Genetic study of cowpea resistance to flower bud thrips (Megalurothrips sjostedti Trybom)

Abstract

Cowpea [Vigna unguiculata (L.) Walp.] is an important and cheap source of protein in the diets of poor people in developing countries. It is cultivated for its leaf and grain, with a grain yield potential of 3000 Kg ha-1. However, the actual yield in farmers’ fields averages 200-400 Kg ha-1. This low yield is due to various constraints. Flower bud thrips is one of most damaging insect pests on cowpea at flowering stage. The yield reduction due to flower bud thrips ranges from 20 to 80% but under severe infestation, grain yield may be almost nil. Chemical control is the most used means to deal with thrips in cowpea. Unfortunately, the rapid development of resistance to insecticides in thrips populations has rendered the chemical treatments ineffective. Host resistance may be deployed to manage thrips and offers the potential to avoid dependence on environmentally toxic chemicals. The main objective of this study was to contribute to improved cowpea productivity in Uganda through generation of deeper knowledge on cowpea genetic resistance. Specifically this study aimed to: (1) determine the reaction of cowpea lines to flower bud thrips in different environments in Uganda; (2) determine the biochemical constituents influencing flower bud thrips resistance in cowpea germplasm; (3) determine the mode of inheritance for resistance to flower bud thrips and (4) identify the quantitative trait loci (QTL) for resistance to flower bud thrips. In order to identify the resistance sources to flower bud thrips among the existing cowpea germplasm, seventy two cowpea lines that contain eight breeding lines from the International Institute of Tropical Agriculture (IITA), sixteen Uganda breeding lines and fourty-eight landraces were screened under natural infestation using alpha lattice design with two replications across six environments (three locations x two rain seasons) in Uganda, for flower bud thrips incidence, damage, and yield components. Thrips species that damaged the flower buds and the flowers of cowpea at flowering stage belonged to Megalurothrips sjostedti species. From the 72 cowpea lines screened for their resistance to flower bud thrips, eleven lines (IT2841*Brown, MU20B, EBELAT*NE39, WC17, WC29, MU24C, WC5, NE46, WC30, NE67, and NE51) were consistently least damaged across the environments. Thrips damage was negatively correlated with the number of days to flowering (r = -0.32), indicating that the resistance in the lines may be explained by their ability to escape flower bud thrips infestation through later flowering. Line MU9 was high yielding (813.87 kg ha-1) and the most yield stable genotype across all the environments; while lines WC26, NE48, and NE5 were the most adapted to Arua and Serere, and WC48A was the most adapted to Wakiso. The eleven resistant lines have a potential of being resistance sources to flower bud thrips. After establishing the resistance status of the lines, three resistant and three susceptible lines were evaluated to determine the biochemical basis of resistance. The test lines were reevaluated for thrips damage using randomized complete block design for two consecutive rain seasons in Wakiso. The leaves, stipules, flower buds and flowers were collected for the extraction of eight biochemical compounds in the biochemistry laboratory of National Crops Resources Research Institute (NaCRRI), Uganda. The results showed that the lines presented different concentrations of total carbon, total reducing sugar, flavonoids, soluble amino acid, antioxidant activity, and tannin in the plants parts. Line TVU-1509 suffered the least thrips damage (score=1.03 out of 9) while WC36 was most severely damaged by thrips (score=7 out of 9). Correlation and regression analyses showed that increase in flavonoids content, total reducing sugar (glucose, fructose) and total carbon (total carbohydrates) in the plants reproductive structures, contributed to the reduction of thrips damage (coefficient of regression = -1.47; -0.61 and -0.48, respectively) while the increase in the concentration of the soluble amino acid allows for more thrips damage (coefficient of regression = 2.10); showing that flavonoids, total reducing sugar and total carbon contributed to resistance of cowpea to flower bud thrips damage. These biochemical compounds can be used as selection traits for enhancement of cowpea resistance to flower bud thrips. To understand the mode of inheritance of cowpea resistance to flower bud thrips, five resistant and three susceptible lines also taking into account others desirable agronomic traits (number of pods per plant, number of peduncles per plant and yield), were crossed in full-diallel mating design. The F2 plants were evaluated with their parents in Arua, Wakiso and Serere. The results indicated that environmental effects were highly significant (P<0.001). Maternal by location interaction effects were also significant (P<0.05). The frequency distributions of the F2 generation revealed quantitative inheritance. Additive and non-additive gene effects made major contributions. One dominant gene, two complementary dominant genes and three complementary recessive genes were involved in the expression of resistance to flower bud thrips. Genotypes TVU-1471, TVU-1509 and TVU-473 were good transmitters of resistance against flower bud thrips in cowpea. Crosses TVU-1509 x NE5, TVU-473 x Sanzi, TVU-123 x Sanzi, TVU-123 x TVU-473, and TVU-473 x TVU-1509 were the most useful crosses that would lead to promising progeny lines as they exhibited high to moderate resistances to flower bud thrips. In order to identify the quantitative trait loci (QTL) that were associated with flower bud thrips resistance genes in cowpea, a F2 mapping population was developed from the genotypes TVU-123 (resistant) and WC36 (susceptible) and evaluated under artificial infestation. Sixty-six cowpea microsatellites (SSR) markers were screened between the two parental lines and the polymorphic markers were used to genotype the F2 populations. The marker-traits association analysis was performed using single marker analysis methods (chi-square independence test on thrips damage scores and analysis of variance on thrips number per flower). The results showed that seven SSR markers were polymorphic between the parents and two QTL (markers CP37/38 and CP215/216) were associated with flower bud thrips resistance genes in cowpea. The identified QTL explained 18.20% of the total variation for flower bud thrips resistance (7% for damage scores and 11.20% for counts). The QTL detected had mainly additive gene effects, therefore can be used for marker-assisted selection. A more detailed study of these loci using recombinant inbred lines and more markers would provide better understanding of this complex trait. This study found the sources and mechanisms of resistance to flower bud thrips in Uganda cowpea germplasm in relation to IITA lines that should serve as a basis for the enhancement of cowpea resistance to flower bud thrips. The eleven fore mentioned resistant stable cultivars identified in this study are recommended for wide adoption by farmers in the growing areas in Uganda. The resistant lines and good combiners identified in this study may serve as parents in breeding programs in Uganda. Breeders may emphasize the increase in the concentration of flavonoids, total reducing sugar, and total carbon in cowpea lines for enhancement of cowpea resistance to flower bud thrips. Further studies are needed to establish the stability of the developed lines in this study and the consistence of the QTL for marker assisted selection.