The winged bean (Psophocarpus tetragonolobus (L.) DC.) is a leguminous vegetable plant of the humid tropics. Its origins are obscure; while the other eight named species of Psophocarpus DC. are African, the winged bean is essentially a crop of Asia and the Western Pacific.
Two hypotheses have been proposed to explain this anomaly. One, supported by much recent taxonomic research, postulates that the progenitor of winged bean arose on the African side of the Indian Ocean whence it was carried East and domesticated through human cultivation. The alternative, implies a wider natural distribution for Psophocarpus, with the winged bean first domesticated within an Indian centre in one model, or within island Southeast Asia and Melanesia in another variation.
This review of literature and of research into genetic variation in the winged bean concludes that the evidence is still insufficient to eliminate either hypothesis, but reveals diverse circumstantial evidence for the antiquity of the domesticate in Asia and Melanesia.
The winged bean, Psophocarpus tetragonolobus (L.) DC., is a climbing, herbaceous, legume of South and Southeast Asia. While all parts of the plant are edible, throughout most of its distribution it is grown mainly for its green pods and beans (Chandel, Pant, and Arora 1984; Masefield 1973; N.A.S. 1975); a minor vegetable in the household garden complex classified as pekarangan in the terminology (borrowed from Bahasa Indonesia) of Sopher (1980). In Eastern Java and Bali, it is very occasionally grown for its ripe seed, planted in small numbers along the bunds of wet rice fields (sawah in Sopher's terminology) and consumed in a variety of specialised ways (Sastrapradja and Aminah Lubis 1975). In highland New Guinea, winged bean is a minor field crop, grown for its above ground vegetable parts and for its edible root tubers (Khan, Bohn, and Stephenson 1977) within a high rainfall swidden agricultural system (ladang). In Myanmar, in the plains south of Mandalay, the winged bean is grown without the usual trellis support, on a field crop scale for its salable tubers (Burkill 1906; Eagleton 1999), in a seasonally irrigated dry field system intermediate between sawah and tegalan within Sopher's schema.
Traditionally, its cultivation has been confined to a broad equatorial belt traversing the Indian Ocean from Mauritius to Melanesia (NAS 1975). Despite this wide and diverse distribution, no evidence for a wild progenitor of the cultivated form of Psophocarpus tetragonolobus has been forthcoming from research within the various countries where it has traditionally been cultivated. Of the other eight named species (plus one unnamed species, reported by Maxted 1990) in the genus Psophocarpus, all are exclusively African, with the exception of P. scandens (Endl.) Verdc. which is also naturally abundant in Madagascar and nearby islands (Verdcourt and Halliday 1978). P. scandens has multiple traditional uses in Africa including as a minor vegetable, and is often planted as a cover crop or companion legume (Harder,Onyembe, and Musasa 1990). It was probably in colonial times, for use as a leguminous cover, that it was carried to India, parts of Southeast Asia and Brazil (Verdcourt and Halliday 1978). There is no clear evidence as to whether it was present outside Africa before colonial times, though this is possible. Prior to the last few years, none of the other Psophocarpus species had been recorded outside Africa.
Although there is no evidence for systematic cultivation of the African species of Psophocarpus species other than P. scandens, Burkill (1906) was drawn by taxonomy to the opinion that cultivated winged bean was likely to have been first domesticated on the western side of the Indian Ocean. In recent years, the view that the ultimate source of Psophocarpus tetragonolobus (L.) DC. is to be found in one of the extant African species has gained support from research (Harder 1996; Harder 1992; Harder,Onyembe, and Musasa 1990; Maxted 1990; Smartt 1980). In the words of Harder and Smartt(1995: 300),this research
"suggests an African progenitor (P. grandiflorus), probably originating in the central African highlands, and subsequent transdomestication to explain the extension of the single cultivated species to its centre of domestication somewhere in Southeast Asia"
In contrast to the African hypothesis, is the Asian hypothesis. On the basis of his observations in Assam, in north eastern India, Vavilov (1951) included winged bean in his Indian centre of crop origins. However, Hymowitz and Boyd (1977) considered New Guinea to be a more likely centre of origin for the winged bean, based on the amount of genetic diversity observed in the Papua New Guinea highlands.
