Aqeel Hasan Rizvi*; Muraleedhar Aski†; Ashutosh Sarker*; Harsh Kumar Dikshit†; Prachi Yadav† * International Center for Agricultural Research in the Dry Areas, South Asia & China Regional Program, New Delhi, India
† Indian Agricultural Research Institute, Division of Genetics, New Delhi, India
The lentil is one of the oldest domesticated and cultivated crops in the world. It plays an important role in human and animal diets, and in modern agriculture, as well. The word lentil comes from the Latin lens, and indeed, this bean cousin is shaped like the double convex optic lens which took its name from the lentil. It has been cultivated for 10,000 years in the most difficult agricultural environments, being perhaps second only to barley in this sense. The plant was given the scientific name Lens culinaris in 1787 by Medikus, a German botanist and physician. Archeological data has revealed the existence of both wild and cultivated lentils in the Near East region, which is evidently the place of origin of the crop.
Recent study using the Genotyping By Sequencing (GBS) method based on phylogenetic tree and STRUCTURE analysis, has identified four gene pools, namely, L. culinaris/L. orientalis/L. tomentosus, L. lamottei/L. odemensis, L. ervoides, and L. nigricans, which form primary, secondary, tertiary, and quaternary gene pools, respectively. Lens orientalis is the wild form out of which the cultigen developed. Lens culinaris forms show, in general, greater height, longer rachis, and a greater number of leaflets per leaf, greater leaflet area, greater number of flowers per peduncle, peduncle shorter or equal to the rachis, a higher frequency of white flowers, and larger pods and seeds. All these characters are related to increase in yield.
Lentil; Origin; Gene pool; Primary; Secondary; Tertiary; Quarternary
The lentil (Lens culinaris ssp. culinaris) is an ancient and early domesticated legume that continues to play an important role in human and animal diets, and in modern agriculture. The lentil plant was given the scientific name Lens culinaris in 1787 by a German botanist and physician, Medikus (Cubero, 1981; Sehirali, 1988; Hanelt, 2001). In different parts of the world, the lentil is known by various names. The most common names are Lentil (English), Adas (Arabic), Mercimek (Turkey), Messer (Ethopia), Masser or Massur (India), Heramame (Japanese), Mangu or Margu (Persian), and Masura, Renuka, Mangalaya (Sanskrit). It is the fourth most important legume crop after beans (Phaseolus vulgaris L.), peas (Pisum sativum), and chickpeas (Cicer arietinum), with an area of 5.5 m ha and production of 6.32 m tons in 2016 (FAO, 2017). Worldwide, production has increased over the last few decades (FAO, 2017). It is an important crop in West Asia, the Indian Subcontinent, Ethiopia, North Africa, Southern Europe, South and North America, and in Oceania. Among the main producers, production has been trending upwards in Canada, the United States (US), Australia, and China, but has been relatively stable in India, Turkey, Syria, Iran, Nepal and Bangladesh. Canada and the United States produce mainly green cotyledon lentils, whereas the rest of the world produces lentils mostly with red cotyledon. Internationally, the trade lies in small-seeded, red cotyledon lentils, which are dominated by Australia, Canada and Turkey, whereas the market in the large-seeded green lentil is held by Canada and the United States. Lentils (Lens culinaris Medik) are mainly grown for grains, used as dhal (whole or dehulled), and in various other preparations like lentil soup or deep-fried and eaten as snack. Its composition and nutritional quality make it an important crop, especially in the developing world. It is a rich source of protein (24%–28%) with an abundance of lysine, which makes it a good supplement with cereals for balancing the human diet. Its seed provides minerals and vitamins for human nutrition (Sarker et al., 2017) and straw for animal feed. Therefore, it is an important dietary source of energy, protein, carbohydrates, fiber, minerals, vitamins, and antioxidant compounds, as well as diverse nonnutritional components like protease inhibitors, tannins, oligosaccharides, and phytic acid. The lentil plant has the ability to fix atmospheric nitrogen and carbon sequestration, improving soil health.
