Deployment of the Cowpea Aphid Resistance Gene for Cowpea Improvement in Ghana
Resistance to the cowpea aphid is important component of integrated pest management of cowpea cropping systems most especially at the vegetative stage. The objective of this study was to demonstrate the effectiveness of the aphid resistance locus identified in advanced breeding line SARC 1-57-2 in reducing damage from the cowpea aphid in Ghana. Using an F2 population developed from Apagbaala x SARC 1-57-2, the resistance locus was tagged with the SSR marker CP 171F/172R with a recombination fraction of 5.91%. Based on the CP 171F/172R, recurrent marker assisted backcrossing was carried out to introduce the resistance locus into the susceptible cultivar, Zaayura. This led to the development of several BC4F3 lines that are isogenic except for the region of the resistance locus. In field tests under no insecticide protection, the BC4F3 lines carrying the dominant marker allele suffered 3% loss of biomass and 4% loss of grain yield compared with plots protected with recommended insecticides. The BC4F3 lines carrying the recessive marker allele recorded 12% loss of biomass and 33% reduction in grain yield compared with the sprayed plots. The resistance locus did not influence the number of days to flowering or maturity and no pleiotropic effects were observed in terms of plant morphology or seed characteristics. In all segregating populations analysed, the locus segregated as a single Mendelian gene. Stability of the resistance locus was conducted at 18 locations covering six important cowpea growing Regions in Ghana. The range of damage by the pest on resistant and susceptible progenies were consistent across locations, and did not support the hypothesis of existence biotypes of the insect (based on differences in feeding damage on different varieties) in Ghana. This stability in performance places a premium on the resistance locus in improving cowpea cultivars developed for different agro-climatic regions of the country for resistance to the pest. The study has demonstrated the effectiveness of an insect resistance locus in significantly reducing insect damage under typical cowpea production conditions in Ghana.
Cowpea, Vigna unguiculata (L.) Walp is an important source of protein for human nutrition in many parts of the semi-arid tropics (SAT) (Rachie, 1985; Githiri et al., 1996; Bashir et al., 2002). It is eaten in the form of dry seeds, green pods, green seeds, and tender green leaves (Githiri et al., 1996). Cowpea is also an important source of protein for animal nutrition; it is used for pasture, hay, silage, or green manure (Singh, 1990). Nigeria, Brazil, Niger and Burkina Faso are among the major producers and account for over 70 % of the world crop (FAO, 2008). Nigeria is the largest producer and consumer of cowpea, with about 5 million ha and over 2 million mt production annually, followed by Niger (650,000 mt) and Brazil (490,000 mt) (Timko et al., 2008: FAO, 2008; Asare, 2012). However, yields at farmers level are low (Jackai and Dacoust, 1986; Motimore et al., 1997: Asare, 2012). The major cause of the low yields are insect pests, diseases, drought and low soil fertility, of which insect pests constitute the major constraint (Nampala et al., 1999; Asare, 2012).
Cowpea suffers serious insect pest infestation from the time of planting through harvesting and during storage (Obeng-Ofori, 2007). The crop therefore suffers severe attack of pre- harvest and post-harvest infestation which if not controlled could lead to total crop failure. The major field pests of cowpea are aphids (Aphis craccivora Koch), flower bud thrips (Megalurothrips sjostedti Trybom), the legume pod borer (Maruca vitrata Fab), pod- sucking bugs including Clavigralla tomentosicollis Stål, Anoplycnemis curvipes Fab., Mirperus jaculus Thunbeng and Nezera viridula Linnaeus (Singh and Jackai, 1985; Jackai and Adalla, 1997; Obeng-Ofori, 2007:ghEo, 2011). The cowpea aphid, A. craccivora, is an important pest of cowpea in Africa (Singh and Jackai, 1985; Kusi et al., 2010a; Souleymane
et al., 2013). The pest primarily infests the seedlings of cowpea and causes direct damage to the crop by sucking plant sap, resulting in stunted plants and distorted leaves (Bohlen, 1978; Jackai and Daoust, 1986; Ofuya, 1997a). Aphids are usually found in clusters around stems, young leaves and on young shoots. The infested leaves are often cupped or distorted and become more or less yellow (Singh and Jackai, 1985). In heavy infestation the plant dies, especially under water stress (Ofuya, 1995). High numbers of cowpea aphids can produce a significant amount of honeydew and sooty mould which reduce the photosynthetic ability of the leaves (Baute, 2004). Indirectly, cowpea aphid transmits aphid- borne cowpea mosaic viruses (Singh and Jackai, 1985; Thottappilly and Rossel, 1985; Shoyinka et al., 1997). Estimated yield losses of 20% to 40% in cowpea due to A. craccivora infestation in Asia and up to 35% in Africa have been reported (Singh and Allen 1980; Kusi et al., 2010b). In eastern region of the Democratic Republic of Congo,phAis craccivora (Hemiptera: Aphididae) is a major pest of cowpea and groundnut (Munyuli et al., 2007) where about 35-65% of yield losses are associated with this pest species (Munyuli et al., 2008; Munyuli, 2009).