The purpose of this paper, then, is to review the evidence for these models of winged bean's beginnings and to scrutinise them in the light of research into the structure of the winged bean gene pool.
It has been generally accepted that Georg Eberhard Rumpf's account of Lobus quadrangularis in Amboina (Rumphius 1747) first unequivocally documented the species Psophocarpus tetragonolobus (Verdcourt and Halliday 1978). Rumphius, himself, pointed to possible recordings in the works of Clusius and Plukenet. Clusius (1605) observed the pod of a plant which bore resemblance to P. tetragonolobus but which he described vaguely, such as to leave identification in doubt. In Plukenet's (1696) Almagestum Botanicum, a pod of an "Indian Phaseolus" of the same general form was described under the name Phaseolus indicus with reference to Lobus cartilagineus from the island of Mauritius. However, the diagram in Clusius' work to which Plukenet referred, is of the species Mucuna gigantea (Verdcourt, personal communication); so it is not clear whether Plukenet had seen our particular plant.
Rumphius believed that winged bean had been recently introduced into Amboina, possibly from Java or Bali. Koenig in 1779 saw a "Dolychos with a winged pod" in the gardens around Malacca town (Koenig 1894:104). This "Dolychos" was almost certainly P. tetragonolobus of which there is a lanceolate-leafed form attributed to Koenig in the British Museum (Verdcourt and Halliday 1978). Today the lanceolate-leafed form appears to be absent from the Malacca area, although such types have been observed in the north of peninsular Malaysia (Eagleton,Thurling, and Khan 1978). It is possible that Koenig obtained his specimen in Southern India or Sri Lanka. Chandel, Pant and Arora (1984) have recorded occasional lanceolate-leafed accessions amongst Indian germplasm.
Loureiro recorded winged bean in Southern China in 1790. Burkill (1906) suggested that he was referring to the neighbourhood of Canton although the name (dau roung) that Loureiro (1790) ascribed to the winged bean is Vietnamese rather than Cantonese. There is a herbarium specimen from Yunan province in China (Verdcourt and Halliday, 1978).
In India, winged bean was recorded by Roxburgh (1814). Burkill's argument that it was already in India by 1799, seems persuasive (but see Hymowitz and Boyd, 1977). Certainly by the end of the 19th century its presence had been documented across the breadth of Peninsular India (Graham 1839; Roxburgh 1814; Watt 1894; Wight and Arnott 1834) as well as in Bangladesh, Burma (Wallich 1826), Thailand, Malaysia (Koenig 1894 ), Indochina (Loureiro 1790), Philippines (Blanco 1837), Guam (Anon. 1905), New Guinea (Warburg 1899; Wirz 1924) , Indonesia (Hasskarl 1842; Rumphius 1747) and the Mascarene Islands (Breon 1820). It had not been recorded in the African mainland at that time (Burkill 1906), but has since been widely distributed there (NAS 1975).
The distribution of winged bean is broadly equatorial (Figure 1). The highest latitudes at which it has been recorded are 210 South in Reunion Island, and 250 North in Assam (Breon 1820; Chandel, Pant, and Arora 1984). It is grown in much of lowland Southeast Asia, but occurs at higher altitudes in Assam, Myanmar, and Papua New Guinea, where it has been recorded in cultivation almost to the frost line at 2000 metres (Khan, Bohn, and Stephenson 1977). It has little drought tolerance and requires well-drained moist soils (Duke 1981; Rachie 1977). It has a reputation for survival on "infertile" soils, which is related to its vigorous nodulation capacity (Masefield 1957).
The cultivation of winged bean reaches its most sophisticated level in the highlands of New Guinea. In Mount Hagen (Figure 2), winged bean is the most important legume, and is grown in rotation with the main starch crop, sweet potato (Powell et al. 1975; Powell 1976). It is planted as a field crop with different strategies according to whether it is grown for tubers or for pods (Khan, Bohn, and Stephenson 1977). Although the area of land planted to winged bean per village is rarely more than half a hectare representing less than 20% of available land, at some times of the year, tubers can provide more than 25% of the crude protein intake (Stephenson et al. 1979).