In India, it is mostly grown in northern and central parts during the winter season as a rain-fed crop after rice, maize, pearl millet, or rainy season fallow. It is mainly cultivated in Uttar Pradesh, Madhya Pradesh, Chhattisgarh, Jharkhand, Bihar, and West Bengal. These states together contribute 85% of the area and 90% of the lentil production. In north-eastern parts of the country, it is also cultivated as a relay crop with rice, in which seeds of lentils are broadcasted into the standing crop of rice just before its harvest. It is grown on a wide range of soils from light loamy sand to heavy clay soil in the northern parts and moderately deep black soils in Madhya Pradesh and Maharashtra. In south India, the lentil is grown in pockets under conserved moisture after Kharif rice in the Belgaum district in Karnataka and has a unique demand in Maharashtra.
The sowing of lentil is popular for mono and sequential cropping, intercropping, mixed cropping, and relay cropping in various countries. In India, Pakistan, Bangladesh, and Nepal, a rice-lentil system is more common, but its cultivation is also done after maize, cotton, sorghum, and pearl millet. In eastern India, the broadcasting of lentil seed in standing rice crops about 15 days before the harvest gives significantly higher grain yield than lentils sown after the harvest of rice. The inclusion of lentils in various cropping systems improves the physical properties of soil and increases the yield of the succeeding cereal crop due to biological nitrogen fixation and other rotational effects.
Lentils are grown as a cool weather or winter crop in the semi-arid tropics, cultivated from sea level to 3800 m. The crop is not suited for the humid tropics. Lentils may survive on a wide range of soils from light loams and alluvial to black cotton soils, and are best on clay soils; they tolerate moderate alkalinity. Salt tolerance is higher during germination than during subsequent development. Lentil seed germinates between 18°C and 21°C, but may germinate at temperature above freezing point. An optimum temperature for growth and yields are around 24°C and temperatures above 27°C are harmful. Waterlogging is more damaging than drought. Lentils are quantitative long-day plants, some cultivars tending to be day-neutral. It is reported to require environments ranging from cool temperate steppe to wet through subtropical dry to moist forest life zones. It tolerates annual precipitation of 2.8–24.3 dm annual mean temperature of 6.3–27.3°C, and pH of 4.5–8.2 (Kay, 1979; Duke, 1981).
The history of the lentil is as old as agriculture (Helbaek, 1963). It has been cultivated along with wheat, barley, peas, and flax. The carbonized remains of the lentil date back to 11,000 BC from Greece’s Franchthi cave, which are the oldest known remains. The ancient Greeks enjoyed lentils as soups and used it to make bread. Pliny has given a detailed account of growing of the crop from seeds, its medicinal properties, and use for various remedies. The famous Apicius also recorded several recipes for lentils. Small-seeded (2–3 mm) types were found at Tell Mureybit in Syria dating to 8500–7500 BC (Zohary, 1972; Hansen and Renfrew, 1978). Lentil remains have been found in Neolithic, aceramic farming villages which were occupied in the 7th millennium BC in the Near-East arc (Helbaek, 1959). The type of agriculture around these lentils cannot be determined, as during this period, small-seeded cultivated lentils could not be differentiated from wild lentil seeds. In an archeological site in northern Israel, the presence of a large storage of lentils clearly established that, by 6800 BC, lentils were a part of farming. Carbonized lentil seeds have been recovered from widely dispersed places such as Tell Ramand in Syria (6250–5950 BC), acreamic Beidha in Jordan, ceramic Hacilar in Turkey (5800–5000 BC), and Tepe Sabz in Iran (5500–5000 BC) (Van Zeist and Bottema, 1971; Helbaek, 1970). In Greece, lentils dating back to 6000–5000 BC have been found in Neolithic settlements such as Argissa-Magula Tessaly (Hopf, 1962) and Nea Mikomedeia, Macedonia (Renfrew, 1969; Van Zeist and Bottema, 1971) and in the same period lentil remains were also seen in Egypt (Matmur, El Omari late 4th millennium, Helbaek, 1963). The archeobotanical remains of lentils have been found in the excavations of the Harappan civilization covering the period of 3300–1300 BC (Sandhu and Singh, 2007). They are mentioned in the Bible’s first chapter, Genesis, in the story of Esau, who lost his birthright over a dish of lentils (Genesis 25: 30–34).
