White Guinea Yam Research Articles

Disease Resistance Attributes Conferred by Mineral Fertilizer in Dioscorea Species

This study investigated proximate content, phenol as well as mineral content effects of an 8-mineral content fertilizer, namely Bounty, on resistance attributes in the nourished tubers of four Dioscorea species varieties, compared to those sourced from farmers plots within Ogun state, which had been nourished with the natural replenishment of at least 2 to 4 years fallow. The experiment was a 4 by 3 factorial arranged in RCBD (randomized complete block design) with 12 treatment combinations and 3 replicates. Main factor was variety that is Dioscorea rotundata varieties Oniyere (from South West Nigeria) and Abuja (from middle belt Benue/Plateau of Nigeria), D. cayenensis var. Igangan/Alo (from South West Nigeria) and Dioscorea alata var. Agbodo (from South West Nigeria).  Treatments were Bounty fertilizer at the rates 0.6 ml l–1 as well as 1.2 ml l–1 and farmers’ plot (FPT, with no fertilizer). The Bounty-fertilized tubers were harvested from the Teaching and Research Farms of the Directorate of University Farms, Federal University of Agriculture, Abeokuta, Ogun State, Nigeria at 6 months after planting (6 MAP). Farmers plot tubers were also harvested at 6 MAP. Analysis was done after processing of the tubers. Among the proximate components, moisture, dry matter, ash, crude fibre and carbohydrate of mean values 14.05%, 85.96%, 2.77% dm, 8.55% dm and 82.71% dm were comparable/similar in the four varieties. However, Oniyere (white guinea yam – South West Nigeria) was lower in fat and higher in crude protein than the other three varieties. Abuja, Igangan and Agbodo were comparable/similar in fat content, but Igangan (yellow guinea yam) had lower crude protein than Agbodo (water yam) whereas Abuja (white guinea yam – Middle Belt Nigeria) and Agbodo were similar/comparable in crude protein. Bounty-fertilized tubers at the 2 concentrations were lower in ash (mean 2.63% dm) and carbohydrate (80.98% dm) than Farmers plot tubers (FPT), (3.06 and 86.17% dm respectively). But they were higher in phenol and calcium, with means of 189.82 and 22.16 mg/100 g dm respectively than farmers plot tubers (means of 120.77 and 16.89 mg/100 g dm). Both calcium and phenol have been reported in increased resistance to disease in plants including Dioscorea spp. in literature. Correlation tests in this study indicated significance between calcium (from the fertilizer) and tuber phenol (r = 0.4523). Calcium correlated significantly also with crude fibre (r = 0.9325) and crude fibre with phenol (r = 0.7110). Calcium correlated significantly also with tuber potassium which was not supplied from the fertilizer (r = 0.6662), indicating that increased uptake of calcium would enhance uptake of potassium. Consequently potassium supported by Nitrogen (the highest component, 15% w/w in Bounty Fertilizer) would have positive effect on tuber yield, as the highest proportion (91% to 94%) of total N and K are reported to be accumulated in yam tubers at harvest. These results underscore effects of mineral nutrition (such as Bounty) for increased disease resistance and increased yield, over natural replenishment after fallow which the smallholder farmer depends on solely in Nigerian traditional yam farming system.

Proximate, Mineral and Phenol Content of Dioscorea Rotundata Var. Amula, Dioscorea Cayenensis Var. Igangan-Alo from Farmers’ Plots and Bounty-Fertilized, Infected with Botryodiplodia Theobromae Pat.

