ugent et al. (1987) proposed the wild species S. maglia as a progenitor. Grun (1990) hypothesized that the Chilean landraces evolved from a cross between subsp. andigena and an unidentified wild species.
Hosaka and Hanneman (1988) identified five plastid genotypes (A, C, S, T, W types) in the Andean and Chilean populations of S. tuberosum. The Andean populations have all five types and native Chilean subsp. tuberosum have three types A, T, and W. The most frequently observed type in Chilean populations is T, which is characterized by a 241 base pair deletion (Kawagoe and Kikuta, 1991). This plastid deletion is associated with specific Chilean mitochondrial DNA types not found in Andean germplasm (Hosaka, 1995; Kawagoe and Kikuta, 1991; Lössl et al., 1999). Hosaka (2002, 2003) found a 241-bp deletion (Kawagoe and Kihuta, 1991) in this region to characterize many accessions of the Bolivian and Argentinean wild potato species S. berthaultii, S. tarijense (these two recently synonymized as the single species S. berthaultii, Spooner et al., 2007a), and S. neorossii, and most landrace populations of S. tuberosum from Chile, but not any other wild species. It is possible, therefore, that the Chilean landraces are of hybrid origins with one of these Argentinean and Bolivian wild species and a cultivated potato from Bolivia or Argentina. The timing or mode of its transport from Bolivia or Argentina to Chile, however, is unknown.
Raker and Spooner (2002) investigated the differentiation of the Andean and Chilean tetraploid landraces with nuclear microsatellites. They included in their analysis the Chilean wild potato species S. maglia and other wild potato species. Solanum maglia is a very narrowly distributed species near the sea near Viña del Mar and nearby areas (32º33’-32º55’S), and 200 km to the west in an isolated canyon (Canyon del Alvarado at 1500 m) in the Andes of Argentina (Spooner et al., 1991; Spooner and Clausen, 1993). The nuclear microsatellite data separated most populations of Andean and Chilean tetraploid potato, and clustered S. maglia with the Chilean populations. This result could be interpreted to support S. maglia as a progenitor of the Chilean cultivated populations, but in agreement with Cribb and Hawkes (1986), Hosaka and Hanneman (1988) and Hawkes (1990), we consider this unlikely. Solanum maglia is narrowly restricted to coastal central Chile, 1000 km north of native S. tuberosum, with a single population in the mountains of Argentina, and most populations are sterile triploids (Hawkes, 1990). Identifications of extant populations of S. maglia in southern Chile by Ugent et al. (1987) are not backed up by voucher specimens and likely are misidentifications of S. tuberosum, based on voucher specimens of others who have collected potatoes extensively in Chile (Contreras, 1987; Spooner et al., 1991). It is possible that S. maglia is a diverged and escaped population of S. tuberosum.
Potato expeditions to Chile (Contreras, 1987; Contreras et al., 1993; Spooner et al., 1991) document that the majority of the potato landraces in the Chonos Archipelago grow along the western chain of the islands near the Pacific shore. Most of these collections appear uniform in morphology. In the majority of the accessions, the tubers are ovoid, small (up to 3 cm in diameter), with blue to purple to light reddish skin and flesh. All have long stolons, some of which are more than two meters from the base of the plant. Most populations lack flowers and fruits, and seedlings are rare. Sprouts of discarded modern cultivars can easily be found at abandoned fishing encampments, showing the apparently ideal climate for potato growth in these islands. In Chiloé Island, cultivation of modern potato cultivars is common. However, farmers maintain small plots of native varieties as curiosities, that show a great variety of colors and shapes (Spooner et al., 1991), unlike the persistent populations of a uniform genotype on the many islands of the Chonos Archipelago farther to the south as described above (Contreras et al., 1993).
What was the Origin of the First Landrace Populations of Cultivated Potato? The origin of crop plants has long fascinated botanists, archaeologists, and sociologists with the following fundamental questions: When, where, how, why, and how many times did crop domestication occur? What are the wild progenitors of these crops? How do crops differ from their progenitors, what selective processes, and how many genetic changes produce these changes? Did crops have single or multiple and separate origins? (Harlan, 1992; Sauer, 1969). Single (diffusionist) vs. multiple origin (in-situ) hypotheses of crop origins have long been the subject of debate (Riley et. al., 1990; Zohary, 1999). Spooner et al. (2005a) investigated these alternatives in potato landraces with Amplified Fragment Length Polymorphisms (AFLPs).
