Inheritance of seed size in cowpea (Vigna unguiculata (L.) Walp.)

Summary The inheritance of seed weight in cowpea was examined in a field planting of the parents, reciprocal F₁s, F₂s and backcrosses to both parents of a cross between TVu 1977-OD (small seeded) and ACC 70002 (large).Seed weight was inherited quantitatively and small seed was partially domiminant to large seed size. Gene action was predominantly additive but dominance and additive × additive epistatic effects were also significant.Broad and narrow sense heritabilities were 85.1±5.3% and 75.4±18.6% respectively. The minimum number of loci involved in the inheritance of seed size was eight, and each gene pair contributed up to 1.02 g increase to seed weight. The estimate of genetic advance from F₂ to F₃ generations with 5% selection intensity was 3.58 g.International Institute of Tropical Agriculture, Semi-Arid Food Grain Research and Development Project/National Cowpea Improvement Program, B.P. 1783, Ouagadougou, Upper Volta.Department of Primary Industries, Hermitage Research Station, Warwick 4370, Queensland, Australia.International Crops Research Institute for the Semi-Arid Tropics (ICRISAT), ICRISAT Center, Patancheru, P.O. 502 324, A.P., India.     

Adaptation and competition in mixtures of cowpea (Vigna unguiculata (L.) Walp.)

Summary Six pure lines and four mixtures were grown in six environments throughout lowland Papua New Guinea. There were no significant transgressive increases in the grain yield of mixtures above their pure line components, and mixture yields were adequately predicted by the mean of components. In yield stability it was found that the individual buffering of pure lines was of more importance than population buffering, and that the magnitude of population buffering varied with the particular combination of components. Competitive effects in all the mixtures were of the compensating type. Dramatic changes in mixture composition resulting from natural selection precluded their use in local agriculture. The outcome of competition in mixtures was strongly influenced by the growing environment, such that selective index was correlated to general fertility.     

Wide hybridization in cowpea: problems and prospects

Summary Attempts were made to cross a wild profusely hairy cowpea (Vigna vexillata) with other cowpea types (both cultivated and non-cultivated) for the purpose of transferring gene(s) for hairiness from the wild to the cultivated genepool. No hybrid was however obtained.     

Evaluation of yield stability of cowpea under sole and intercrop management in Nigeria

Summary Nine cowpea (Vigna unguiculata (L.) Walp.) breeding lines were grown in 13 experiments under sole crop and intercrop management, with and without insecticide application, in Nigeria. Cowpea was intercropped with cassava (Manihot esculenta Cranz), maize (Zea mays L.) and maize-cassava in the forest zone and with sorghum (Sorghum bicolor Moench.) and millet (Pennisetum glaucum (L.) R.Br.) in the savanna zone. Line by cropping system interactions were significant in 5 of 13 experiments and line by insecticide treatment interactions were significant in 2 of 13 experiments. Cowpea yield was severely depressed in intercropped plots and in plots to which no insecticide was applied. Among-environment variation in cowpea yield was greater when no insecticide was applied. Within each management treatment, the yield performance and stability of the cowpea lines was examined. Stability was assessed both by examining among-environment coefficients of variation and by regression analysis. Several of the breeding lines evaluated appeared promising under different levels of management and in a diversity of environments.     

Genetics of resistance to Aphis craccivora in cowpea

Summary Inheritance of aphid resistance and allelic relationships among sources of resistance was studied in the parents, F₁, F₂, F₃, and backcross populations of cowpea crosses. Each 4-day old seedling was infested with five fourthinstar aphids. Seedling reaction was recorded 14–16 days after infestation when the susceptible check was killed. The segregation data from eight crosses between resistant and susceptible cowpea cultivars indicated that aphid resistance was inherited as a monogenic dominant trait. Segregation data from crosses among eight resistant cultivars indicated that one or two loci and modifier(s) were involved in the expression of resistance to aphids. It was suggested that further studies on allelism among sources of resistance needed to be conducted in order to resolve this.     

Evaluation of parent selection methods for yield improvement of cowpea (Vigna unguiculata (L.) Walp.)

