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Yam (Dioscorea spp.) plants are mostly dioecious and sometimes monoecious. Low, irregular, and asynchronous flowering of the genotypes are critical problems in yam breeding. Selecting suitable pollen parents and preserving yam pollen for future use are potential means of controlling these constraints and optimizing hybridization practice in yam breeding programs. However, implementing such procedures requires a robust protocol for pollen collection and viability testing to monitor pollen quality in the field and in storage. This study, therefore, aimed at optimizing the pollen germination assessment protocol for yam. The standard medium composition was stepwisely modified, the optimal growth condition was tested, and in vivo predictions were made. This study showed that the differences in yam pollen germination percentage are primarily linked to the genotype and growing conditions (i.e., medium viscosity, incubation temperature, and time to use) rather than the medium composition. The inclusion of polyethylene glycol (PEG) in the culture medium caused 67–75% inhibition of germination in D. alata. Although the in vivo fertilization was dependent on female parents, the in vitro germination test predicted the percentage fruit set at 25.2–79.7% and 26.4–59.7% accuracy for D. rotundata and D. alata genotypes, respectively. This study provides a reliable in vitro yam pollen germination protocol to support pollen management and preservation efforts in yam breeding.

Yam (Dioscorea spp.) is a monocotyledonous herbaceous vine cultivated for its starchy underground or aerial tubers in the tropics and subtropics. It is an allogamous and polyploid species that reproduces by both sexual and asexual mechanisms. However, many of the landrace cultivars, including most of the popular varieties, reproduce exclusively by vegetative propagation (planting the tubers). These varieties are either sterile or produce sparse and irregular flowering with high flower abortion rate, low fruit and seed set. Production of crossbreed seeds for genetic improvement and for maintaining genetic diversity in yams is, therefore, mainly achieved through natural or managed pollination. Flowering in yam is mostly dioecious and, in some instances, monoecious. Flowering asynchrony, sticky nature of the pollen grains, and cross incompatibility are among the challenges in making genetic progress in yam breeding. There are many limitations in basic and applied knowledge of yam flower biology and pollination. This paper, therefore, reviews the flowering biology, pollination, and methods of improving pollination efficiency in yam breeding programs.

Crosses were made in line × tester mating design between a set of five IITA soybean released varieties and three plant introduced (PI) accessions obtained from World Vegetable Center, Taiwan. In order to produce sufficient seeds, F1 crosses were selfed, subsequently F2 populations along with their parents were planted in a randomized complete block design at two locations in Nigeria with three replications. Agronomic traits viz. days to flowering, days to poding, plant height, number of pods/plant and seed yield/plant were measured. Testers and lines showed significant differences for all the measured traits except days to flowering for testers. Considering the significance and magnitude of general combining ability (GCA) effect, line TGx 1988-5F was observed desirable for earliness, while line TGx 1989-19F was the best combiner for number of pods/plant and seed yield/plant. On the other hand, best tester for seed yield was PI 230970. Crosses TGx 1835-10E × PI 459025B and TGx 1987-62F × PI 459025B had significant and highest SCA effect for seed yield/plant. These two crosses appeared to be most promising for soybean yield improvement programme.

Yam (Dioscorea spp.) species are predominantly dioecious, with male and female flowers borne on separate individuals. Cross-pollination is, therefore, essential for gene flow among and within yam species to achieve breeding objectives. Understanding genetic mechanisms underlying sex determination and cross-compatibility is crucial for planning a successful hybridization program. This study used the genome-wide association study (GWAS) approach for identifying genomic regions linked to sex and cross-compatibility in water yam (Dioscorea alata L.). We identified 54 markers linked to flower sex determination, among which 53 markers were on chromosome 6 and one on chromosome 11. Our result ascertained that D. alata is characterized by the male heterogametic sex determination system (XX/XY). The cross-compatibility indices, average crossability rate (ACR) and percentage high crossability (PHC), were controlled by loci on chromosomes 1, 6 and 17. Of the significant loci, SNPs located on chromosomes 1 and 17 were the most promising for ACR and PHC, respectively, and should be validated for use in D. alata hybridization activities to predict cross-compatibility success. A total of 61 putative gene/protein families with direct or indirect influence on plant reproduction were annotated in chromosomic regions controlling the target traits. This study provides valuable insights into the genetic control of D. alata sexual reproduction. It opens an avenue for developing genomic tools for predicting hybridization success in water yam breeding programs.