Fertilizer effects on infection and weight loss by Botryodiplodia theobromae Pat, was determined in white guinea yam Dioscorea rotundata variety Amula from the middle belt-region of Nigeria, and yellow guinea yam D. cayenensis variety Igangan/Alo from South West Nigeria. Tubers of both varieties were sourced from farmers’ plots from within Ogun State ( South West Nigeria) where they had been planted on three or more years previously fallowed plots, and harvested at six months after planting. Another lot of tubers of both varieties were sourced from a yam project plot at the Teaching and Research farms of the Directorate of University Farms (DUFARMS), Federal University of Agriculture (FUNAAB) Abeokuta, where they had been fertilized at the rate of 0.60 ml l-1 of Bounty fertilizer, an 8-mineral component fertilizer, and harvested also at six months after planting. Tubers of both varieties from farmers’ plots were analysed for proximate content (dry matter, moisture, ash, fat, crude protein, crude fibre and carbohydrate) phenol content and ten minerals namely Ca, N, P, K, Mg, Fe, Mn, S, Zn and B. Another set of tubers of the two varieties from farmers plots, as well as those from the Bounty-fertilized yam project plot, were inoculated with Botryodiplodia theobromae in an infection and weight loss experiment. The infection and weight loss experiment was a 2 by 2 factorial arranged in RCBD (randomized complete block design) with 4 treatment combinations and 5 replications. Tubers of both varieties from both sources inoculated with a 7-day old pure culture of B. theobromae were the four treatment combinations. Incubation period was 3 weeks. Infection in Amula (white guinea yam) was over 2500% higher than in the yellow guinea yam, Igangan. Phenol (107.89 mg/100 gdm) and calcium (16.85 mg/100 gdm) were higher in Igangan than in Amula (89.04 and 16.70 mg/100gdm respectively), and is adduced as the reason for the high infection in variety Amula. Weight loss however was 154% higher in Igangan than in Amula. Crude fibre had a negative significant correlation with weight loss (r = -0.9285, P=0.001), and crude fibre with the value 4.49% dm was higher in variety Amula than in variety Igangan (4.02%dm). This is responsible for the higher weight loss in var. Igangan. Phenol content also had a significant negative correlation with weight loss (r = -0.9586, P = 0.05), suggesting that the effect of high crude fibre in reducing weight loss outweighs that of phenol in reducing weight loss. There was high reduction of infection in the Bounty-fertilized tubers, 3.72% compared to farmers’ plot tubers which had 27.22%. Weight loss was also 0% in Bounty-fertilized tubers compared to a high 63.54% in the tubers from farmers’ plots. This indicates that the 8-mineral component fertilizer, Bounty will prolong shelf life in the white and yellow guinea yams Amula and Igangan. Five minerals namely Mg, K, Zn, Mn and B had significant correlation with weight loss (r = 0.9461, 0.9802, 0.9214, 0.9898 and 0.9122) respectively. Three of these minerals namely Zn, Mn and B content were all low with mean values of 0.25, 0.44 and 0.07 mg/100gdm respectively but Mg and K were relatively higher with mean values of 21.59 and 806.67mg/100gdm respectively. This suggests that mineral nutrition to enhance prolonged shelf life in D. rotundata var. Amula and D. cayanensis var. Igangan may be established with a balance of Mg, Zn, Mn and B in lower proportions than K, as potassium is essential in optimum yield in Dioscorea species.

Calcium Nitrate Fertilization Effect on Anthracnose Caused by Colletotrichum Gloeosporioides Penz., Tuber Rot By Rhizopus Stolonifer Ehrenb. and Yield in Dioscorea Alata Variety Atoroku After Fallow

Calcium nitrate soil amendment fertilization effects on anthracnose disease caused by Collettrichum gloeosporioides, tuber rot by Rhizopus stolonifer and yield were determined in Dioscorea alata variety Atoroku (a local variety from South West Nigeria). The field design was RCBD (randomized complete block design) with 3 treatments and 3 replicates. The 3 treatments were calcium at 0, 2 and 4 kg ha–1 from calcium nitrate (analar grade) applied at 3 MAP (months after planting). N in the experimental site was below the critical determined for optimum yam production in South West Nigeria, but Ca, Mg and K were above the critical. At 4 MAP, shoot number (mean, 3.68) was comparable/similar across treatments, but shoot length (3.43 m) was higher in the 2 kg ha–1 treatment than others. At 5 MAP the control and 2 kg ha–1 calcium fertilized plants had higher shoot number, a mean of 5, than the 4 kg ha–1 calcium fertilized plants, whereas shoot length had become similar in the treatments. Tubers were inoculated and incubated for two weeks with Rhizopus stolonifer, an economic yam storage pathogen in Nigeria, after the 5MAP and 6MAP harvests. There were no significant differences in weight loss or infection in the incubated tubers at 5MAP. Weight loss of 11.01% in the 2 kg ha_1 incubated calcium-fertilized tubers were significantly higher than in the control (3.20%) and the 4 kg ha_1 treatment (1.90%) at 6MAP. However infection was significantly lower in the 2 kg ha_1 (0.78%) and 4 kg ha_1 (0.86%) calcium-fertilized tubers than the control (2.25%) at 6MAP. Calcium nitrate at 2 kg ha_1 and 4 kg ha_1 soil amendment may be used in smallholder farmers plots to reduce infection in D. alata variety Atoroku tubers if the six months after planting is desired for income as is the practice for white guinea yam 6MAP milk harvest in Nigeria. Anthracnose severity scored at 8MAP was moderately susceptible in the 4 kg ha_1 treatment (3.17), control (2.83) and the 2 kg ha_1 treatment (2.75) and were not significantly different from one another. Reduction of anthracnose may be further determined with one or two foliar sprays in addition to the 3MAP calcium nitrate soil amendment, as well as leaf calcium analysis to ascertain the optimum concentration commensurate to anthracnose resistance. Shoot number per plant at 8MAP was in the range 2.07 to 2.54 with no significant differences among the three treatments. Tuber number per plant was also not significantly different and was 3.55, 4.27 and 3.18 in the 0, 2 and 4 kg ha_1 treatments. Plant weight above the mound of 4.78 kg and tuber weight of 5.35 kg in the 4 kg ha_1 treatment at 8MAP harvest was significantly higher than the control (3.25 kg and 3.55 kg respectively), and the 2 kg ha_1 treatment (3.05 kg and 3.25 kg respectively), whereas both were similar in the control and the 2 kg ha_1 treatment. The 8MAP yield of 3.55 kg, 3.25 kg and 5.35 kg in the 0, 2 and 4 kg ha_1  treatment is the equivalent of 35.5, tons ha_1, 32.5 tons ha_1 and 53.5 tons ha_1 and are higher than the expected 15 to 25 tons ha_1 with 400 kg (8 bags) NPK 15-15-15 (ICS-Nigeria) and also in the range of the estimated potential of 30 to 75 tons ha_1 (Zinsou, 1997; Diby et al., 2004). Calcium nitrate soil amendment at 4 kg ha_1 is recommended for increase in yield in Dioscorea alata variety Atoroku after fallow in South West Nigeria or areas of similar soils.