Summary The mean aim of this study was to identify an effective method of predicting cross-potential in respect of yield improvement of the cowpea (Vigna unguiculata (L.) Walp.) through hybridization and subsequent selection.Three prediction tests based on data collected from experiments with ten cowpea lines and the F1 and F2 generations of all possible crosses between these lines were evaluated. The three prediction tests were based on: i Yields of prospective parents. ii. General combining abilities estimated from F1 and parental data iii. Frequencies of superior plants in F2 populationsEach of the three procedures was used to identify the crosses with the lowest or highest yield potential, i.e. those crosses most likely to produce the greatest number of lines with yields higher or lower than those of either parent. Crosses selected on the basis of these criteria were evaluated in the following growing season on the basis of biometrical analysis of F2 and backcross populations and the performance of the respective F3 and F4 generations.Six different crosses were selected from the 45 tested on the basis of the three prediction tests. Analyses of F2 and backeross populations from each cross provided estimates of [d] the difference in parental means and 1/2D the additive genetic component which, in turn, were used to predict the frequency of transgressive inbred lines obtained by single-seed descent from the F2. Both crosses selected on the basis of F2 data were expected to produce a much higher frequency of transgressive lines than those selected on the basis of parental yields. The high potential cross selected on the basis of general combining ability was expected to produce a higher frequency of transgressive lines than the corresponding low yield potential cross. Replicated trails of F3 bulks of the selected crosses showed that high yield potential crosses were significantly higher yielding than low yield potential crosses for all prediction criteria. In a trial with 22F4 lines of each of the two crosses selected on the basis of parental yields and frequency of high yielding F2 plants, the high yield potential cross in each case had a greater mean and variance for seed yield than the corresponding low yield potential cross.It was concluded that parental yields provide a sound basis for an initial screening of prospective parents. This screening should identify a high yielding line which can be crossed with a number of contrasting lines to produce F2 populations for the final screening phase.     

The effectiveness of early generation (F3) yield testing and the single seed descent procedures in two cowpea (Vigna unguiculata (L.) Walp.) crosses

Summary Two breeding procedures were compared in two cowpea crosses. Both procedures were started from the same selected F₂ plants in each cross. For the early generation yield testing, the F₃ lines were yield tested and a pedigree and bulk methods followed in F₄ and F₅. Each F₅ line was bulked to provide seed for a yield test in F₆. In the Single Seed Descent (SSD) procedure, a single seed was taken from each F₂ plant to produce the F₃ generation. The procedure was repeated for the F₄ and F₅. The F₆ SSD lines and the F₆ yield testing lines were compared in yield tests.The results showed that differences in yielding ability of F₃ lines persisted over generations indicating that selection was effective. This was confirmed by the high significant correlations between F₃ yields and those of later generations which ranged from r=0.51^* to 0.85^**.The grain yields of lines derived by the single seed descent procedure were as good as those derived from early generation yield testing.Significant linear correlation between visual rating of F₃ and F₆ yields with actual yields revealed that it is possible to identify promising lines of cowpea visually.     

A stability analysis of time to flowering as a screen for responsiveness to temperature and photoperiod in cowpea (Vigna unguiculata)

Summary Twenty-one genotypes of cowpea (Vigna unguiculata), comprising landraces and varieties, were grown in 22 photothermal environments in Nigeria and Niger, West Africa, and a stability analysis of days from sowing to flowering (f) was carried out. Cowpeas are rarely insensitive to photoperiod; they are typically quantitative shortday plants wherein f is delayed when photoperiod (P) is longer than the critical photoperiod (P _c ). Therefore, in order to quantify genotypic variation in temperature sensitivity, genotype f was regressed against the mean trial f in circumstances where P c (i.e. approximately 13 hd^-1) and mean temperature (T) was between 19° and 28° C. Correspondingly, in order to assess genotypic variation in photoperiod sensitivity, trials where T was near optimal (25°–28° C) but where P ranged from 10–14.5 hd^-1 were used. These stability analyses detected no significant differences (P>0.05) between genotypes 9n temperature sensitivity but revealed significant differences (P<0.001) in photoperiod sensitivity. Regression coefficients from the stability analysis were strongly correlated (r=0.94, 19df) with a photoperiod sensitivity constant, c, determined from a photothermal flowering model. A stability analysis of f from field trials can therefore identify and quantify genotypic variation in response to temperature and photoperiod in cowpea.Abbreviations f days from sowing to flowering - P mean photoperiod - P _c critical photoperiod - P _ce ceiling photoperiod - T mean temperature - T _b base temperature - T _o optimum temperature - SDP short-day plant     

Combining ability analysis and association of yield and yield components among selected cowpea lines