The type of gene action involved in the inheritance of combined resistance to two stem borer species (Sesamia calamistis and Eldana saccharina), and the interrelationship among agronomic and stem borer damage parameters were studied in a tropical maize population (DMR ESR-W). The 225 full-sib progenies generated in the maize population using the North Carolina Design II (NCD II) mating scheme were evaluated with three checks under artificial infestation with the two borer species in two seasons using a 12 × 19 rectangular lattice design with two replications. Genotypic variance was significant for stalk breakage, ear damage, leaf feeding at 5 weeks after infestation, and for most agronomic traits. Narrow-sense heritability estimate was very low for damage parameters except for stalk breakage (23.26%), but it was moderate for grain yield under infestation (43.84%). Expected gains/cycle were low for damage parameters, but moderate for grain yield (10.59% per cycle). Damage parameters were negatively correlated with grain yield and some agronomic traits, while correlations among the damage parameters were positive. Stalk breakage had the highest genotypic correlation with grain yield (rg = −0.47*) among damage parameters, followed by tunnel length (−0.42*). Breeding scheme such as S1 or full-sib selection method would be effective in improving this maize population for combined resistance to the borer species, as well as grain yield using directselection for increased grain yield under infestation as the criterion. The low heritability as well as low expected gains/cycle for damage parameters indicate that progress from selection for resistance may be slow.

Late root bulking is a major factor leading to rejection and abandoning of improved cassava genotypes in sub-Saharan Africa. Early bulking (EB) varieties shorten the growth period from planting to harvesting, better fit into environments with short rainy season, and reduce exposure to biotic and abiotic stresses thereby increasing productivity. This study was carried out to identify molecular markers linked to EB in cassava. Nine cassava hybrid populations (COB-1–COB-9) were developed using six elite varieties (TMS 30572, TMS 97/2205, TMS 98/0505, TMS 30555, NR 8212 and NR 8083) from the African cassava germplasm as parents. The progeny in each of the nine populations (101–272 genotypes per population) were evaluated for EB at 7 months after planting at seedling, clonal, and preliminary stages of breeding evaluation at Umudike. The parameters measured are fresh root yield, harvest index, fresh shoot weight and number of storage roots per plant. The progeny in each of the nine populations were genotyped at 542 simple sequence repeat (SSR) marker loci. Bulked segregant analysis was used to identify the SSR markers associated with EB in the populations. Nine SSR markers (SSRY 106, (ESTs)SSRY 292, SSRY 239, (ESTs)SSRY 7, NS 194, (ESTs)SSRY 47, SSRY 63, SSRY 250, and NS 323) were found to be closely linked (r = 0.3–0.5; p < 0.05) to EB in six of the nine hybrid populations. Seven of the markers with 10 % or more coefficient of determination (R2) were linked to major quantitative trait loci associated with EB in cassava. The molecular markers identified in this study provide useful materials to select for EB in cassava and for further target-traits-improvement by pyramiding.

SUMMARYIn 2004 and 2005 through field trials in over 2500 locations in several agroecological zones, 40 new cassava varieties were introduced in Nigeria. The trials were managed by scientists, extension workers and farmers. The aim in introducing these new varieties was to pre-emptively manage cassava mosaic disease, to avert an imminent threat from the Ugandan strain of the pathogen and prevent damage to the Nigerian cassava economy. From these trials, 10 new varieties were selected and officially released for high root yield, high dry matter content and acceptability for food, industry and livestock. Cassava root rot is a major source of yield loss in Nigeria, especially where farmers practice late harvest because they wait for a good price from the market before harvest and sales. This paper investigated how the 40 varieties together with three old improved varieties currently in farmers' fields during the formal trials responded to root rot disease. No absolutely resistant variety was found, indicating that early harvest (9–11 months after planting) is still the best way to reduce losses from root rot. Root rot is also significantly (p ≤ 0.05) influenced by genotype and environment interaction. Most root rot was recorded in the humid forest and the least was in the Sudan savanna agroecological zone. Using a rank-sum method, the 43 varieties were separated into highly resistant, resistant, moderately resistant, moderately susceptible, susceptible and highly susceptible classes.