Winged bean is grown throughout the highlands of the island, although its importance declines away from the more established agricultural centres. Schumann (1899) recorded winged bean on the north east coast of New Guinea, but the first certain record of the winged bean in the highlands was from the Kremer expedition of 1921-22 (Wirz 1924) which observed cultivation in the Baliem Valley of Irian Jaya. Lam (1945) believed that it had been introduced into the highlands, in the same way as sweet potato, maize and tobacco, which are thought to have arrived less than four centuries ago (Yen 1974). Watson (1965, 1967) on the other hand, suggested that the winged bean might belong with species like yam, taro and Pueraria lobata (Willdenow) Ohwi, to an earlier agricultural phase preceding the introduction of sweet potato.
There has been much debate around the concept of a 'sweet potato revolution' in the highlands, and it is instructive to examine winged bean alongside Pueraria lobata, a tuberous legume found throughout Melanesia and Polynesia (Barrau 1958, 1961). Its distribution and use as a starch source prompted Watson (1964) and Barrau (1965) to consider it as a part of an early agricultural phase in the Western Pacific. It is a vigorous (weedy) creeper in secondary scrub, yet its failure to regularly set seed at these equatorial latitudes suggests that its origins are outside Melanesia.
In constrast to P. lobata, the winged bean has not been recorded as an escape in the New Guinea highlands, nor does it appear to have reached Polynesia prior to European intervention in the Pacific (Barrau 1961). It is not likely that the winged bean was grown in the southern part of the New Guinea island before recent times. The main point of comparison between the two leguminous species, is their edible tubers. Yen (1973) has argued that there was no important plant in the pre-European phase of South Pacific agriculture that was reproduced by seed. Winged bean, however, is never propagated other than by seed, so it fits awkwardly into the model of a "pre sweet potato" phase in highland agriculture.
Although in many parts of the Papua New Guinea highlands, tuber production is an important objective of growing winged bean, this is not the case in all parts of the highlands. Clarke (1971) reporting about winged bean in one location in the Western Highlands Province noted:
"In the Ndwimba Basin the leaves and beans are eaten; but the pod is rarely, and the tuberous 'yam' never eaten".
In January 1995, in a survey of five winged bean growing villages in the Baliem Valley of Irian Jaya, the consumption or sale of winged bean tubers were never seen by the current author; no farmer interviewed in this part of the highlands indicated awareness that the tubers can be eaten. On the other hand, pods both in the fresh green stage and in the early 'bean' stage, were a daily item in local markets and in the central market at Waimena.
Throughout island Southeast Asia, winged beans are cultivated for pods, and not for tubers. Heyne (1927) noted that apart from Rumphius, no one had recorded winged bean tubers being eaten in Indonesia proper. Root crops do not dominate the subsistence agriculture of Indonesia: cultivated rice was probably part of the diet by 2000 BC (Glover 1977; Glover and Higham 1993). From historical and archaeological records, winged bean seems at no stage to have been more than a minor crop in Indonesia (Sastrapradja and Aminah Lubis 1975). For example, despite its common appearance in the main population centres of Indonesia and Philippines, winged bean seems to be absent or little known in the Toraja areas of central Sulawesi (author's observations).
It is only in Myanmar that winged bean appears as important as in the New Guinea Highlands, and again, it is mainly for tubers that it is planted (Figure 3). Its cultivation was described in detail by Burkill (1906) and more recently by Eagleton (1999). In Burkill's time, the most advanced area of tuber cultivation was in Kyaukse, south of Mandalay in the central Burmese plain (Figure 3). He recorded that the seed for these plantings came from the Shan hills at latitudes 200 to 230N. This is still largely the case today.
In 1969, Gorman reported finding carbonised plant fragments of such genera as Cucumis, Lagenaria, Aleurites and possibly certain 'leguminous' species amongst archaeological remains dating back to 7000 BC at a cave site in the North West of Thailand 60 km north of Mae Hong Son (Gorman 1969; Solheim 1972; Gorman 1977). It is interesting to note that winged bean is present today in the area of Mae Hong Son (Gypmantasiri and Stern, unpublished survey) and that seed samples, even in the fresh state, can be confused with other legumes e.g. Glycine. However, the preliminary interpretation of the archaeological remains was criticised by Harlan and de Wet (1973). This led to a careful review of the evidence, summarised by Yen (1977). The specimens tentatively identified as 'leguminous' remain of doubtful status. Furthermore, in the light of more recent archaeological work, it seems very unlikely that the plant remains found in various 'Hoabinhian' sites in North West Thailand were other than those of uncultivated plants left behind by pre-agricultural peoples (Glover and Higham 1993 ; Higham and Maloney 1989 ).
