Mohar Singh ICAR-National Bureau of Plant Genetic Resources, Regional Station, Shimla, India
Lentil genetic resources, including wild taxa, are potential reservoirs of useful genes/alleles for both broadening the existing genetic base and synthesizing a new gene pool. To maximize the genetic gains and sustain lentil production, new gene sources must be identified and introgressed into the elite genetic background. Therefore, accumulating target characters for widening the cultivated gene pool would be the most appropriate strategy for solving the problems associated with stressed crop production and plateaued yields. Further, expansion and deepening of our understanding of lentil genomics, along with comprehensive research in other related crop species, provide suitable guidelines to achieve remarkable progress in lentil genetic improvement. Further, proteomics and metabolomics are emerging technologies which can be used to characterize the functional mechanisms behind crop-breeding targets.
Genetic gains; Characterization; Wild lentil; Introgression; Genetic mapping; Genetic transformation
Domesticated lentils (Lens culinaris ssp. culinaris) is an annual, herbaceous, self-pollinating, true diploid (2n = 2 × = 14) species with an estimated genome size of 4063 Mbp/C (Arumuganathan and Earle, 1991). The crop is one of the first domesticated grain legume species originated from the Near East center of origin (Zohary, 1999) and it is the most appreciated grain legume of the Old World (Smartt, 1990). It is an important cool-season legume species grown in Mediterranean and semi-arid climates. It provides an affordable source of dietary protein (22%–25%), minerals (K, P, Fe, and Zn), carbohydrates, and vitamins for human nutrition (Bhatty, 1988; Kumar et al., 2018). Lentil grains are also rich in lysine and tryptophan content (Erskine et al., 1990). The genus Lens belongs to family Fabaceae, and a total of seven annual species have been recognized, including the cultivated species L. culinaris subsp. culinaris. The other wild taxa include L. culinaris ssp. odemensis Ladizinsky; L. culinaris ssp. orientalis (Boiss) Ponert; L. ervoides (Brign) Granade; L. lamottei Czefr; L. nigricans (Bieb) Godron; and L. tomentosus Ladizinsky. As far as the cross-compatibility of Lens taxa is concerned, L. culinaris subsp. orientalis is fully cross-compatible with the domesticated lentil (Roberson and Erskine, 1997) and has been proposed as its putative ancestor (Barulina, 1930; Mayer and Soltis, 1994). Globally, the lentil ranks sixth in production among grain legumes after dry beans, peas, chickpeas, faba beans, and cowpeas (FAO, 2015). However, the world’s lentil production constituted 6% of total dry pulse production during 2010–15, with an average productivity of 926 kg/ha. India is the biggest lentil-producing country in the world, followed by Canada and Turkey, which collectively accounted for 66% of total global lentil production (FAOSTAT, 2016). Further, average lentil yield in Asia is 817 kg/ha, which is significantly below the world average of 926 kg/ha. Lentils, despite their tremendous significance in human food, animal feed, and cropping systems (in the Indian sub-continent, West Asia, Ethiopia, North Africa, parts of Southern Europe, Oceania, and North America), have remained an under-exploited and under-researched crop until recently. Modern lentil commercial varieties have some superiority over traditional ones in terms of their yield potential and disease-resistance. A small number of improved local landraces have contributed significantly to the development of a majority of improved varieties through pure lines and mass selection, following hybridization between lines adapted to specific environmental conditions across lentil-growing regions. Notwithstanding the number of lentil varieties released, there has been slow progress in production and productivity of this important crop over the decades. Besides a very high influence of environmental factors, cultivated lentil germplasm has low genetic variation as compared to wild species (Ford et al., 1997; Duran et al., 2004). Among different accounts across lentil-growing regions in India, the pedigree analysis of 35 released lentil varieties has been traced back to only 22 ancestors, and the top 10 contributed 30% to the genetic base of released cultivars (Kumar et al., 2003). This situation could lead to crop vulnerability to pests, disease epidemics, and unpredictable climatic factors, limiting progress in increasing lentil production. Furthermore, the narrow genetic base of lentil varieties owes their vulnerability to several biotic and abiotic stresses and loss of yield-contributing traits because of their cultivation on marginal lands, mostly in developing countries (including India). To meet the dietary requirements of a growing human population, consolidated efforts are necessary to increase the genetic potential of existing lentil varieties. Therefore, there is an immediate need to broaden the genetic base of lentil cultivars by introgression of diverse gene sources, which are available in distantly related wild Lens taxa. To broaden the genetic base and maximize gains from selection, it is imperative to accumulate favorable genes and alleles from the potential germplasm in elite lentil backgrounds.