Effect of leaf thinning on shoot growth and tuber yield of white Guinea yam

ABSTRACT Mutual leaf shading can inhibit the growth of yam, reducing tuber yield. To improve light utilization, approximately 25% of leaves in a plant were thinned during the period of maximum shoot growth. Shoot dry weight was estimated every two weeks using a non-destructive method. Leaf thinning caused higher shoot growth rates (SGRs) after thinning, while control plants had SGRs close to zero. The higher SGRs in the thinned plants was attributed to an increase in new leaf development. This indicates that the plateau in shoot growth commonly observed during the late growth period is reversible and could be improved artificially. In thinned plants, there was a positive relationship between shoot dry weight and SGR, although no such relationship was observed in control plants after the middle growth period. This positive correlation indicates a higher shoot growth per unit leaf area in the thinned plants than in the control plants, presumably due to improved light utilization and a higher photosynthetic rate of new leaves. However, leaf thinning reduced tuber yields, presumably because of a lower total carbon assimilation per plant and greater growth competition between shoots and tubers. High correlations between shoot dry weight and tuber yield indicated that a high shoot biomass is more important than improving light utilization for increased tuber yields.

Genetic parameter estimation and selection in advanced breeding population of white Guinea yam

ABSTRACT White Guinea yam (Dioscorea rotundata Poir.) is an important tuber crop grown extensively in tropical regions of West African yam belt. Tuber yield, dry matter content, and tolerance to yam mosaic virus are key traits used for identification and selection of superior varieties for commercial deployment. In this study, we estimated genetic parameters for fresh tuber yield, tuber dry matter content, and quantitative field tolerance to yam mosaic virus in 49 clones grown in multi-environment trials (METs). We conducted genomic prediction involving 6337 single nucleotide polymorphisms (SNPs) and phenotypic field evaluation of data collected on the three traits from four sites. Additive genetic and non-genetic factors contributed significantly to phenotypic variation of studied yam traits in METs but to varying degrees. The non-genetic effects were relatively high for most of the measured traits. Narrow-sense heritability values were low (<0.30) for all studied traits. Further analysis of the performance of the clones at test sites with additive main effects and multiplicative interaction (AMMI) analysis exhibited significant genotype by environment interactions (GEI) for the three traits. The AMMI identified TDr10/00412, TDr11/00055, and TDr09/00135 clones with lowest mean trait stability index and outstanding performance for fresh tuber yield (t ha−1), tuber dry matter, and mosaic virus resistance across sites. The elite clones identified could serve as useful source of alleles for the genetic improvement of the crop and possibly considered for release to farmers.

Low Soil Nutrient Tolerance and Mineral Fertilizer Response in White Guinea Yam (Dioscorea rotundata) Genotypes.