In island New Guinea, excavations in the Kuk swamp at Mount Hagen have led to a claim for the field scale manipulation of land and vegetation in the highlands as early as 7000 BC (Golson 1977; Golson and Hughes 1980). 'Gardening features' began to appear in the drained swamp around 4000 BC, by which time the pig had been introduced into the island ultimately from Southeast Asia. These were interpreted as reflecting mixed cultivation of dry- and wet-land plants, with taro (Colocasia esculenta), an important tuber crop, nominated amongst the latter. Although there is good taxonomic and ethnological evidence (Powell 1976; Simmonds 1966; Simmonds 1976; Yen 1973; Yen 1990) to support an indigenous origin for some New Guinea cultigens such as the Australimusa bananas (Musa spp.) and sugar canes (Saccharum spp.) and perhaps also taro, there has been no direct archaeological support for indigenous plant domestication (Golson 1989).
The Kuk investigators were confident that the gardening systems that made their appearance in the swamp around 2000 years ago signified monoculture of taro and that the present staple of the highlanders, the tropical American sweet potato, introduced from island Southeast Asia as a result of European penetration in the 16th century, began to make an impact in the Mount Hagen area around 250 years ago. Unfortunately, there are no plant remains, including pollen, which would confirm the biological inferences about New Guinea plant domestication. Similarly, there is no record of winged bean in the archaeological finds of New Guinea, nor in island South east Asia and the Pacific, even though a rice-based agriculture had reached mainland and insular Southeast Asia, by around 4000 BC, at least three millennia after it appeared in Central China (Bellwood 1993; Glover and Higham 1993).
Linguistic evidence has proven no more fruitful than the archaeological, in giving firm clues to the origins of winged bean domestication. In a list of some 90 different names compiled by the author, there is no indication of continuity across cultures as has been taken to occur, for example, with the Malay word for yam, 'ubi', or the Sanskrit word for sesame, 'tila' (Burkill 1953; Bedigian and Harlan 1986).
Rumphius' (1747) believed that the Indonesian name 'kacang botor' ('kacang babotor' in modern day Amboina) was derived from the Arabic but other commentators have doubted this (Hymowitz and Boyd 1977). On linguistic grounds, Burkill (1906) believed that the cultivation of winged bean in India was a relatively late phenomenon. However,his suggestion that the Tamil name 'morisu-avarai' might point to Mauritius as a source of Indian winged bean, is scarcely more convincing than that the English name Goa bean might indicate a Goan origin.
The list compiled by the author includes some 30 different names attributed to winged bean in New Guinea. Yet Jardin's (1974) lexicon listed 133 names for sweet potato, a plant which has probably been in New Guinea less than 400 years. The number or uniqueness of names is little proof of antiquity, although it may indicate something of the importance and diversity in the crop.
Of the nine species of Psophocarpus DC. described in Verdcourt and Halliday's (1978) review, it is only winged bean (P. tetragonolobus (L.) DC.) that is found extensively outside of Africa (Figure 1). Pollen morphology (Poole 1979), and a detailed multivariate analysis of up to 97 morphological characters measured on 126 herbarium specimens representing the known Psophocarpus species (Maxted 1990), suggest that the nearest African species to winged bean are P. scandens (Endl.) Verdc., P. palustris Desv. and P. grandiflorus Wilczek.
On morphological grounds, the west African P. palustris and east African P. scandens, with predominantly lowland distributions (0 to 1000 m) that overlap in the centre of the continent, are distinct but closely related species (Verdcourt and Halliday 1978; Maxted 1990 ). P. grandiflorus, with different morphology and a much more restricted central African distribution, is predominantly an upland species (1600 to 2300 m).