The study was carried out to determine combining ability and association of yield and yield components among crosses derived from seven selected cowpea (Vigna unguiculata (L.) Walp) parents. Twenty-one hybrids were generated from diallel crosses excluding reciprocals. Hybrids along with seven parents were studied for combining ability and phenotypic correlations for seed yield and eight yield components. The result indicated that the general combining ability (gca) and specific combining ability (sca) were significant for most characters indicating the importance of both additive and non-additive genetic components. Additive gene action was important for eight characters except pod number, whereas non-additive gene action was not important for nodule number and grain yield. Parental lines IT86D-716 and IT81D-985 were found to be good general combiners for grain yield and other traits. The most promising specific combiners for yield and yield components were from crosses including IT87D-697-2 × IT86D-716, IT88D-867-11 × IT86D-716, IT93K-624 × IT87D-697-2, and IT87D-697-2 × IT92KD-405-1. Significant positive phenotypic correlations were observed between seed yield with pod length (POL), number of pods per plant (PON), and number of seeds per pod (SPD). POL, pod number per plant, SPD, and grain yield were identified as the best selection criteria that could be used in cowpea breeding programs.     

Linkage relationships among loci controlling morphological traits in cowpea (Vigna unguiculata (L.) Walp.)

Summary Linkage among loci controlling various morphological traits in cowpea were determined using F₂ progenies. Data were collected on individual plants of four crosses segregating for several loci. Recombination estimates between the following pairs of loci were as follows: Sw (swollen vs normal stem base)-Fbc (cream vs green flower buds) (41±4.8%), Pu^s(purple vs green stems)-Cbr (cocoa-brown vs straw-yellow dry pods) (31±5.7%), Pu^p(purple vs green immature pods)-Cbr (30±5.7%), Pu^s-Pu^p (4±1.5%), Ndt (non-determinate vs determinate)-Pd (peduncle colour) (26±2.8%), Ndt-Hg (semi-erect vs erect plant type) (26±2.8%), P^t(purple vs green pod tips)-Bk (greyish-black vs straw-yellow dry pods) (19±2.4%) and Hg-Bpd (normal vs branching peduncle (24±9.5%). Four linkage groups (LG) were identified in these studies. The proposed LG I contained loci Sw and Fbc; LG II loci Pu^s, Pu^p, and Cbr; LG III loci Pd, Ndt, Gh, and Bpd; and LG IV loci P^tand Bk.     

Digestive enzyme inhibitors and storage pest resistance in cowpea (Vigna unguiculata) seeds

Summary Amylase and trypsin inhibitors are proteins which inhibit digestive enzymes. The loss of the activity of these enzymes produces a reduction of starch and protein digestion. Amylase and trypsin inhibitor activity have been investigated in seeds of 20 cowpea lines in trying to establish their role in the storage pest resistance. A broad variation has been observed for both the inhibitors. Correlation analysis between inhibitor levels and extent of attack has shown that neither amylase nor trypsin inhibitors can separately explain the resistance to Callosobruchus maculatus (F.). Lines bruchid resistant in fact, have high levels of both inhibitors. Conversely lines with low levels of amylase and trypsin inhibitors are bruchid susceptible. The breeding for high contents of both amylase and trypsin inhibitors can be an effective way to obtain lines with resistance to storage pest.     

Analysis of host genotype x rhizobial strain interaction in N2 fixation

Summary A method of analyzing host genotype x rhizobial strain interaction for N₂ fixation potential, based on the principle of structural relationship analysis, is proposed. When this interaction is significant, selecting genotypes for high N₂ fixation potential is incorrect, because the N₂ fixation efficiency is dependent on the rhizobial strain used. Under such circumstances, grouping genotypes based on average fixing ability (AFA), a linear response to the effectiveness of rhizobial strain, and specific fixing ability (SFA), deviation from the linear response in terms of the magnitude of the error variance, is useful before initiating a breeding program for enhanced N₂ fixation. Host genotype x strain interaction effect is partitioned into two components, and two parameters are derived which estimate AFA and SFA. N₂ fixation data from 3 mungbean genotypes and 6 rhizobial strains are used to illustrate this method.     