Understanding of origin of any biological species encounters three main problems: (i) where and when the biological species originated; (ii) where and when that species became a crop; and (iii) how it has evolved as a crop (Cubero et al., 2009). Cubero, in 1981, with the help of archeological data, briefed the places of evidence. The oldest remains of wild lentils were found in Hacilar in Turkey, Ramad in Syria, Jarmo in Iraq, Jericho in Palestine, Beidha in Jordan, and Ali Kosh in Iran, dated around 9000 BP, in aceramic Neolithic layers. Even older remains were found in Mureybit in Syria, c. 10,500 BP. The oldest Greek lentils were dated around 8000 BP, then Central Europe in 5000–7000 BP, from classical Neolithic to early Bronze; in both regions there are some doubtful nigricans seeds. Late arrivals were those of India during 3000–4000 BP and Western Europe countries like France, Germany in 3000–3500 BP. Egypt also shows a later arrival (c. 5000 BP) than in Greece and Central Europe, but conditions in the Nile Delta are not favorable for preserving agricultural remains. Archeological data has revealed the existence of both wild and cultivated lentils in the Near East region, which is evidently the place of origin of the crop.
Ladizinsky (1999) suggested that considerable polymorphism was found to exist in the wild accessions of ssp. orientalis. However, three accessions from eastern Turkey and northern Syria were shown to share these characteristics with the cultigen, and therefore, can be regarded as members of the genetic stock from which the cultivated lentil was domesticated. Based on founder effects revealed by chromosome and DNA polymorphisms, as well as evidence from domestication traits and species diversity that the lentil was domesticated once or only a few times (Zohary, 1999). Lev-Yadun et al. (2000) argued that the lentil might have originated in a place close to or overlapping the area in the Fertile Crescent where einkorn and emmer wheats were domesticated.
In contrast, Barulina (1930), based on the Vavilovian criterion to define centers of origin, suggested the eastern border of southwest Asia as a possible center of origin of the cultivated lentil, as the region between Afghanistan, India, and Turkistan (i.e., the Himalaya-Hindu Kush junction) showed the highest proportion of endemic varieties. She also noticed that the area of distribution of wild lentils did not overlap much with that of the domesticated ones, but she maintained her idea considering that the eastern part of the orientalis area of distribution reaches Turkmenia. Thus, the overlap between the wild ancestor and the cultigen and the archeological remains connecting both are sine qua non conditions to establish such a center of origin. Three groups are restricted to very concrete areas: pilosae to the Indian subcontinent, aethiopicae to Ethiopia and Yemen, and subspontanea to the Afghan regions closest to the Indian subcontinent. All three have very small and dark-colored seeds, violet flowers, few flowers per peduncle, calyx teeth much shorter than the corolla, few leaflets per leaf, and dwarf plants. Each one of these three shows a distinct character: pilosae—a strong pubescence; subspontanea—very dehiscent pods, being purple colored before maturity; and aethiopicae—pods with a characteristic elongated apex.
These particular traits are shown together with a cluster of primitive traits closely related to orientalis. The other three microsperma groups (europeae, asiaticae, and intermediae) and macrosperma are rather cosmopolitan and are clearly intermixed, even when intermediae has a rather restricted area. Seeds are variable in size, but in general, they are wider than 4 mm. They have more flowers per peduncle, more leaflets per leaf, and the calyx teeth are equal to or larger than the corolla. White flowers are common, with seeds being diverse in color. The subspontanea overlaps only with orientalis, and aethiopicae with ervoides; pilosae does not overlap with any wild lentil. All other microsperma as well as macrosperma lentils overlap to a greater or lesser extent with all the known wild lentils. On the contrary, no lentils have been found in the sites dating back to the seventh millennium BP in Turkmenia. The high degree of endemism that exists in the Afghanistan-Indian-Turkmenian area is better explained, as in all other species, by an intense genetic drift, typical of highly diversified environments, coupled with artificial selection carried out by very diverse human populations, with drastic genetic fixation and losses providing secondary centers of diversity. In conclusion, the area from western Turkey to northern Iraq contains not only all the wild lentils but also “lentoid” characters such as flattened pods and seeds present in other species like V. montbretii (possible bridge between Vicia and Lens) and V. lunata. The known evidence suggests that from southern Turkey to northern Syria region is the most likely place of lentil domestication. Some populations of orientalis were unintentionally subjected to automatic selection here, leading to a new crop, Lens culinaris.