Cytogenetic studies by Pickersgill (1980) determined the diploid number as 2n=18 for P. tetragonolobus from Papua New Guinea, Thailand, Philippines and Indonesia, as well as for two specimens of P. scandens. More recently, Harder (1992) confirmed that the diploid number of 2n=18 extends to other species of Psophocarpus (P.lancifolius Harms, P. lecomtei Tisserant, P. grandiflorus and P. palustris). The diploid number of 18, and the distinctive bimodal chromosomal complement (3 pairs of short chromosomes and 6 pairs long), found in all six species so far examined, support the conclusion arising from general phenetic analysis of morphological characters, that Psophocarpus DC. is a somewhat isolated and closely knit genus within the tribe Phaseoleae (Maxted 1991; Harder 1996).
Given the similarities in chromosomal complement (Pickersgill 1980) and morphology (Maxted 1990), between winged bean (P. tetragonolobus) and P. scandens, it is notable that all attempts to hybridise the two have so far failed (Erskine 1978; Eagleton, Thurling and Khan 1978; Pickersgill 1980; Harder 1992). While intraspecific crosses between winged beans of widely separate geographic origins present little difficulty (Eagleton, Thurling and Khan 1978), Pickersgill (1980) observed that flowers drop within 48 hours of interspecific cross-pollination with P. scandens, "regardless of the direction in which the cross was made". Pickersgill (1980) related this lack of success to small observable differences in chromosomal morphology between the species. However, it is important to realise, that until now, only a tiny sample of the geographic range of P. scandens has been available for use in hybridisation studies. Before it is concluded that the two species are reproductively incompatible, more study is required.
The possibility of hybridisation between winged bean and other species of Psophocarpus has been inhibited by limited availability of seed of the African species, difficulties in growing some of these outside their natural habitats, and difficulties in arranging for suitable cross-pollination conditions. For example, when it has been possible to grow winged bean and P. grandiflorus alongside one another, lack of synchronisation in flowering has presented a barrier to experimentation, due to the very late and sparse flowering habit of P. grandiflorus (Eagleton, unpublished observations).
The hypothesis that P. grandiflorus is the wild progenitor of winged bean was first proposed by Smartt (1980) on the basis of the phytogeographical evidence. Since that time, Harder and Smartt (1992) and Harder (1996) have gathered additional evidence apparently in support of the P. grandiflorus hypothesis.
Following the success of Harder, Onyembe and Musasa (1990) in obtaining seed of P. grandiflorus from Kivu Province in eastern Zaire, the current author visited eastern Zaire, South Western Uganda and Southern Ethiopia, where P. grandiflorus is found.
At first sight, this species resembles winged bean more closely than does P. scandens: it has a climbing rather than creeping habit, and has an open inflorescence with larger floral parts, pods and seeds (Figure 4.) However, it is a rare species, found only in the humid mountainous pockets of Teza, Virunga, Ruwenzori, Mt Elgon and Southern Ethiopia. Its habitat is quite specific. In the Kigesi region of South Western Uganda, it grows on acid soils of volcanic origin, in valley bottoms rich in organic matter. The altitude of these valleys is over 1500 m with a mean maximum temperature of 23oC and a mean minimum of 10oC (Mukasa and Tiley 1967; Tothill 1958). The species is found within a few metres of water or else on the edges of humid rainforest settings, where precipitation exceeds 1300 mm. The nearest stretch of coast is over 500 km from its present distribution.
There is no direct evidence of P. grandiflorus cultivation in Africa. Westphal (1974) found P. grandiflorus (mistakenly named P. palustris in the original paper) growing alongside gardens in Sidamo province of Southern Ethiopia. In 1988, the present author visited the site of these observations, and like Westphal, saw plants growing on a garden fence. The farmers in the household were questioned, and they reported that they did not eat this plant or its parts as food. Similarly, at Lake Bunyonyi in South Western Uganda, the author found instances of P. grandiflorus growing on the verges of planted fields of sweet potato and common bean, but no examples of its being cultivated.
Traditional uses of the African species are varied; e.g. leaves, pods and roasted seeds of P. grandiflorus , and the tuberous roots of P. lancifolius Harms are consumed as food sources (Harder,Onyembe, and Musasa 1990; Verdcourt and Halliday 1978). There is no unequivocal evidence of either of these species being systematically planted or cultivated by human hands in modern times.