Virulence characteristics of a new race of the parasitic angiosperm,Striga gesnerioides,from southern Benin on cowpea (Vigna unguiculata)

An in vitro growth system was used to determine the virulence of two samples of Striga gesnerioides from Zakpota in southern Benin. Cowpea variety B301, previously considered resistant to all races of S. gesnerioides, was susceptible to both samples of the parasite. Two other cowpea varieties, 58–57 and IT81D-994, were totally resistant. Resistance in 58–57 was associated with a hypersensitive necrosis of infected roots, whilst IT81D-994 supported production of small S. gesnerioides tubercles with stems which failed to develop. Striga gesnerioides from southern Benin is the fourth race of the parasite to be identified, and the first with virulence on variety B301. The implications for breeding cowpeas with resistance to S. gesnerioides are discussed.Abbreviations IITA International Institute of Tropical Agriculture - LARS Long Ashton Research Station - SAFGRAD Semi-Arid Food Grain Research and Development     

Genetic analysis of yield and fruit traits in cantaloupe cultivars

The purpose of this research was to evaluate the combining ability of six local Iranian and one imported cantaloupe cultivar. Heritability of traits was estimated using a half‐diallel mating design. Seven parents and their crosses were evaluated in 2013 and 2014. The greatest general combining ability (GCA) for yield and fruit number was for “Rish‐baba” (0.53 kg/plant and 0.3, respectively). The cultivar “Ananasi” had the highest GCA for fruit weight and soluble solids content (SSC) (0.088 kg and 1.4, respectively). “Ananasi” presented the highest GCA values for fruit firmness, chlorophyll a and b and carotenoid content, as well as the highest total chlorophyll content. The cross Garmak × Rish‐baba showed the highest specific combining ability (SCA) for yield with heterosis value of 99%. High heritability estimates for SSC (0.52), flesh thickness (0.61) and concentration of chlorophyll a (0.7) were obtained. Although there were significant SCAs for yield, the parents are suggested to be improved prior to hybrid development. The parent “Ananasi” appears to be a suitable donor in breeding programmes.     

RFLP mapping of an aphid resistance gene in cowpea (Vigna unguiculata L. Walp)

Summary Restriction fragment length polymorphism (RFLP) analysis has several advantages over traditional methods of genetic linkage mapping, one of these being the starting point for map-based cloning. The recent development of an RFLP map of cowpea (Vigna unguiculata L. Walp) has allowed the investigation of associations between genes of interest and RFLP markers. A cross between an aphid (Aphis craccivora Koch) resistant cultivated cowpea, TT84S-2246-4, and an aphid susceptible wild cowpea, NI 963, was screened for both aphid phenotype and RFLP marker segregation. One RFLP marker, bg4D9b, was found to be tightly linked to the aphid resistance gene (Rac ₁) and several flanking markers in the same linkage group (linkage group 1) were also identified. The close association of Rac ₁ and RFLP bg4D9b presents a real potential for cloning this insect resistance gene.     

Genetics of host reactions to three races of the bacterial pustule pathogen in cowpea

Summary Inheritance of the brown hypersensitive resistant (BHR), non-hypersensitive resistant (R) and susceptible (S) host reactions produced by three races of the bacterial pustule pathogen (Xanthomonas campestris pv. vignaeunguiculatae) was studied in 45 F₁, F₂ and testcross progenies, using the infiltration inoculation method BHR reaction was dominant over R and S reactions, and R was recessive to S reaction. Two genes appeared to be involved in BHR reaction; one governing BHR reaction to race 1 and the other to races 1 and 2. Both were ineffective against race 3. R reaction, effective against all the races, appeared to be controlled by one, two or three recessive genes. One cowpea line had one BHR gene and two duplicate recessive R genes. Reaction expression in the segregants was clear and as expected with races 2 and 3 but was modified with race 1, possibly due to interactions between dominant or recessive alleles of the BHR genes and the homozygous recessive allele of the R genes. Gene symbols Bp-1 and Bp-2 are proposed for the BHR genes and bp-3, bp-4 and bp-5 for the recessive R genes. The genes present in each of the differential cowpea line are suggested.Contribution from the International Institute of Tropical Agriculture, Ibadan, Nigeria and Crop Development Division, Ministry of Agriculture, P.O. Box 9071, Dar es Salaam Tanzania.     