The genus Lens Miller is a member of the tribe Vicieae, subfamily Papilionaceae, family Leguminosae. Beside Lens, three other genera are included in the Vicieae: Vicia L., Lathyrus L., and Pisum L. From a morphological point of view, a range exists between the genera Lens and Vicia. However, Lens is a much smaller genus, characterized by an annual growth habit, small flowers, calyx deeply divided into subulate, subequal teeth, and a broadly rhomboid compressed legume with one or two orbicular flattened seeds. The genus Lens comprises seven taxa in six species (Ferguson, 1998; Ferguson et al., 2000). Lens orientalis is the presumed progenitor of Lens culinaris, and the two species are crossable and produce fully fertile progeny (Muehlbauer et al., 2006). According to crossability, phenetic relations, and chromosomal diversity, Ladizinsky and Abbo (1993) suggested two biological species in the genus Lens: Lens culinaris and Lens nigricans, with a few subspecies. However, additional information now indicates that some of the proposed subspecies are species in their own right. In 1997, two new species were recognized in genus Lens. Lens tomentosus was separated from Lens culinaris subsp. orientalis on the basis of its tomentose, as opposed to puberulent, pods, and a relatively small asymmetrical chromosome which bears a minute satellite (Ladizinsky, 1997). Lens lamottei, originally described by Czefranove (1971), was found to be the same as a differentiated cytotype identified within Lens nigricans by Ladizinsky et al. (1983, 1984) and is now recognized as a separate taxon (Van Oss et al., 1997). Thus, as a result of combined evidence of crossability, phenetic relations, and morphological markers (Ferguson and Erskine, 2001; Ferguson et al., 2000), the genus Lens consists of the six species. From the standpoint of crossability for use in breeding, the Lens species can be divided into three groups: L. culinaris and L. odemensis make up the primary genepool, L. ervoides and L. nigricans belong to the secondary genepool, and L. lamottei and L. tomentosus belong to the tertiary genepool (Muehlbauer and McPhee, 2005). Crosses between members of the different genepools generally fail, because the hybrid embryos abort. However, embryo rescue has been used successfully to obtain viable hybrids between groups (Ladizinsky et al., 1985). The basic chromosome number of the genus Lens is n = 7. All the Lens species share more or less the same karyotype, which includes three pairs of metacentric, or submetacentric chromosomes, a pair of metacentric chromosome with a secondary constriction very close to the centromere, and three pairs of acrocentric chromosomes (Ladizinsky and Abbo, 1993).
Varietal identification was realized by choosing convenient, nongeographical, and sometimes utilitarian characteristics. Lentil in taxonomy is as follows (Anonymous, 2012):
Kingdom: Plantae-Plants,
Subkingdom: Tracheobionta-Vascular plants,
Superdivision: Spermatophyta-Seed plants,
Division: Magnoliophyta-Flowering plants,
Class: Magnoliopsida-Dicotyledons,
Subclass: Rosidae,
Order: Fabales,
Family: Fabaceae-Pea family,
Genus: Lens Mill.-lentil,
Species : Lens culinaris Medik.-lentil.
Taxonomic analyses based on morphological and/or biochemical markers ranged from four species in 1979, namely, L. culinaris, L. orientalis, L. nigricans, and L. ervoides; to two species in 1984: L. culinaris (with subspecies culinaris (the cultigen), orientalis and odemensis) and L. nigricans (with ssp. nigricans and ervoides); again to four species in 1993, that is, L. culinaris (ssp. culinaris and orientalis), L. odemensis, L. nigricans, and L. ervoides; to also four species in the year 2000, that is, L. culinaris (ssp. culinaris, orientalis, tomentosus, and odemensis), L. nigricans, L. ervoides, and L. lamottei; and six species after 2000, which are L. culinaris (ssp. culinaris and orientalis), L. odemensis, L. tomentosus, L. nigricans, L. ervoides, and L. lamottei. The contradictory results of different studies are a consequence of the evolutionary process itself. Lens species share many common structural and biochemical features, and the distances obtained in different studies are a function of the origin of the accessions involved in them, as well as of the particular characters and molecular markers chosen. Subtle differences can place accessions in distinct clusters. It is not a mistake of the experimental method; rather, it is a consequence of a radiating evolution (Cubero et al., 2009).