The wild species progenitors of these Andean landraces have long been in dispute. All hypotheses center on a group of about 20 morphologically similar wild species referred to as the Solanum brevicaule complex, distributed from central Peru to northern Argentina. (Correll, 1962; Grun, 1990; Miller and Spooner, 1999; Ugent, 1970; Van den Berg et al., 1998). Members of the S. brevicaule complex are morphologically similar to the landraces. Potato domestication from these wild species involved selection for underground characters of shorter stolons, larger tubers, (often) colored and variously shaped tubers, and the reduction of bitter tuber glycoalkaloids; above ground characters of wild and cultivated species are similar, but with cultivated types exhibit high vigor and extensive segregation for flower and foliage traits.
Literally all hypotheses have suggested complex hybrid or multiple origins of the cultivars from both northern and southern members of the S. brevicaule complex (Brücher, 1964; Hawkes, 1990; Hosaka, 1995; Huamán and Spooner, 2002; Ochoa, 1990, 1999; Ugent, 1970). The AFLP study of Spooner et al. (2005a) included a wide range of accessions of the cultivars (98 accessions), and included a more complete sampling of wild species (362 accessions) and molecular markers (438) than prior studies. The initial intention of that AFLP study was to gain corroborative evidence for the two geographic subsets of the S. brevicaule complex 1) Peruvian, 2) Bolivian and Argentinean), and to see if a larger database of accessions and molecular markers would provide better support for species in the complex. The results fully supported the north/south components of the complex, and like earlier studies intermixed accessions of many (but not all) species, suggesting a need for synonymy of many of them. However, the results gave an unexpected and quite surprising new result. It grouped all cultivated species examined into a cohesive (monophyletic) branch of a phylogenetic tree, and supported their origin from the northern members of the Solanum brevicaule complex in central to southern Peru. Prior hypotheses suggested many and separate origins of the cultivated species from many species in the complex from both the north and the south.
What was the Origin the First “European” Potato? The first record of cultivated potato outside South America was in the Canary Islands in 1567 (Hawkes and Francisco-Ortega, 1993; Ríos et al., 2007), and shortly thereafter in continental Spain in 1573 (Hawkes, 1990; Hawkes and Francisco-Ortega, 1992; Romans, 2005). The potato rapidly spread throughout Europe and worldwide, and is here referred to as the “European” potato. It was slow to be adopted as a major food crop until about 100 years later, and in some European countries it was rejected as an acceptable food well into the late 1700’s.
The origin of the European potato has long been a subject of controversy. Russian investigators (Juzepczuk and Bukasov, 1929) first proposed that the European potato was introduced from tetraploid landraces from Chile, while British investigators (Salaman, 1937; Salaman and Hawkes, 1949) suggested that it came from the Andes and persisted until the potato late blight epidemics beginning in the UK in 1845, after which it was replaced with Chilean germplasm through introductions and breeding efforts. Potato landraces from the high Andes and from lowland Chile can be distinguished, although sometimes with difficulty, by the following five traits: 1) cytoplasmic sterility factors: hybrids of Chilean landraces as females with Andean landraces as males have male sterility, but the reciprocal cross is fertile (Grun, 1979); 2) morphology, with the Chilean landraces having wider leaflets held more outward from the plant, and other minor morphological differences (Huamán and Spooner, 2002); 3) the Chilean landraces tuberize under long days, and the Andean landraces under short days (Glendinning, 1975); 4) a suite of microsatellite markers (Raker and Spooner, 2002; Spooner et al., 2005b); and 5) a 241-bp deletion in the trnV-UAC/ndhC intergenic region of the plastid DNA molecule, which is absent in 94% (or 95%) of the Andean tetraploid landraces and present in 86% (or 81%) of the tetraploid Chilean landraces, depending on the studies of Hosaka (2004) (or Spooner et al., 2007b).
The Chilean origin hypothesis was proposed because of similarities among Chilean landraces and modern European cultivars with respect to morphology and tuberization under long days (traits 2 and 3 described above). Alternatively, the Andean origin hypothesis suggests that these two traits in European potato evolved rapidly, in parallel, from Andean landraces to a Chilean-type potato through selection following import to Europe, and the late blight epidemics beginning in the United Kingdom in 1845 killed the Andean forms that later were replaced by breeding and introductions with the Chilean landraces.
The Andean origin hypothesis has been generally accepted over the last 60 years (Glendinning, 1975; Hancock, 2004; Hawkes, 1990; Hawkes and Francisco-Ortega, 1992; Simmonds, 1964, 1995; Swaminathan, 1958). It was supported, in part, by the identity of putative long-remnant landrace introductions of potato in India (Swaminathan, 1958) and in the Canary Islands as Andean potatoes. However, the Indian landraces were shown to be of Chilean origin (Spooner et al., 2005b) and the Canary Island landraces as of Andean and Chilean origins (Ríos et al., 2007).