On the other hand, P. scandens is planted for a range of uses (Paulus et al. 1981; Harder, Onyembe and Musasa 1990). Burkill (1906) claimed that P. palustris (closely related to P. scandens) was cultivated in tropical Africa and cited Welwitsch's observations in Angola. However, Dalziel (1937) reported no certain evidence of its cultivation in West Africa.
Plant pests and diseases can give clues to plant origins (Leppik and White 1975). One of the important diseases of winged bean is the false rust caused by the fungal organism Synchytrium psophocarpi (Rac.) Gaumann. A number of members of the subgenus Woroninella to which S. psophocarpi belongs, are found on tropical and subtropical legumes including Pueraria, Vigna and Phaseolus genera (Karling 1964). However, S. psophocarpi has only been found on the genus Psophocarpus. Gaumann (1927) and Alicbusan (1965) were unable to transfer S. psophocarpi from winged bean to a wide range of other legume and non-legume genera.
Synchytrium psophocarpi has been recorded on winged bean in Papua New Guinea, Indonesia, Philippines, Malaysia and to a minor extent Bangladesh (Drinkall 1978; Haq, Chowdhury, and Rashid 1980). However, there is no record of it in Myanmar, or India (Burkill 1906; Chandel, Pant, and Arora 1984; Eagleton 1999).
Some winged bean accessions show genetic resistance to the disease. Thompson and Haryono (1979), Aminah Lubis and Sastrapradja (1981) and the current author independently observed field resistance in the perennial lines from Bogor in West Java. Aminah Lubis and Sastrapradja (1981) produced evidence that this resistance is conferred by the interaction of two separate gene loci.
The evidence for Synchytrium psophocarpi on other members of Psophocarpus has been less well researched. Erskine (1978), Thompson and Haryono (1979) and Price (personal communication) reported on the failure of S. psophocarpi to transfer from infected winged bean accessions to Psophocarpus scandens accessions growing in adjacent plots. The current author examined a large number of wild plants of P. scandens growing along the streams feeding into the Lake Tanganyika in Bujumbura, Burundi, but there were no symptoms of S. psophocarpi on any of the plants. In contrast, almost every plant of Psophocarpus grandiflorus observed in the North Kivu area of Zaire, at Lake Bunyonyi in South Western Uganda, and the Kefa province in Ethiopia, was infected by a false rust with the appearance of S. psophocarpi. Similar observations were made independently by Harder,Onyembe, and Musasa (1990).
A herbarium specimen of the plants observed by the author at Lake Bunyonyi was sent to Dr T. Price of La Trobe University who took scanning electron micrographs of the fungal sporangia and forwarded samples to Dr G. Hall of the CAB International Mycological Institute. Dr Hall reported in a communication to Dr Price:
"Herb. IMI 327691: Synchytrium psophocarpi (Raciborski) Gaumann.
This organism has produced only sporangia in leaf tissues, and is therefore a microcyclic species. The dimensions and shape of the sporangia agree well with the description of S. psophocarpi in Karling (1964), which is believed to be restricted to Psophocarpus spp. There are no records of S. psophocarpi on P. grandiflorus in ROPP/RAM, and there is only one specimen from Uganda (on P. palustris) in Herb. IMI."
This last comment hints of the possibility that false rust in Uganda is not restricted to P. grandiflorus but might also to be found on other African Psophocarpus species. However, it should be noted that the identification of P. palustris referred to by Dr Hall is probably in error, since that species has not previously been recorded in Uganda (Verdcourt and Halliday 1978). Only carefully controlled inoculation studies across a range of genotypes representative of the genus will resolve the issue of host specificity. However, on balance, it would seem that current evidence about Synchytrium psophocarpi gives support to the hypothesis of a close relationship between winged bean and P. grandiflorus, first suggested by Smartt (1980).
Verdcourt and Halliday (1978) have commented on the uniformity in the large number of herbarium specimens of P. scandens. The author has observed a similar uniformity for P. grandiflorus in the field.