The lentil is a diploid species (2n = 14) (Muehlbauer, 1991). It is a self-pollinating annual species with a haploid genome size of an estimated 4063 Mbp (Arumuganathan and Earle, 1991). The botanical features of Lens culinaris (cultivated lentil) can be described as annual, bushy herb, slender almost erect or suberect, much-branched, softly hairy; stems slender, angular, 15–75 cm height (Duke, 1981; Muehlbauer et al., 1985). Ten to sixteen leaflets are subtended on the rachis (40–50 mm); upper leaves have simple tendrils, while lower leaves are mucronate (Muehlbauer et al., 1985). “The leaves are alternate, compound, pinnate, usually ending in a tendril or bristly; leaflets 4–7 pairs, alternate or opposite; oval, sessile, 1–2 cm long; stipules small, entire; stipels absent; pods oblong, flattened, or compressed, smooth, to 1.3 cm long, 1–2-seeded; seed biconvex, rounded, small, 4–8 mm × 2.2–3 mm, lens-shaped, green, greenish-brown, or light red speckled with black; the weight of 100 seeds range from 2 to 8 g; cotyledons red, orange, yellow, or green, bleaching to yellow, often showing through the testa, influencing its apparent color” (Kay, 1979; Duke, 1981; Muehlbauer et al., 1995). Flowers are small, pale blue, purple, white, or pink, in axillary 1–4-flowered racemes; 1–4 flowers are borne on a single peduncle, and a single plant can produce from 10 to 150 peduncles each being 2.5–5 cm long (Muehlbauer et al., 1985). Flowering proceeds acropetally. The size of seeds increases from the types grown in eastern regions to western types. Two types, namely, macrosperma, found mainly in the Mediterranean region and the New World (seed size ranging from 6 to 9 mm in diameter and yellow cotyledons with little or no pigmentation), and microsperma (2–6 mm with red-orange or yellow cotyledons) found on the Indian subcontinent and in the Near East and East Africa, respectively, are known (Hawtin et al., 1980; Muehlbauer et al., 1985). The first one includes the Chilean or yellow cotyledon types, while the latter includes the small-seeded Persian or red cotyledon lentils (Kay, 1979). Germination is hypogeal, and this keeps the developing seedlings below ground level, which reduces the effects of freezing and other desiccating environmental conditions (Muehlbauer et al., 1985).
Being first cultivated in the Fertile Crescent, then moving to Greece and Central and Western Europe along the Danube, to the Nile Delta, and eastward to India, lentils followed the spreading route of the Neolithic agricultural techniques (Harlan, 1992). From Greece, lentils found their way to Central Europe through the Danube. Since the early Neolithic, there are lentil archeological remains in the Iberian Peninsula, the oldest ones corresponding to the eastern part of the Peninsula. In particular, in the cave, “de les Cendres,” there are remains of several crops (Triticum monococcum, Triticum dicoccum, Triticum aestivum, barley, peas, grass peas, lentils, and faba beans, that is, a typical Near-East crop complex) dated by 14C to 7540 ± 140 BP (Cubero et al., 2009). The archeological lentil findings in the Iberian Peninsula, conferring to the seed size, fall within the range of the microsperma sizes (Buxo, 1997). Intermediae forms reached Sicily, and asiaticae forms reached Sardinia, Morocco, and Spain, suggesting the arrival in these countries of lentil stocks from Central Europe, or from the route of the isles (Cubero et al., 2009). De Candolle (1882) suggested that lentils may have reached the cradle of the Indo-European people after the Greek ancestors split on linguistic grounds: Greek for lentil is phakos, but lens in Latin, lechja in Illyrian, and lenzsic in Lithuanian. It is likely that the ancient Greeks took the word from the aboriginal Mediterranean populations they conquered. Central Russia, then Siberia, was more likely reached from the western coast of the Black Sea or from the Danube valley rather than from Mesopotamia or Central Asia. The arrival of lentils to the Nile Delta had to be much earlier than the archeological remains suggest due to its proximity, both geographical and cultural, to the Fertile Crescent; but the Delta environment is not helpful in preserving organic materials. Ethiopia was probably reached from the Arabian coast (at that time it was the Arabia Felix, more humid and fertile than now), rather than by the Nile. Lentils likely travelled in the Near East complex (along with barley, wheat, chickpeas, faba beans, etc.), established in the Ethiopian highlands since primitive times, and have been evolving since then in isolation, producing much endemism that allowed Vavilov to place the centers of origin of crop plants that only arrived there by farming. Indeed, aethiopicae exhibits very primitive characters, meaning that lentils probably also arrived at a very primitive stage of domestication (Cubero et al., 2009). Lentils did not reach India before 4000 BP, probably carried by the Indoeuropean invasion (De Candolle, 1882) through Afghanistan, but again there is the problem of availability of archeological findings. Primeval introduction was probably performed by very small samples of a common origin as the variability found in the Indian subcontinent in the local landraces is very limited, in spite of being the largest lentil-growing region in the world; the asynchrony in flowering of the local pilosae ecotype, probably a consequence of a long reproductive isolation period, is now being broken by plant breeding methods in order to widen the genetic base available (Erskine et al., 1998).
