Advanced backcross-QTL strategy Since the mid-1990s, convincing evidence at both morphological and molecular levels has accumulated for the utility of wild progeni-tors and related species as donors of produc-tivity alleles. Producproduc-tivity-enhancing genes/
QTLs (quantitative-trait loci) have been intro-gressed in oats from Avena sterilis (Frey et al., 1983), in tomato from Lycopersicon pimpinelli-folium and L. parviflorum (Tanksley et al., 1996;
Fulton et al., 2000), in rice from Oryza rufipogon (Xiao et al., 1996) and in chickpea from Cicer reticulatum (Singh et al., 2005). Novel breeding strategies such as AB-QTL (advanced back-cross-QTL) have been deployed to exploit the worth of the progenitor and related species as this helps minimize the negative effect of linkage drag associated with alien gene intro-gression (Tanksley and Nelson, 1996). The related species of mung bean, such as Vigna umbellata and V. angularis, have compara-tively higher productivity and their relation-ship with mung bean offers an opportunity for the introgression of some productiv-ity alleles using AB-QTL strategy. Another related species, V. mungo, and the wild pro-genitor of mung bean, V. radiata var. sublobata, may also contribute some productivity alleles to the elite mung bean lines using the same approach.
Looking for genes based on molecular maps
The traditional approach in utilizing exotic germplasm is to screen the phenotype of entries from a gene bank for a clearly defined character and to use them in a crossing pro-gramme in order to introduce the genes into cultivated germplasm. Although effective for qualitative traits, only a small proportion of the genetic variation has been exploited for crop improvement as a result of this strategy (Tanksley and McCouch, 1997). Availability of genetic linkage maps based on molecular markers has opened up new opportunities in
Distant Hybridization and Alien Gene Introgression 97
the utilization of hitherto unexploitable exotic germplasm. This requires a paradigm shift from selecting potential parents on the basis of phenotype to evaluating them directly for the presence of useful genes, through the integra-tion of molecular tools. A gene-based approach to screening exotic germplasm has already been successfully used in rice and tomato for improving yield levels (Tanksley et al., 1996;
Xiao et al., 1996). Recently, good progress has been made in generating genomic resources for food legume crops that will be very useful in genetic mapping and QTL analysis in these crops (Varshney et al., 2009). With the use of DNA profiles, the genetic uniqueness of each accession in a gene bank can be determined and quantified. Molecular marker technol-ogy allows a targeted approach to the selec-tion and introgression of valuable genes from a range of genetic resources while retaining the integrity of valuable genetic background through forward and background selection.
Recombination DNA technology Transgenic approaches provide new options for broadening the genetic base in those cases where current options are lacking in their effi-cacy or existence. Plant genetic transformation techniques such as Agrobacterium-mediated transformation and direct gene delivery sys-tem (biolistics) allow the precise transfer of genes from any organism into either plant nuclear or chloroplast genomes. Many iso-lated plant genes are now being transferred between sexually incompatible plant spe-cies. In chickpea and pigeon pea, helicoverpa pod borer is a major insect pest for which no genetic solution exists. This requires devel-opment of transgenics having Cry genes from the soil bacterium Bacillus thuringiensis to combat the menace of helicoverpa pod borer. The recent report of a Bt. chickpea is an encouraging step towards improvement of food legumes for difficult traits such as pod borer resistance (Acharjee et al., 2010).
Similar is the case for botrytis grey mould in chickpea, where efforts are under way to construct a resistance against this disease. For gene introgression purposes, difficult species
falling in tertiary and quaternary gene pools may turn out to be important sources of alien genes. For example, identification and clon-ing useful genes from Phaseolus filiformis, P. angustissimus and P. lunana and successful regeneration and transformation of common bean may facilitate gene introgression in the future.