On the other hand, the domesticated winged bean exhibits considerable diversity across its distribution (Haryono, Soedarsono, and Thompson 1978; Khan 1976; Khan 1982; Eagleton 1999; Sastrapradja and Aminah Lubis 1975). In the Mount Hagen area of New Guinea, 48 named varieties of winged bean have been recorded, which compares favourably with over 40 for sweet potato and 31 for taro (Powell et al., 1975). Khan (1976) produced evidence of polymorphism for a range of morphological characteristics in the New Guinea germplasm, while Erskine (1978) estimated the mean percentage of heterozygosity at 4 gene loci, to be in the order of 80%, within 14 distinct samples from the highlands. This genetic heterogeneity led to the conclusion that the highlands represent a principal centre of diversity within the species. Hymowitz and Boyd (1977) considered that New Guinea might be the Centre of Origin for the species.
The present author set out to test this hypothesis. A germplasm collection of 209 accessions, representing traditional winged bean growing areas of Southeast Asia and New Guinea, was assembled through the efforts of many workers. In 1979, these were planted out at the University of Agriculture in Malaysia. Of these accessions, 135 of known origin were planted under artificially extended daylengths, as well as natural daylength conditions. Two replicate hill plots were planted per accession under each daylength regime. Altogether, 47 variables were recorded per plot. These included metric variables, ordered multistate variables and un-ordered multistate variables. The data were subject to preliminary univariate analyses to compare accessions from different countries of origin for particular characters of interest. The entire data matrix of 47 by 135 elements was then subject to a multivariate analysis in order to see whether a taxonomic structure could be discerned amongst the accessions, making no a priori assumptions as to the origins of the accessions or the relative importance of the recorded variables. The variables were first brought to equal variance. Euclidean distances were computed between the 135 accessions within the 47 dimensioned space created by these standardised variables. The matrix of standardised distances between accessions was subject first to Principal Coordinates Analysis which assumed no discontinuities amongst accessions, and then, when it was clear that discontinuities existed, to a Classificatory analysis based on Burr's (1970) minimising incremental sums of squares strategy. Because many accessions had still not flowered at harvest 165 days after planting, the analysis was repeated with 22 post-reproductive characters dropped from the data matrix. A full description of the methods, results and analyses is given in Eagleton (1983).
These analyses confirm the hypothesis that the highlands of New Guinea represent a unique centre of diversity for the winged bean (Table 1). It is likely that this genetic diversity is the result of selection pressure and random drift, in the diversity of a highland environment and culture. The evidence suggests that the evolution of the New Guinea gene pool has taken place in relative isolation from the Southeast Asian (Fig 5). However, there is little support in the data for the hypothesis that New Guinea is the "Centre of Origin" for the species.
Papua New Guinea accessions (particularly those comprising Groups 1 and 2 in the Cluster Analysis, Fig 5) were earlier to mature, had fewer low growing branches, higher mature pod yield, and lower root yields at harvest 165 days after planting than Southeast Asian accessions (Table 2). In contrast, one group of Southeast Asian accessions - included in Group 8 in the Cluster analysis, and supplied by the Bogor workers in Indonesia (Sastrapradja and Aminah Lubis 1975; Sastrapradja 1978), failed to flower within 165 days, under either artificially long daylengths or normal daylengths at 3o latitude. This distinct group of accessions (referred to here as the Bogor lines) exhibited a range of characteristics which appear to be agriculturally undeveloped - perenniality; indeterminate growth habit, horizontal branching with rooting from secondary leaf nodes; and late, sparse, flowering pattern.
New Guinea accessions, with their smaller stature and quick maturity, are typical of more advanced domesticates according to the model presented by Smartt (1978). Similarly, the North West Thai accessions (Group 7) show evidence of advanced domestication for tuber production, as is found in neighbouring Myanmar (Eagleton 1999).
Winged bean may have originated in Africa since all the other members of Psophocarpus are located there. The spread of many crops from Africa to Asia is substantiated: the long bean form of Vigna unguiculata (L.) Walp. is believed to have reached Asia over the Sabaean Lane through Ethiopia in prehistoric times (Burkill 1953; Steele 1976), and Bambara groundnut Voandzeia subterranean (L.) Thoars is known to have been taken by Arab traders to Madagascar at an early date and was seen in Cochinchina by Loureiro prior to 1779 (Doku and Karikari 1971). The progenitor of winged bean could have reached Asia in a similar way; through Madagascar and the Mascarene chain as suggested by Burkill (1906) or through North Eastern Africa as proposed by Smartt (1980), Harder and Smartt (1995)and Harder (1996).