The evidence produced on the basis of crossability between the crop species and the subsequent cytogenetic, phylogenetic, and molecular studies has been used to characterize the crop species into different gene pools (Harlan and De Wet, 1971). The wild relatives can be divided into primary, secondary, and tertiary gene pools, according to their relatedness to L. culinaris ssp. culinaris and their ability to produce fertile hybrids when intercrossed with cultivated lentils. The species of lentils have been grouped into primary (Lens culinaris ssp. culinaris, L. culinaris ssp. orientalis, L. odemensis), secondary (L. ervoides, L. nigricans), and tertiary gene pools (L. lamottei, L. tomentosus) (Ladizinsky et al., 1984; Ladizinsky, 1999; Muehlbauer and McPhee, 2005). Hancock (2004) grouped lentil species into three groups only on the basis of crossing, and placed intercrossable species L. nigricans, L. ervoides, and L. lamottei into a secondary pool or Group II. Crossing of Group I species with L. nigricans produces nonviable seeds in hybrids due to irregular meiosis (Ladizinsky et al., 1984, 1985). However, the use of embryo rescue can produce viable seed in hybrids derived from crossing between L. culinaris and L. ervoides (Ladizinsky et al., 1985). They also put L. tomentosus in Group III or the tertiary gene pool as a single species group, and this species does not produce viable seed in hybrids derived by crossing with other group of species. Schaefer et al., 2012 noted that the genus Lens (2n = 14) is phylogenetically comes under the tribe “Vicieae,” which are cool-season legumes belonging to subfamily Papilionoideae of family Fabaceae. The genus Lens is comprised of seven closely related taxa, namely, L. culinaris, L. orientalis, L. tomentosus, L. odemensis, L. lamottei, L. ervoides, and L. nigricans. Most of the past taxonomic studies based on crop morphology, cytogenetics, hybridization studies, and/or molecular markers have frequently disagreed with respect to classification at the species and subspecies level. The most recent classification by Kole et al. (2011), identified seven taxa grouped into four species, namely, L. culinaris ssp. culinaris, L. culinaris ssp. orientalis, L. culinaris ssp. tomentosus, L. culinaris ssp. odemensis, L. ervoides, L. lamottei, and L. nigricans. Despite the taxonomic reorganizations, all studies generally agreed that L. culinaris ssp. orientalis is the most closely related wild progenitor of L. culinaris ssp. culinaris, while L. nigricans is the most distant relative.
For genetic enhancement by generating new variability, valuable genes recognized in the primary gene pool have readily been used for crop improvement. Nevertheless, for incidence, the useful genes have been observed much more in the species of the secondary and tertiary gene pools (Collard et al., 2001; Tullu et al., 2006). Accordingly, more efforts are required on deployment of novel techniques for using these gene pools for the enhancement of lentils. Fratini and Ruiz (2006) suggested that a hybrid between L. culinaris ssp. culinaris and L. nigricans, L. ervoides, and L. odemensis developed through embryo rescue can be viable with a rate of 3%–9%. Based on these observations, L. odemensis has been considered to be a member of secondary gene pool, and L. nigricans and L. ervoides were classified in the tertiary gene pool (Ladizinsky, 1993). Further, Cubero et al. (2009) also suggested placing L. odemensis in the secondary gene pool, while L. nigricans and L. ervoides can also be part of secondary gene pool as hybrids which could be obtained by means of embryo rescue. They also suggested that there is a need to study hybridization in order to establish place of L. tomentosus and L. lamottei in secondary or tertiary gene pool. Some early studies, based on cross compatibility and cytogenetic evidence placed L. orientalis in the primary gene pool, while the secondary gene pool consisted of L. odemensis, L. ervoides, and L. nigricans. Ladizinsky and Muehlbauer 1993 noted that the gene pool placement of two more recently identified species L. lamottei and L. tomentosus is inconsistent among studies. These two species were placed in the secondary gene pool by Muehlbauer and McPhee (2005). Fratini and Ruiz (2006) suggested that L. ervoides and L. nigricans should be placed in the tertiary gene pool. Nonetheless, the gene pools proposed by all these studies are not in concordance with the species classification from the study of Ferguson et al. (2000), suggesting more work is needed to clarify these relationships.