The aim of the Integrated Cassava Project (ICP) of the International Institute of Tropical Agriculture was to pre-emptively manage the cassava mosaic disease (CMD) to avert an imminent and increasing possible threat of the Ugandan strain of the CMD virus of the pathogen from doing damage to the Nigerian cassava economy. The strategy was to engage in activities that would lead to cultivar-substitution by replacing the susceptible varieties on farmers’ fields with superior genotypes that are not only CMD resistant or tolerant but also high yielding with good dry matter content. A fast track participatory selection approach was used in 2 years to release nine new lines in Nigeria. It was intensive and several lessons were learnt. The varieties released after 2 years were TMS 98/0510, TMS 98/0581, TMS 97/2205, TMS 98/0505, TME 419, TMS 92/0326, TMS 96/1632, TMS 98/0002, and TMS 92/0057.

The identification of superior genotypes and mega-environments on the basis of multiple traits is a key objective of multi-environment trials in cassava (Manihot esculenta Crantz). The objective of this study was to examine the genetic variation and genotype × environment interaction (GEI) effects for fresh root yield, six other agronomic traits, and severity ratings for cassava mosaic disease (CMD) and cassava green mite (CGM) in 40 genotypes of cassava. Experiments were conducted using a randomized complete-block design with four replications for 2 yr in three representative agro-ecological zones in Nigeria. Site regression (SREG) analysis revealed that GEI was a major source of fresh root yield variation and the different testing sites discriminated among the genotypes. Genotypes TMS 98/0581, TMS 97/4763, TMS 98/ 0002, TMS 99/3073, and M98/0068 were highest yielding at Otobi and Umudike, whereas TMS 98/0510, TMS 97/4779, and TMS 92B/00068 yielded the most at Ishiagu. TMS 98/2226, TMS 92/0325, and M98/0028 had the poorest performance across all locations. Genotypes with the highest yield showed the lowest CMD scores, whereas very tall (well above 2 m) plants had low harvest index on the basis of multiple trait analysis. We identified optimally adapted genotypes for commercial cassava production in different areas in Nigeria.

Treatment with indolebutyric acid (IBA) significantly increased the number of roots produced by vine cuttings of ten Dioscorea rotundata cultivars. Some cultivars could be propagated by layering, especially with IBA treatment, but layering does not appear to be feasible for use on farms. Copyright © 2004 Whurr Publishers Ltd

Expanded and mature leaves of the fluted pumpkin (Telfairia occidentalis Hook. fil.), grown in fields, were studied to derive a predictive regression equation for use in estimating the leaf lamina area (LA) during crop growth studies. A suitable equation based on the number of leaflets in a leaf (Nt) combined with the length (Lc) and maximum width, (Wc) of only the central leaflet in 156 leaves representing diverse seasonal, cultural, and genetic backgrounds with 2–5 leaflets was LA = 0.9467 + 0.2475LcWc + 0.9724LcWcNt ( r 2 = 0.92∗∗∗ ). Variation in the different attributes of lamina reduced the percentage of the variation in LA accounted for by use of the equation. Leaf shapes varied widely depending on the number of leaflets, extent of lamina separation, type of tip, degree of lobing, as well as the flatness or waviness of lamina surface. Overall, the central leaflet was largest with other leaflets being 80, 70, 64 and 54% the size of the central leaflet.

Investigations were conducted on the identification and distribution of yam natural pollinators, flowering pattern and hybridization of white yam (Dioscorea rotundata Poir.) at the International Institute of Tropical Agriculture (IITA), Ibadan, Nigeria, in order to provide informations for producing hybrid seeds needed for vam genetic improvement.

Studies during 1980–1987 relating to seed production of the fluted pumpkin (Telfairia occidentalis Hook. f.) are summarized. Essentially, the dioecious nature of the crop makes sex-ratios of plants affect the number of fruit-bearing females. Overall, about 50% of plants are female, of which some 15% are fruitless. Each fruit-bearing female has 1.5 fruits that contain 60 seeds. The mean weight of seeds is 15 g, and 74% of this total seed weight comprise the seed kernel which contains about 40% oil. Thus, a yield estimate of seed kernels reaches 1.9 t/ha; an output that is superior to that of most other oil seeds now being exploited for seed oil and cake in Nigeria. Measures to raise seed yields and the possibility of breeding work are discussed.