Resistance of cowpea and cereals to the parasitic angiospermStriga

Summary Striga species are parasitic angiosperms that attack many crops grown by subsistence farmers in sub-Saharan Africa and India. Control of the parasite is difficult and genetically resistant crops are the most feasible and appropriate solution. In cowpea, complete resistance toStriga gesnerioides has been identified. Breeding for resistance in sorghum has identified varieties with good resistance toS. asiatica in Africa and India. One variety was also resistant toS. hermonthica in W. Africa. No such resistance toStriga has been found in maize or millets.Resistant varieties have usually been sought by screening germplasm in fields naturally infested withStriga. However, laboratory techniques have also been developed, including anin vitro growth system used to screen cowpeas for resistance toS. gesnerioides. Two new sources of resistance in cowpea have been identified using the system. The technique has also been used to investigate the mechanisms of resistance in this crop. Two mechanisms have been characterised, both were expressed after penetration of cowpea roots by the parasite.The resistance of some sorghum varieties toStriga is controlled by recessive genes. In cowpea, resistance toStriga is controlled by single dominant genes. The genes for resistance are currently being transferred to cowpea varieties which are high yielding or adapted to local agronomic conditions. OneStriga resistant cowpea variety, Suvita-2, is already being grown widely by farmers in Mali. Reports of breakdown of resistance in cowpea toStriga have not yet been confirmed, but a wider genetic base to the resistance is essential to ensure durability ofStriga resistance.Abbreviations ICRISAT International Crops Research Institute for the Semi-Arid Tropics - IITA International Institute of Tropical Agriculture - LARS Long Ashton Research Station - SAFGRAD Semi-Arid Food Grain Research and Development     

New sources of resistance of cowpea (Vigna unguiculata) toStriga gesnerioides,a parasitic angiosperm

Summary Thirty-seven accessions of cowpea and yard-long bean were assessed for resistance toStriga gesnerioides. Cowpea plants were grown using anin vitro method, then inoculated with young seedlings ofS. gesnerioides produced from seed from three West African countries. Resistance was assessed by comparing the number and size ofS. gesnerioides tubercles on these accessions with those on a known susceptible cowpea, cv. Blackeye. Two cowpea landraces, APL-1 and 87-2, were completely resistant toS. gesnerioides from Burkina Faso, Mali and Cameroon and partially resistant toS. gesnerioides from Niger. Complete resistance was expressed either as a hypersensitive response of infected root tissues or as a severely retarded development of successful infections. All other accessions, including three samples of yard-long bean were susceptible toS. gesnerioides. The original 87-2 plants segregated for resistance and susceptibility. However, uniformly resistant progeny were obtained by producing seed from vegetatively propagated clones of single resistant 87-2 plants. Resistance of APL-1 and 87-2 toS. gesnerioides was confirmed in pot and field trials. Neither of these cowpeas were resistant toAlectra vogelii. Varieties APL-1 and 87-2 provide additional sources of resistance to most races ofS. gesnerioides, including a newly discovered virulent race from Benin.Abbreviations ICRISAT International Crops Research Institute for the Semi-Arid Tropics - IITA International Institute of Tropical Agriculture - SAFGRAD Semi-Arid Food Grain Research and Development     

Inheritance of useful traits in cassava grown in subhumid conditions

A diallel study among nine parental clones of cassava was conducted in the subhumid environment on the northern coast of Colombia. Analysis of variance suggested significant effects for the six variables analysed: fresh‐root yield, harvest index, root dry matter content, height of first branching, reaction to thrips and plant‐type scores. General and specific combining ability effects and their interaction with the environment were significant for most of the variables as well. Results suggested that dominance plays a particularly important role in the cases of fresh‐root yield and harvest index but had relatively little importance in the reaction to thrips, dry matter content or height of first branching. Specific breeding approaches are suggested for these traits, depending on the relative importance of additive or non‐additive effects in their inheritance. The correlations among different traits were also analysed and in several cases their magnitude reached statistical and biological significance.     

Quantitative resistance to Cercospora leaf spot disease caused by Pseudocercospora cruenta in cowpea

Six populations (Parent 1, Parent 2, F1, F2, BC1 and BC2) generated from each of four crosses involving four resistant and two susceptible varieties of cowpea (Vigna unguiculata L. Walp) were evaluated for resistance to Cercospora leaf spot (CLS) disease caused by Pseudocercospora cruenta under induced epiphytotic conditions, in four separate field experiments. Climatic conditions determined the onset of CLS disease in the susceptible cultivar and varied in the four experiments from 35 to 48 days after planting (DAP). Genetic analysis revealed that the mode of inheritance of resistance to P. cruenta can be oligogenic or polygenic depending upon the cross. This is the first report of polygenic inheritance of CLS resistance. Number of nodes infected fitted a simple additive dominance model with predominance of additive effects based on generation mean analysis. Oligogenic resistance was observed for the other three crosses, with the most plausible models being: a single gene model with incomplete dominance in CB27 × IT87D-939-1; a single gene model with complete dominance in CB27 × VRB-10; and a triger model in Los Banos Bush Sitao × IT86D-792, based on segregation analysis of symptomatic : non-symptomatic plants. The role of minor genes was also indicated in the above crosses. Suggested approaches to breeding for resistance to CLS are discussed.