On the other hand, there is a problem of mechanism. Unlike Voandzeia subterranea, winged bean was never cultivated in Africa. It was recorded as "cultivated and casually subspontaneous" in Mauritius and Reunion (Baker 1877), but De Sornay (1916) felt that it had probably reached there from Asia. Winged bean seems not to have been documented in Madagascar as such (Verdcourt and Halliday 1978), and while P. scandens grows wild in Madagascar and the Comoros, it is probably a late introduction to Mauritius (De Sornay 1916). The genetic distance between P. scandens and P. tetragonolobus, as indicated by differences in morphology and responses to the disease S. psophocarpi, as well as the failure to hybridise, makes it unlikely that the former was the immediate forerunner of winged bean. Investigations with a wider range of P. scandens genotypes are necessary to consolidate this point.
The Ethiopian route is the other possibility. Cyamopsis senagalensis, the proposed wild progenitor of Guar (Cyamopsis tetragonoloba (L.) Taub.), may have passed at some stage along this track and then been carried as horse fodder to India where it was domesticated in quite recent times (Hymowitz, 1972). Smartt's (1980) similar proposal for Psophocarpus grandiflorus poses problems, however. Unlike the semi-arid distribution of C. senegalensis, P. grandiflorus is limited to humid pockets far from the coast. There is a discontinuity in the distribution of Psophocarpus across the Indian Ocean, which is not to be found in the African-West Asian distribution of Cyamopsis.
Two hypotheses seem most likely: either Psophocarpus grandiflorus is the wild progenitor of the winged bean, and was carried from East Africa to Southern Asia where it under went domestication; or alternatively, the wild progenitor of winged bean, perhaps resembling P.grandiflorus, has become extinct or has yet to be located within Asia.
In favour of the first hypothesis, is the accumulating evidence from taxonomy, cytology and plant pathology of close similarities between P. grandiflorus and the winged bean, the belief of Rumphius (1747) that winged bean was a late introduction into Eastern Indonesia, and the belief of Burkill (1906) that it was likewise late into India.
Against the first hypothesis, and in support of the Asian hypothesis, is the modern day fragility of P. grandiflorus in Africa, its absence from low altitude sites, and the lack of any clear evidence for its domestication or spread by human hands within North East Africa. On the other hand, the genetic studies referred to in this paper, point to a long and rich encounter between plant and man in Southeast Asia and especially the Indonesian archipelago that encompasses both lowland and highland environments and a diversity of domestication trajectories.
In the literature, there are many examples of discontinuities in plant taxa across the Indian Ocean. The trans-oceanic distribution of Psophocarpus may well turn out to be the result of ancient geological events, rather than of dispersal by human hands (McKenzie and Sclater 1973; Pickersgill 1980; Schuster 1976). However, there can be little doubt though, that the modern-day distribution of winged bean is predominantly an artifact of human hands, hands that have molded the plant to their own purposes, and in so doing, obliterated, perhaps completely, its natural origins.
One can expect that further field exploration in Africa and Asia, hybridisation studies with Psophocarpus grandiflorus, and the application of molecular labelling techniques to representative collections of the genus, will in time, provide a test of these ideas.
The research data reported in this paper were drawn from trials supported by the Departments of Agronomy of the University of Western Australia and the Universiti Pertanian Malaysia.
Mr. D. Tukahabwa of the Forestry Service, Uganda, and Mr. Melchior Nahimana of the Institut Recherche Agronomique et Zootechnique in Burundi, and Dr. Jean Hanson of ILCA in Ethiopia, assisted the investigations into P. grandiflorus. Dr T. Price, of La Trobe University, identified the false rust disease in P. grandiflorus.
Many people contributed seed for the trials and several people assisted in correcting earlier version of this paper.
Particular thanks go to Dr Tanveer Khan of the West Australian Department of Agriculture for his support over many years.
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