Telfairia (Cucurbitaceae) is usually dioecious, but one monoecious plant was observed. Plants produce floral buds 15–17 weeks after planting. There were > 800 open male flowers to one open female flower. Male flowers open and shed pollen at night, followed by an apparent inefficient insect transfer of pollen to receptive stigmas by midday. Although a female plant may produce five open flowers. only two or three set fruit and only one or two are retained and develop fully. Fruit growth is sigmoidal over 8 weeks, but is most rapid 1.5–5.5 weeks after successful fruit set. Mature fruits are harvested once shoots senesce at 31–33 weeks from planting.

Chloroplast DNA (cp) and nuclear ribosomal DNA (rDNA) variation was investigated in 45 accessions of cultivated and wild Manihot species. Ten independent mutations, 8 point mutations and 2 length mutations were identified, using eight restriction enzymes and 12 heterologous cpDNA probes from mungbean. Restriction fragment length polymorphism analysis defined nine distinct chloroplast types, three of which were found among the cultivated accessions and six among the wild species. Cladistic analysis of the cpDNA data using parsimony yielded a hypothetical phylogeny of lineages among the cpDNAs of cassava and its wild relatives that is congruent with morphological evolutionary differentiation in the genus. The results of our survey of cpDNA, together with rDNA restriction site change at the intergenic spacer region and rDNA repeat unit length variation (using rDNA cloned fragments from taro as probe), suggest that cassava might have arisen from the domestication of wild tuberous accessions of some Manihot species, followed by intensive selection. M. esculenta subspp flabellifolia is probably a wild progenitor. Introgressive hybridization with wild forms and pressures to adapt to the widely varying climates and topography in which cassava is found might have enhanced the crop's present day variability.

During 1978–1979, viability of mixed hand-collected pollen from 6–10 genotypes of white yam (Dioscorea rotundata Poir.), stored under various combinations of relative humidity (Rh) and temperature, as well as the relation of pollen germination in vitro with fruit set were investigated. Yam pollen stored at 0% RH, −5°C, remained highly viable for over one year (from one flowering-season to the next). Fluctuations in storage conditions accelerated loss of pollen viability. Pollen germination in vitro was not significantly correlated with the degree of fruit set, and pollen samples with low percent germination did give satisfactory fruit set.

Flowering behaviour and the morphology of flowers in white yam,Dioscorea rotundata Poir. is described in relation to hand pollination. Plants flowered between June and September with flowering intensity depending largely on factors associated with shoot vigour in genotypes. The high predominance of male plants coupled with irregular flowering necessitate the establishment of many plants to ensure sufficient flowering individuals for a hybridization programme. In order to achieve good fruit set and seed yields, pollen should be collected from open male flowers around 14.00 h and mature female flowers pollinated within a week after they are bagged particularly during the early flowering months. Low fruit set in white yam (12%) is primarily due to inefficient pollination by insects. With hand pollination fruit set is improved three-fold.

To identify the principal characters which account for the major variation among yellow yams, Dioscorea cayenensis Lam., for use in routine groupings, 20 accessions were studied using principal component analysis (PCA) with 49 characters. Based on PCA, six characters were chosen for constructing the metroglyph, whose groupings of accessions highly corresponded with those indicated by a PCA ordination on axes I, II and III. The main vegetative characters that distinguish between yellow yam accessions were: leaf retension in October, mean leaf area, size of tuber crown, leaf virus infection, plant leafiness, and fresh tuber yield. However, some accessions consistently failed to flower and can thus be easily differentiated.