Most recently Wong et al. (2015) used next-generation, sequencing-based genotyping methods for screening of the lentil germplasm collection using a two-enzyme GBS approach to clarify the doubts on gene pool classification in lentil. They constructed two 96-plex GBS libraries with a total of 60 accessions where some accessions had several samples, and each sample was sequenced in two technical replicates. They detected a total of 266,356 genome-wide SNPs by automated GBS pipeline. Based on haplotype information, they filtered low quality and redundant SNPs, and constructed a maximum-likelihood tree using 5389 SNPs. The phylogenetic tree grouped the germplasm collection into their respective taxa with strong support. Based on phylogenetic tree and STRUCTURE analysis, they identified four gene pools, namely, L. culinaris/L. orientalis/L. tomentosus, L. lamottei/L. odemensis, L. ervoides, and L. nigricans, which form primary, secondary, tertiary, and quaternary gene pools, respectively.
The lentil was utilized by man during the early stages of the Neolithic Revolution. Remains of lentil seeds in archeological digs suggest that it was one of the first plants to be exploited by man (Zohary and Hopf, 1988). The oldest seed remains come from the Middle East, hence, the prevailing idea that lentil domestication occurred here, together with that of other pulses and cereals. Attempts to identify the place of domestication based on the occurrence of carbonized lentil seeds in archeological sites are also not without complications, because there is no way to distinguish between wild and cultivated forms found in these remains. By accepting L. nigricans as the wild progenitor of lentils, Renfrew (1969, 1973) concludes southern Europe was the place where the lentil evolved. But Zohary (1972) and Williams et al. (1974), favoring Barulina’s (1930) idea that lentil originated from L. orientalis, interpreted the occurrence of carbonized lentil seeds in Neolithic settlements in the Middle East as a sign that the lentil was domesticated throughout the Fertile Crescent. Archeological studies, have confirmed the presence of lentil in the Turkey-Syria-Iraq region as far back as 8500–600 BC. This region probably played an important role in lentil domestication and the further spread to the Nile, Greece, Central Europe, and eastward to South Asia (Nene, 2006). At the same time, lentils also spread to Ethopia, Afghanistan, India, Pakistan, China, and later to the New World, including Latin America (Cubero, 1981, Duke, 1981, Ladizinsky, 1979a). Bahl et al. (1993) suggested that lentils’ domestication was probably the oldest among grain legumes. Domestication may have started with the selection of plants from wild species that retain their seeds in pods before harvesting, and continuous selection for large seed size. Two main traits for lentils were involved in domestication, pod dehiscence and seed dormancy, and both are reported to be under the control of a single recessive gene. A third major trait, seed size, appears to be under more complex control (Sonnante et al., 2009). Many authors consider seed dispersal to have been the first character entity selected by man, because of its consequences on harvest effectiveness, thus yield, and seed increase efficiency (Zohary, 1999). Certainly, seed size increase followed later, because archeological relics show no seed size increase until the Bronze Age (Fuller, 2007). To establish how and when domestication took place, evidence tends to suggest that cultivation was carried out by man far before domestication traits were fixed (Pringle, 1998; Balter, 2007). The most possible pattern is that man consciously targeted those cultivated wild species which were carrying the genes targeted by domestication. Therefore, consider domestication as a guided evolution, in which natural selection was substituted with agricultural pressure. The evidence that domestication may not be a sudden process might also come from the observation that seed size increase is surely successive to the establishment of a real crop (Sonnante et al., 2009).