Quantitative trait loci and genomic prediction for grain sugar and mineral concentrations of cowpea [Vigna unguiculata (L.) Walp.].

Development of high yielding cowpea varieties coupled with good taste and rich in essential minerals can promote consumption and thus nutrition and profitability. The sweet taste of cowpea grain is determined by its sugar content, which comprises mainly sucrose and galacto-oligosaccharides (GOS) including raffinose and stachyose. However, GOS are indigestible and their fermentation in the colon can produce excess intestinal gas, causing undesirable bloating and flatulence. In this study, we aimed to examine variation in grain sugar and mineral concentrations, then map quantitative trait loci (QTLs) and estimate genomic-prediction (GP) accuracies for possible application in breeding. Grain samples were collected from a multi-parent advanced generation intercross (MAGIC) population grown in California during 2016-2017. Grain sugars were assayed using high-performance liquid chromatography. Grain minerals were determined by inductively coupled plasma-optical emission spectrometry and combustion. Considerable variation was observed for sucrose (0.6-6.9%) and stachyose (2.3-8.4%). Major QTLs for sucrose (QSuc.vu-1.1), stachyose (QSta.vu-7.1), copper (QCu.vu-1.1) and manganese (QMn.vu-5.1) were identified. Allelic effects of major sugar QTLs were validated using the MAGIC grain samples grown in West Africa in 2017. GP accuracies for minerals were moderate (0.4-0.58). These findings help guide future breeding efforts to develop mineral-rich cowpea varieties with desirable sugar content.

Identification of sources of resistance in cowpea lines to Macrophomina root rot disease in Northern Ghana.

Macrophomina root rot disease (MRRD) caused by Macrophomina phaseolina is an emerging threat to the profitable cowpea production in northern Ghana. Recommended control methods including the use of fungicides are ineffective and expensive for resource poor farmers whilst biocontrol options are not commercially available. An integrated method based on host plant resistance is considered the cheapest and most effective method of managing the disease. This study sought to confirm and characterize previously identified MRRD isolates from Northern Ghana using molecular technology, and to identify cowpea with potential sources of resistance to the MRRD. A PCR assay of ten isolates of the cowpea root rot pathogen revealed all isolates belonged to the species M. phaseolina, whilst a nucleotide BLAST of eight isolates showed 98% similarity with the sequences of Macrophomina isolates from other host available in GenBank. A sick pot method evaluation of 49 cowpea lines found 10 lines resistant to MRRD on a 1-9 disease severity scale (disease score, less than 5). A selection of eight resistant lines (Suvita 2, Abagbaala, IT97K573-1-1, IT93K-503-1-1, Hewale, AV2 3224, Nhyira and T2T4), and a susceptible check (Songotra) were evaluated against 10 isolates of M. phaseolina using a sick pot method. All the genotypes except for the susceptible check were resistant to MRRD. Thus, these genotypes could be used in cowpea MRRD resistance breeding programs.

Genetic control of earliness in cowpea (Vigna unguiculata (L) Walp).

Global climate change is expected to further intensify the already harsh conditions in the dry savannah ecological zones of sub-Saharan Africa, posing serious threats to food and income security of millions of smallholder farmers. Breeding cowpea for improved earliness could help minimize this risk, by ensuring that the crops complete their lifecycle before the cessation of rainfall. In this study, we crossed two sets of cowpea lines showing contrasting phenotypes for earliness in terms of days to 50% flowering (DFF). One set of the lines comprised three extra-early parents (viz.: Sanzi-Nya, Tobonaa and CB27, 30-35 DFF), and the other set consisted of three early-to-medium maturity lines (viz.: Kirkhouse-Benga, Wang-Kae and Padi-Tuya, 42-45 DFF). The derived crosses and their parents were evaluated for key earliness-related traits at Nyankpala and Manga sites of CSIR-Savanna Agricultural Research Institute (SARI), Ghana. To unravel the genetic control of measured traits, we compared the appropriateness of Chi-square goodness of fit tests using classical Mendelian ratios, and frequency distribution (histogram)-related statistics such as skewness and kurtosis. The Chi-square test suggested a single dominant gene mode of inheritance for earliness, whereas the quantitative methods implicated duplicate epistasis and complementary epistatic gene actions. Our results show that coercing segregating lines to fit into classical Mendelian ratios to determine the genetic control of earliness could be misleading, due to its subjectivity. Thus, the genetic control of earliness in cowpea is governed by complementary and duplicate epistasis. The most applicable breeding approach for traits influenced by duplicate epitasis is selection of desirable recombinants from segregating populations developed from bi-parental crosses. Complementary epitasis, as found in the Wang-Kae × CB27 cross, could be exploited in developing improved extra-early lines through backcrossing. Heritability and genetic advance estimates were high for days to first flower appearance (DFFA) and days to 95 % pod maturity (DNPM) in the Padi-Tuya × CB27 and Kirkhouse-Benga x CB27 crosses, indicating that breeding for extra-earliness is feasible. CB27 could be a good donor for introgression of earliness into medium to late maturing improved cowpea varieties, because crosses developed from it had high heritability and genetic advance estimates.

Genetic variability, heritability and correlation analysis among maturity and yield traits in Cowpea (Vigna unguiculata (L) Walp) in Northern Ghana.

Experimental studies were conducted to evaluate 16 advanced breeding lines of cowpea (Vigna unguiculata (L) Walp) for genetic variability, heritability and correlation between maturity and yield related traits. The genotypes exhibited significant (P < 0.05) differences for the eight traits evaluated viz., number of days to 50 % flowering, number of days to 90 % pod maturity, plant height at maturity, number of pods per plant, number of seeds per pod, pod yield, grain yield and hundred seed weight. SARI-3-11-100, SARI-6-2-6, SARVX-09-004 and IT07K-299-6 had grain yields of 1.99 ± 0.30, 1.88 ± 0.20, 1.95 ± 0.30 and 1.91 ± 0.20 t/ha, respectively, which were significantly higher than the check (Songotra) (1.68 ± 0.01 t/ha). In addition, SARI-5-5-5 matured significantly earlier than the check but no significant difference was observed for grain yield. The higher value of phenotypic component compared to the corresponding genotypic component for all the traits suggest that there was an environmental influence on the performance of the genotypes. Hence, the need for multi-location evaluation of the promising lines for onward release if found stable. The information provided in this study, can be exploited in cowpea breeding program.