GENETIC ANALYSIS OF TRAITS RELATED TO BIOLOGICAL NITROGEN FIXATION IN COWPEA [Vigna unguiculata (L.) WALP] UNDER LOW SOIL PHOSPHORUS.

ABSTRACT Studies were undertaken to obtain information on the genetics of traits related to biological nitrogen fixation in cowpea, in soils deficient in phosphorus prevailing in the humid forest zone (HFZ) of Cameroon. In this zone, farmers are not usually consulted in the development of new varieties in cowpea breeding programmes. Participatory rural appraisal (PRA) technique through focus group discussions and questionnaire administration was used to assess farmers‟ knowledge on the role of grain legumes in improving soil fertility in cropping systems; and to identify farmers‟ preferred cowpea traits in five sites in the HFZ of Cameroon. Ninety percent of the farmers were of the view that legume root nodules were disease causing agents, which they referred to as soil “cysts”. This means productivity on farmers field may be declining due to lack of knowledge. Farmers grew four varieties of cowpea, the white seed coated being the dominant
(75%) and preferred variety. They also mostly grew and preferred the erect and early maturing cowpea type and were ready to adopt cowpea for soil fertility improvement if residual effects of legumes is demonstrated on-farm. Three cowpea varieties (Dsch MMBr, Vya niebe and 58-77) were used as trap crops to estimate the population of indigenous Bradyrhizobia spp in two study sites.  The results of the Most Probable Number (MPN) counts indicated that the total Bradyrhizobia population in Nkoemvone soil was between 1.0 x 105 and 5.8 × 105 cells per gram of soil sample while in Nkometou, it was between 5.8 x 103 and 1.0 x 104.. These levels of Bradyrhizobia populations observed in the two sites were adequate to give satisfactory nodulation and nitrogen fixation. This suggests that cowpea production in the HFZ of Cameroon may not require inoculation. Morphological characterization of 50 cowpea genotypes including 10 Cameroon landraces was carried out in three environments: low phosphorus (LP; 0 mg P kg-1 soil), high phosphorus (HP; 30 mg P kg-1 soil) and high phosphorus and nitrogen (high P and N; 30 mg P and 90 mg N kg-1 soil). Data collection involved 20 morphological traits comprising phenological, nitrogen fixation and yield related traits. Thirteen out of the 20 traits were selected by the principal component analysis and explained 55.3% of the variation. Correlation studies indicated N-fixed and grain yield were positively correlated only under low P conditions, while shoot N was highly and positively correlated with N-fixed in all three environments. Shoot N was considered the best nitrogen fixation related trait and was used in place of N-fixed in subsequent studies. A study was undertaken to determine gene effects controlling nitrogen fixation related traits through generation mean analysis (GMA). The parents included a high N2 fixing genotype, 58-77 and a low N2-fixing type Lori-niebe. Data collected included nodule number (NN), nodule dry weight (NDW), shoot dry weight (SDW), and shoot nitrogen (shoot N) content. All the traits showed significant additive and/or additive x additive gene effects, while additive x dominance effect was significant for SDW and shoot N, suggesting that early generation selection for improved nitrogen fixation would be effective only for NN and NDW. QTL were mapped using 372 polymorphic EST-SNPs on 88 F11 recombinant inbred lines (RILs) from 58-77 (high N2-fixing cowpea) and Yacine (poor N2-fixing cowpea) cross. The traits under study included: NN, NDW, SDW, number of pods per plant (Npod), and weight of hundred seeds (HUN_SDW) phenotyped in eight environments. Win Cartographer identified a total of 31 QTL for NDW, 12 QTL for HUN_SDW, 26 QTL for NN, 32 QTL for SDW, and 14 QTL for Npod across the eight environments. QTLnetwork identified a total of 13 main QTL for all the traits across the eight environments. Fifteen pairs of epistatic QTL were also identified for all the
traits. The results demonstrated the importance of epistasis as a genetic basis of the quantitative traits studied. All the traits revealed epistatatic gene effects with more that 90% of QTL found with significant additive x additive effects. Therefore, in marker assisted selection, markers linked to epistatic QTL should also be taken into consideration. QTL  x environment  interaction effects were detected and might indicate that gene expression could be greatly influenced by environments.