All of the above data and arguments bearing upon the origin of the European potato have been inferential. Ames and Spooner (2008) addressed this question for the first time with direct evidence through an analysis of the 241-bp deletion marker from historical (1700 - 1910) herbarium specimens. They obtained plastid DNA deletion data by both PCR and DNA sequencing from 49 herbarium specimens collected before 1910 from 11 European herbaria. Twenty-one of the 49 specimens were collected before 1850, and 28 from 1850-1910. Their results showed that the Andean potato (lacking the DNA plastid deletion) first appeared in Europe around 1700 and persisted until 1892, long after the late blight epidemics, while the Chilean potato (possessing the deletion) first appeared in Europe in 1811, long before the late blight epidemics and persisted until the present day, when over 99% of extant modern potato cultivars possess Chilean cytoplasm (Hosaka, 1993, 1995; Powell et al., 1993; Provan et al., 1999). Of the period from 1811-1850, 9 of the 16 specimens (56.3%) possessed Chilean cytoplasm.
These results refute the idea that the late blight epidemics beginning in Europe in 1845 stimulated introductions of Chilean germplasm as breeding stock to combat this disease, or eliminated the Andean potato, which persisted up until 1892. Chilean potatoes became predominant by at least 1811, fully 34 years before the late blight epidemics.
There were always problems with the idea that Chilean potatoes were germplasm sources subsequent to the late blight epidemics. First, Chilean potatoes are not noted as sources of late blight resistance (Glendinning, 1975; Jansky, 2000). Second, plastids are not transferred in pollen in the Solanaceae (Corriveau and Coleman, 1988), so only crosses of Chilean potatoes as female with Andean potatoes as male would produce the over 99% of extant modern varieties having Chilean type plastid DNA. However, this cross is hindered by the unilateral incompatibility of Chilean and Andean potatoes described above (Grun, 1979).
Summary. The studies highlighted here have greatly changed our understanding of the origin and diffusion of cultivated potato:
1. Cultivated potato was domesticated, perhaps up to 8,000 years ago, from a group of wild potato species today most closely related to the northern representatives of the Solanum brevicaule complex.
2. Based on present day distributions of these progenitors, cultivated potato originated in southern Peru.
3. Cultivated potato taxonomy is complicated by similarity among both the cultivated potato species and their wild progenitors in the Solanum brevicaule complex, by different ploidy levels, and by interspecific hybridization.
4. There is no “easy” taxonomic solution to classify potato landraces. However, morphological and molecular evidence supports a classification of four species: (1) S. tuberosum divided into the Andigenum (including diploids, triploids and tetraploids) and Chilotanum (tetraploid) cultivar-groups, 2) S. ajanhuiri (diploid), 3) S. juzepczukii (triploid), and S. curtilobum (pentaploid).
5. The Andean potato first appeared in Europe around 1700 and persisted until 1892, long after the late blight epidemics, while the Chilean potato first appeared in Europe in 1811, fully 34 years before the late blight epidemics and persisted until the present day, when over 99% of extant modern potato cultivars possess Chilean cytoplasm. These results support original introductions from the Andes, but refute the idea that the late blight epidemics beginning in Europe in 1845 stimulated introductions of Chilean germplasm as breeding stock to combat this disease, or eliminated the Andean potato.
6. These results have a variety of practical and theoretical impacts, as discussed below.
A maintenance of the old classifications of cultivated potato, to recognize seven (Hawkes, 1990) or even more species would only perpetuate confusion by the users, in genebanks, and in the literature. Potato genebanks are in great need of an integrated and comprehensive program of ploidy determinations; controlled and replicated studies of tuber dormancy (that they suspect would highlight grades of dormancy, not the present/absent determinations that exist today); photographically documented determinations of tuber and flesh colors and tuber shapes; and determinations of tuber pigments, glycoalkaloid contents, carbohydrates, proteins, amino acids, minerals, and secondary metabolites, using functional genomics approaches, with all data publicly integrated into a readily searchable web-based bioinformatics database. Such a multi-component system will serve the breeding community much better than the outdated, unstable, and phylogenetically indefensible traditional classifications that exist today.
The study of the origin of the origin of the European potato by Ames and Spooner (2008) demonstrated the impact of understanding plant origins to practical breeding programs. Years of effort were put into creating “Neo-Tuberosum” populations through artificial selection of long-day adaptation from Andean potatoes. The goal was based on the unquestioned assumption that the European potato was solely of Andean origin and was mass selected for long day length adaptation like the Chilean potato, but in the process of selection, many desirable characters for general adaptation, such as resistance to potato late blight, were lost (Glendinning, 1975; Plaisted, 1972; Simmonds, 1964). The results of Ames and Spooner (2008), however, document that European potato germplasm was derived from high latitude Chilean forms of Solanum tuberosum in Europe long before the potato blight epidemics. Their study provides the first direct evidence to bear upon the long-held controversy of the extra-Andean origin of this major food plant and changes our understanding of the history of the potato outside of South America.
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