BREEDING COWPEA [VIGNA UNGUICULATA (L.) WALP.] FOR STRIGA [STRIGA GESNERIOIDES (WILD.) VATKE] RESISTANCE USING MARKER ASSISTED SELECTION IN NIGER - ABSTRACT
Striga gesnerioides (Wild.) Vatke is the main biotic constraint restricting yield of cowpea in Sub-Saharan Africa (SSA) in general and particularly in Niger. The available resistant varieties developed so far often lack farmers‟ and end-users‟ desired traits and they also show levels of breakdown in resistance. Understanding farmers‟ knowledge regarding production constraints and varietal preferences is invaluable in breeding for cowpea improvement. The development of cowpea varieties by introgressing multiple Striga resistance into adapted genotypes will facilitate the achievement of food security in the country. This study aims to (i) assess farmers‟ preferred traits and perceptions of cowpea production and constraints (ii) identify new sources of Striga resistance in the national germplasm and to select high yielding genotypes (iii) identify different races of Striga gesnerioides in Niger (iv) introgress the resistance gene Rsg1 into adapted cultivars and (v) apply Marker Assisted Selection (MAS) to select Striga tolerant genotypes.
A participatory rural appraisal was performed in order to identify the place of cowpea in the farming system and its production constraints as well as farmers‟ knowledge on Striga and their cowpea preferred traits. Farmers ranked cowpea as the second most important crop after millet in Niger. Cowpea was grown for both food (56 %) and cash (44 %). The most important variety selection criteria used by farmers were high yielding potential, early maturity, white-colored grain and good taste. The major constraints to production were insects, Striga, drought and low soil fertility. Farmers had a good knowledge on Striga and their preferred grain characteristics were large size and white seeded cowpea varieties.
A screening was conducted to evaluate the response of 80 genotypes to natural Striga infestation in the field. There was significant variation in the resistance of cowpea lines to Striga. The cowpea lines IT93K-693-2, IT99K-573-1-1 and IT98K-205-8 were free of Striga emerged shoots m-2 while the lines 2491-171, 2472-154 and Suvita-2 supported only a few Striga shoots m-2. The other lines supported higher numbers of emerged Striga shoots. Striga infestation resulted in yield losses of the susceptible genotypes as compared to the resistant and tolerant lines. The screening did not reveal new sources of Striga resistance; however the varieties IT93K-693-2, IT99K-573-1-1 and IT98K-205-8 were confirmed as potential sources of resistance and good donor parents to incorporate Striga resistance in well adapted genotypes. The cultivars B2/16/2378, B1/18/2542 and B1/12/2525-234 were tolerant to Striga with a significant yield potential compared to that of the resistant and the susceptible lines. Therefore, they could be used as donor parents in breeding cowpea for yield improvement in Niger.
Field and pots experiments were conducted during two years in order to identify the different Striga races that infest cowpea in Niger. Fifteen genotypes were screened under natural infestation as well as inoculated with three samples of the parasitic weed collected in three hot spot regions in Niger. The test revealed Striga attack on three multiracial Striga resistant varieties, B301, IT97K-499-35 and IT98K-205-8 suggesting the presence of other races other than SG3 in the studied area; or the breakdown of the resistance to Striga race SG3 in these cowpea varieties. Also, the resistance of varieties HTR and Suvita 2 observed, respectively in field at Kollo and in pots with a sample of inoculum from Magaria suggested the prevalence of Striga race SG1 in these sites. The geographical distribution of Striga races needs to be clarified in Niger. iv
Marker assisted backcrossing (MABC) was used to transfer Rsg1 Striga resistance gene from the breeding line IT93K-693-2 into three farmer preferred varieties, IT90K-372-1-2, KVx30-309-6G and TN5-78. The microsatellite marker SSR1 was used to track and introgress the resistance gene Rsg1 in the varieties IT90K-372-1-2 and TN5-78. The marker which produced a single band in resistant lines and amplified a 150 bp fragment was validated with the parents and the subsequent F1, F2 and BC1F1 populations. Ten promising lines with the resistance marker were selected in BC2F3, BC3F3 and F6 populations derived from the crosses IT90K-372-1-2 x IT93K-693-2 and TN5-78 x IT93K-693-2. Further evaluations and improvement of these genotypes will probably accelerate the release of varieties combining farmers‟ preferred traits with stable resistance to Striga.