To enrich the genetic resource base, it is clear that germplasm must be collected from the diversity rich regions across the globe. The establishment of ex-situ germplasm collections (gene bank) has been the result of rigorous global efforts over several decades to conserve diverse genetic resources, including crop wild relatives (CWRs). In the case of lentils, the International Centre for Agricultural Research in Dry Areas (ICARDA) holds the largest collection of lentil germplasm, including wild species collected from more than 46 countries (Furman et al., 2009). The first ex-situ gene bank of lentil collection including wild relatives was established in the Vavilov Institute for plant industry in Russia. The survey of global ex-situ collection of lentil germplasm accessions lists more than 43,214 and about 40,000 other accessions, which are preserved in the different gene banks around the world (GCDT, 2016). For wild lentil germplasm collection, ICARDA has undertaken more than 120 explorations and collected more than 400 wild accessions from diversity-rich areas of the world (Redden et al., 2007). Turkey has the largest holdings of lentil landraces (Atikyilmaz, 2010), followed by Nepal and Pakistan (Sultana and Ghafoor, 2008), Bangladesh, Spain, Syria, Ethiopia, and China (Fikiru et al., 2007; Liu et al., 2008). An exact status of on-farm in-situ conservation of diverse germplasm is not well documented (Furman et al., 2009). Recently, wild species of lentils have received positive attention for being an invaluable genetic resource for widening the genetic base of cultivated lentil varieties. Because these species hold a wealth of useful genes and alleles, it is possible to break yield barriers and enhance tolerance to various biotic and abiotic stresses. The important regions for in-situ on-farm conservation of wild germplasm comprise West Turkey for L. nigricans, Southeast Turkey, Southwest Syria, and Jordan for L. culinaris ssp. orientalis, South Syria for L. culinaris ssp. odemensis, and the border areas of Turkey and Syria for L. ervoides (Ferguson et al., 1998).
Each of these species has its own morphological characteristics and shows specific ecological affinities and typical geographic distribution. L. culinaris ssp. orientalis and odemensis are members of the primary gene pool, whereas L. tomentosus, L. lamottei, L. nigricans, and L. ervoides species belong to the secondary gene pool. A recent study using Genotyping by Sequencing (GBS) method based on the phylogenetic tree and STRUCTURE analysis 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. Germplasm enhancement approaches have also been used to study variation among important traits of interests, namely, high pods/plant, seeds/pod, number of clusters/plant, harvest index (%), biological yield (g), and early maturity, etc. Sincere efforts have also been made to characterize and evaluate various agro-morphological traits in large numbers of lentil accessions, including global wild accessions, for which remarkable variation was recorded. Many attempts have been made in the past to transfer desirable genes from wild lentil species to cultigens using embryo rescue technique and cold-tolerant genes from L. orientalis and some disease-resistant genes from Lens ervoides have also been transferred to the cultivated lentil. However, an understanding of the genetic potential of both cultivated and wild lentils is essential for mining useful genetic variations, as well as for adding considerable resources to lentil breeding programs. The chapter also addresses the knowledge on available lentil genomics derived through traditional cytogenetics and the potential to improve our understanding through applications of modern cytogenetic manipulations, including the use of fluorescence in-situ hybridization (FISH), genome in-situ hybridization (GISH), and distant hybridization, coupled with embryo ovule rescue and haploid breeding to widen the genetic resources of lentil.
Furthermore, molecular breeding technologies including marker-assisted selection, marker-assisted backcrossing, marker-assisted recurrent selection, gene pyramiding, marker-assisted backcross gene pyramiding, and genomic selection have been used to introgress single or multiple genes. Multiple-trait selection using selection indices based on information from both phenotypes and markers distributed across the whole genome has recently been practiced in various crops including lentils. Multiple-trait selection is a realistic approach that can be exploited in lentil breeding programs to improve multiple traits simultaneously. There has been considerable progress on in-vitro regeneration as well as Agrobacterium-mediated genetic transformation to integrate fungal diseases resistant gene in microsperma varieties of lentils. A complete protocol for Agrobacterium-mediated genetic transformation was established by utilizing GUS and nptII genes. Among various explants studied, cotyledon-attached decapitated embryo (CADE) appeared to have the best response toward in-vitro regeneration compatible to Agrobacterium-mediated genetic transformation. Those in-vitro regenerated shoots failed to produce effective roots were subjected to induce in-vitro flowers as well as seeds to develop an alternative regeneration system for lentil avoiding the in-vitro root formation stage. For enhancing the utilization of lentil germplasm, a Focused Identification of Germplasm Strategy (FIGS) has also been developed. In this strategy, germplasm for a target trait is identified from that region, where it is frequently occurred. The strategy has been useful in identification of genotypes having desirable traits such as tolerance to biotic and abiotic stresses, superior grain quality and nutritional traits, improved grain size, and early maturity. Thus, the development of core sets of diverse origin after their screening at multiple locations for useful traits may lead to their utilization in the improvement of yield and productivity.