The cowpea aphid can be controlled by various methods including the use of insecticides, cultural practices and biological control (Singh and Jackai, 1985). However, growing of aphid resistant cultivars offers one of the simplest and most convenient methods of pest control for the resource-poor farmers (Dent, 1991; Orawu et al., 2013).
Host plant resistance as indicated by Painter (1951) is a relationship between the plant feeding insects and their host-plants. It is the property that enables a plant to avoid, tolerate or recover from injury by insect populations that will cause greater damage to other plants of the same species under similar environmental conditions (Kogan, 1975; Tingey, 1986). Kumar (1984) and Dent (1991) defined host plant resistance as the inherent ability of crop
plants to restrict, retard or overcome pest infestation and thereby improve yield and/or quality of the harvested product. Host plant resistance has proved to be a successful tool against insect pests that attack many crops (van Emden, 1991; Thomas and Waage, 1996; Felkl et al., 2005; Orawu et al., 2013). Plant genotypes, either due to environmental stress or genetic makeup, possess physiological and biochemical differences which alter the nutritional value (primary metabolites) and may also cause changes in the levels of secondary metabolites that could affect the behaviour of phytophagous insects (Eckey- Kaltenbach et al., 1994; Karban and Baldwin,9179: Siemens et al., 2002; Städler, 2002; Theis and Lerdau, 2003).
Three mechanisms of plant resistance originally defined by Painter (1951) are non- preference, antibiosis and tolerance. The non-reference has since been replaced by antixenosis (Kogan and Omar, 1978). Antixenosis is the inability of a plant to serve as host to an insect herbivore. The basis of this resistance mechanism can be morphological (e.g. leaf hairs, surface waxes, tissue thickness) or chemical (e.g. repellents or antifeedants) (Kogan and Omar, 1978). These plants would have reduced initial infestation and/or higher emigration rate of the insect than susceptible plants.
Antibiosis is the mechanism that describes the negative effects of a resistant plant on the biology of an insect which has colonized the plant (e.g. adverse effect on development, reproductive and survival) (Painter, 1951; Kogan and Omar, 1978). Both chemical and morphological plant defences can induce antibiosis effects (Painter, 1951; Kogan and Omar, 1978). The consequences of antibiosis resistance may vary from mild effect that influences fecundity, development time and body size to acute direct effect resulting in death (Painter, 1951; Kogan and Omar, 1978).
Plant tolerance is the degree to which a plant can support an insect population that under similar conditions would severely damage a susceptible plant (Painter, 1951; Kogan and Omar, 1978). When two cultivars are equally infested the less tolerant one will produce low yield.
Cowpea aphids are easily controlled by the use of aphid resistant varieties (Singh, 1977; Obeng-Ofori, 2007). Several aphid-resistant cowpea lines have been identified at the IITA and have been tested against aphid populations from several locations in Africa and Asia (Chari et al., 1976; Dhanorkar and Daware, 1980; Karel and Malinga, 1980; MacFoy and Dabrowski, 1984; Manawadu, 1985; Ofuya, 1988a; 1993). Antibiosis has been shown as the main mechanism responsible for aphid resistance in cowpea (Singh, 1977; Ansari, 1984; Ofuya, 1988b,) and is controlled by a single dominant gene (Singh and Ntare, 1985; Bata et al., 1987; Ombakho et al., 1987; Singh, et al., 1987; Pathak, 1988). Additionally, a large number of aphid-resistant lines have been developed, and have been evaluated in international yield trials (MacFoy and Dabrowski, 1984; Manawadu, 1985; Ofuya, 1988a; 1993).
Farmers have over-relied on chemical insecticides over the years to control cowpea aphid which has resulted in misuse of chemicals, high cost of production, poisoning of human beings, the environment and development of resistance to most of the insecticides leading to resurgence of the aphids (Dent, 1991; Singh and Jackai, 1985).
More recently, Kusi et al. (2010a) identified new sources of cowpea genotypes (SARC1- 57-2) resistant to A. craccivora. Segregation ratio in F2 population generated between a resistant line and Apagbaala (a susceptible parent) suggested that a single dominant gene
controlled resistance to the aphid in the breeding line. This presents a valuable source of resistance for developing cowpea cultivars with resistance to the cowpea aphid in the field. Ongoing efforts at mapping the cowpea genome presents an opportunity to tag the resistance locus with co-dominant PCR based markers to facilitate marker-based selection of aphid resistant progenies in large segregating populations.
The main objective of the study was to demonstrate the effectiveness of an aphid resistance locus in the cowpea line SARC 1-57-2 at controlling aphid in Ghana and specifically:
- Identify DNA marker(s) tightly linked to locus controlling resistance to the cowpea aphid in a resistant breeding line, SARC 1-57-2.
- Deploy the DNA marker(s) to improve at least one cowpea cultivar through marker- assisted
- Assess the stability of the cowpea aphid resistant line in the major cowpea growing regions in
- Determine yield loss due to aphid infestation in near isogenic lines developed from the resistant line SARC 1-57-2 and Zaayura
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