Protoplast technology
Somatic hybridization using protoplast fusion has potential to overcome pre- and post-zy-gotic barriers to interspecific hybridization (Powers et al., 1976; Davey et al., 2005). It is possible to regenerate plants from a number of legume species, including Pisum (Ochatt et al., 2000), Trifolium (Gresshoff, 1980), Lotus (Ahuja et al., 1983) and Melilotus (Luo and Jia, 1998), and asymmetric protoplast fusion has been used for Medicago improvement (Tian and Rose, 1999; Yuko et al., 2006). However, only a few reports of successful regeneration of plantlets are available in legumes (Li et al., 1995). Initially, protoplast-derived tissues in rice bean were obtained although no shoot regeneration could be obtained. Shoot regen-eration from protoplasts of Vigna sublobata has more recently been reported by Bhadra et al. (1994), with the maximum protoplast yield being obtained from 5-day-old seed-lings. There are no reports at the time of writ-ing of successful growth or regeneration of protoplasts from Lens species. Rozwadowski et al. (1990) cultured protoplasts from lentil epicotyl tissue, and around 6% of protoplasts developed into cell colonies.
Doubled haploids
Doubled haploid breeding is an important approach in many crop species, including wheat, barley, rice, maize and canola, to fix the hybrid immediately. Implementation of dou-bled haploids increases selection efficiency and allows new varieties to be bred up to 5 years faster than with conventional breeding methods alone. Haploids may be produced from either immature pollen cells, immature
egg cells or following asymmetric chromo-some elimination after interspecific hybridi-zation. Several attempts have been made to develop anther and microspore culture sys-tems for chickpea (Huda et al., 2001; Vessal et al., 2002; Croser et al., 2006), common bean (Peters et al., 1977; Munoz-Florez and Baudoin 1994a, b), field pea (Croser et al., 2006) and pigeon pea (Pratap et al., 2009). In chickpea, cultivars responsive to isolated microspore cultures have been identified and the induc-tion of sporophytic development achieved in uninucleate microspores via the application of heat stress (32.5°C) pre- treatment to the buds (Croser et al., 2006). Due to difficulty in derivation of green haploid regenerants these species have been defined as recalcitrant to androgenesis, although some progress has been made towards standardizing callus induction media and culture conditions in some of these crops. However, the produc-tion of a successful double haploid system in chickpea has been reported (Grewal et al., 2009). A review of the literature on doubled haploid production in Fabaceae (Croser et al., 2006) indicated that none of these approaches had been successful in producing haploid plants in food legumes, but the early stages of isolated microspore division have been observed.
6.7 Prospects
Productivity of food legume crops is affected by various biotic and abiotic stresses. There is thus an urgent need to widen the cultivated gene pool of these crops by incorporating genes for economically important traits from diverse sources. Wild species have proved to be an important reservoir of useful genes, and offer great potential for the incorporation of
such genes into commercial cultivars. Many of the useful alien genes are expected to be different from those of the cultivated species, and are thus useful in broadening the base of resistance to various stresses. Recently, QTLs (oligogenic traits) that have been identified for yield traits in wild species of pulse crops may enhance agronomic and market values of cultivated varieties. The molecular marker technique can also be used for authentication of interspecific hybrids (Yamini et al., 2001).
There is a need to identify high-crossability genes in food legumes, as has been identified in wheat cultivars such as Chinese Spring (Luo et al., 1993; Sharma, 1995). Identification of such genes in food legumes can bring non-crossable species within the ambit of alien gene transfer technology. There are major gaps in germplasm collections of wild spe-cies and their evaluation in food legumes that need to be filled, in order to progress further inroads in alien gene introgression.
Continuing advances in wide-crossing tech-niques, such as embryo culture and develop-ment of novel crossing strategies, are creating greater accessibility in wild gene pools of many crops. The success rate of gene trans-fer in such wide crosses can be increased by knowledge of chromosome pairing mecha-nisms and their genetic control. The modern tools of molecular biology, such as mono-clonal antibodies and in situ hybridization using various DNA probes, may soon make it possible to study the switching on and off of various genes in diverse tissues of the fer-tilized ovule, and control over the levels and movements of both exogenous and endog-enous growth substances within the develop-ing seed. It is likely that continudevelop-ing advances in structural genomics and genetic engineer-ing will result in new strategies for alien gene introgression.
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