Cassava (Manihot esculentaCrantz) is a carbohydrate staple and cash crop for about 800 million people in the tropics. As food, its use is influenced by its content of potentially toxic cyanogenic glycosides that release hydrocyanic acid when enzymatically hydrolysed (Conn, 1969). Cyanide plays important role in the protection of cassava plants from attacks by animals and insect pests. The glycosides are synthesized mainly in leaves and translocated to all parts of the cassava plant (Bediako, Tapper & Pritchard, 1981). De Bruijn (1973) noted more than a 100% increase in the HCN content of stem bark above the incision after ringing, especially during the first 2 days, and a continued increase for at least 2 months. However, when leaves were removed, no such increase was observed. De Bruijn (1973) observed that younger plants showed a greater increase (165%) than older plants (65%). Leaf HCN did not increase after ringing, whereas root HCN decreased by about 20% in 2 weeks. The literature on cyanogenic glycoside translocation is relatively recent and limited. Consequently, the exploitation of the supposed direct relationship between leaf HCN and root HCN in the selection of low HCN clones in cassava breeding programmes needs to be carefully assessed. The assessment is complicated because the methods used by different workers for the determination of HCN vary in their efficiency. Thus, Cooke, Howland & Hahn (1978) found no correlation (r2= 0·13) between leaf HCN and root HCN among 108 clones, yet over 88000 genotypes have been screened by analysis of leaf HCN, and leaf analysis has been used routinely in cassava improvement for 9 years (Hahn, 1983). Hahn also reported the range of HCN concentration within each leaf-picrate class (1 = 80, 2 = 80−200, 3 = 200 mg/100g fresh weight) but the ranges within each of these classes were too wide to enable effective selection for low HCN clones.

One 3-to 5-year clonal and two recent seedling-derived populations of white yam cultivars were grown and studied for the association of sex, flowering intensity, number of fruits and seed yield with vegetative characters and tuber yield. Yield was lowest in non-flowering plants and increased in the following order: male, monoecious and female plants; with females yielding almost twice as much as the non-flowering plants. The plants produced from small tuber setts, with higher leaf virus infection, low plant leafiness either had low flowering intensity, fewer fruits and seeds or even failed to flower. The sequence and timing of vegetative and reproductive processes in relation to the difference in yield between male and female plants is discussed. There is a strong indication that the intensity of flowering and fruiting is lower in plants from previous vegetative propagation compared to recent seedling-serived clones.

Phenotypic and genotypic coefficients of variation, heritability, and genetic advance were estimated for: time to vine emergence, leaf size, leaf virus infection, plant leafiness, shoot height, vine dry weight, number of tubers per hill and tuber yield per hill in white yam (Dioscorea rotundata Poir.) using plants recently derived from seedlings (SP) as well as those that have been clonally propagated for 3-5 years (CP). Estimates for these parameters were much lower in SP than in CP plants. Mean squares of characters in SP families were significant for leaf virus infection, plant leafiness, vine dry weight and tuber yield; and for all characters in CP except number of tubers per hill. Although in CP, heritability (%) was high for leaf size (67.8), plant leafiness (66.9) and fresh tuber yield (58.1), only tuber yield had a correspondingly high expected genetic advance (74.3%). The study indicated that rapid genetic advance can be achieved for tuber yield, under more uniform environment, by single plant selections followed by clonal propagation.

Phenotypic variances, genetic coefficients of variation, repeatability, expected genetic advance, correlation coefficients and path coefficients were estimated for seven agronomic traits in yellow yam. Plant leafiness, leaf virus infection, number of tubers per hill and tuber yield showed higher expected genetic advances associated with higher repeatability. Positive and highly significant correlations of tuber yield with plant leafiness, shoot height and vine dry weight were observed. Correlations of time to vine emergence and leaf virus infection with yield were negative. Path analysis showed that leaf virus infection had a large negative direct as well as indirect effect on yield. Based on these studies, it is suggested that resistance to leaf virus infection, as expressed by foliage vigour, is the chief criterion for selecting high yielding plant types in yellow yam.

Pollination and fertilization success in white yam, Dioscorea rotundata Poir., depend on knowledge and good management of factors influencing the availability of open male flowers from which good pollen can be collected for effective use in pollinations of genetically compatible female plants under non-xeric conditions. In vitro germination of pollen from numerous genotypes demonstrated varied viability (0.3–85.0%). However, pollen from bisexual flowers on monoecious plants was inferior to pollen from unisexual flowers from dioecious plants. Natural pollination by thrips is inefficient, such that populations with higher ratios of male to female plants set more fruits. To improve fruit set, hand pollinations should be made with pollen from diverse genotypes, early in the flowering period when net water balance is positive. In crossing-blocks, male plants should be closely interpositioned with female plants to reduce pollination distance, thereby improving pollination and fruit set.