Induced mutations for resistance to aphid (Aphis craccivora Koch) in cowpea

Aphid is a major insect pest of cowpea and causes heavy yield losses throughout the world. Two mutants, ICV11 and ICV12, resistant to aphids were developed from the M3 generation of ICV1 seeds irradiated with 20 kR of gamma rays. These mutants are true breeding and have a high level of seedling resistance to aphids under screenhouse conditions. The mutants have a high level of antibiosis, causing the death of the pest. They have improved plant type (semi-erect) and longer pod size (20 cm), with 19 seeds, compared with ICV1, which has a pod size of 14 cm with 13 seeds. However, the mutants are late in flowering and maturity compared with the control ICV1. Grain yields of mutants under 'unspread' field conditions are rather low, but they could give higher yields if planted early or protected from other insects using insecticides. The genetics of resistance of mutants was studied in crosses with the susceptible ICV1 and other resistant cultivars. Five day old seedlings were infested with ten aphids per plant in the screenhouse. The reactions of the F1, F2 and BC1 (F1 back-crossed to the susceptible parent ICV1) populations from the crosses of the resistant mutant ICV11 with ICV1 revealed that resistance was governed by a single dominant gene. Analyses of the allelic relationship of the gene for resistance in ICV11 were undertaken in the F1 and F2 populations from the crosses of ICV11 with other resistant cultivars TVu310 and ICV10. Results indicated that the single dominant genes of TVu310 and ICV10 are the same and allelic, while the single dominant gene of the mutant cultivar ICV11 is non-allelic to, and independent of, the resistance genes of TVu310 and ICV10. The dominant gene of TVu310 and ICV10 is designated as Ac1, while the dominant gene of ICV11 is designated as Ac2. Use of resistance genes in breeding for aphid resistance is discussed. (author). 19 refs, 3 figs, 9 tabs