Yam (Dioscorea spp.) is a major food security crop for millions of resource-poor farmers, particularly in West Africa. Soil mineral deficiency is the main challenge in yam production, especially with the dwindling of fallow lands for the indigenous nutrient supply. Cultivars tolerant to available low soil nutrients and responsive to added nutrient supply are viable components of an integrated soil fertility management strategy for sustainable and productive yam farming systems in West Africa. This study’s objective was to identify white Guinea yam (D. rotundata) genotypes adapted to available low soil nutrients and responsive to externally added nutrient supply. Twenty advanced breeding lines and a local variety (Amula) were evaluated under contrasting soil fertility, low to expose the crop to available low soil nutrient supply and high to assess the crop response to added mineral fertilizer (NPK) input at Ibadan, Nigeria. The genotypes expressed differential yield response to low soil fertility (LF) stress and added fertilizer input. Soil fertility susceptibility index (SFSI) ranged from 0.64 to 1.34 for tuber yield and 0.60 to 1.30 for shoot dry weight. The genotypes R034, R041, R050, R052, R060, R100, and R125 combined lower SFSI with a low rate of reduction in tuber yield were identified as tolerant to LF stress related to the soil mineral deficiency. Likewise, the genotypes R109, R119, and R131 showed high susceptibility to soil fertility level and/or fertilizer response. Genotypes R025 and R034 had the tuber yielding potential twice of that the local variety under low soil nutrient conditions. Shoot dry weight and tuber yield showed a positive correlation both under low and high soil fertility conditions (r = 0.69 and 0.75, respectively), indicating the vigor biomass may be a morphological marker for selecting genotypes of white Guinea yam for higher tuber yield. Our results highlight genotypic variation in the tolerance to low soil nutrients and mineral fertilizer response in white Guinea yam to exploit through breeding and genetic studies to develop improved genotypes for low and high input production systems in West Africa.

Current Status of Yam (Dioscorea Sp) Genetic Resources-Case Study of Selected Locations in South-West Nigeria

Yam is an important tuber crop in Nigeria. Some locations in South west Nigeria (Oyo State, Ondo State, Osun State, and Ogun State) were surveyed to find out the existence and the abundance of diversity of white guinea yam (Dioscorea rotundata). It was discovered that there are also diversity in other species of landrace yams (Dioscorea cayenensis, Dioscorea alata, Dioscorea dumentorum). One unidentified species of yam was found in Ondo State. This species also exist in Osun State. Some of the yam diversities are endangered. Genetic erosion of these yam diversities is imminent because of the decline of the traditional farmers growing these diversities; also because of modern commercial farms that tend to mono-cropping of improved variety and also because of problem of storage of yams and especially ex-situ conservation of yam genetic resources of yam in Nigeria.

Simple sequence repeat‐based mini‐core collection for white Guinea yam (Dioscorea rotundata) germplasm

AbstractA core subset with a small number of accessions representing the genetic diversity of the base collection plays a vital role in facilitating efficient utilization of plant genetic resources. This is particularly relevant for vegetatively propagated large plant size tuber crops with a long growing period, such as white Guinea yam (Dioscorea rotundata Poir.). For the efficient utilization of D. rotundata genetic resources, this study was aimed at developing a mini‐core collection from a core collection of 447 D. rotundata accessions maintained at the International Institute of Tropical Agriculture (IITA). Accordingly, a D. rotundata mini‐core collection representing 102 accessions was selected using 16 simple sequence repeat (SSR) markers, retaining ∼98% of the SSR allelic diversity of the base collection. A similar level of diversity was captured within the mini‐core collection and the base collection with respect to 21 morphological traits, ploidy level, and geographic origin. The mini‐core collection demonstrated a wide range of variation in agronomic traits such as growth period, number of tubers, average tuber weight, and total yield per plant. This variation was considerable when compared with the variation observed for the same traits among the 10 lines or genotypes conventionally used in the breeding program at IITA, which were included in this study as checks. The selected mini‐core accessions could serve as a working collection to broaden the genetic variation for use in practical breeding programs, as well as in future genomic analyses aimed at the genetic improvement of D. rotundata in West Africa.

Genetic parameters, prediction, and selection in a white Guinea yam early-generation breeding population using pedigree information.

Better understanding of the genetic control of traits in breeding populations is crucial for the selection of superior varieties and parents. This study aimed to assess genetic parameters and breeding values for six essential traits in a white Guinea yam (Dioscorea rotundata Poir.) breeding population. For this, pedigree‐based best linear unbiased prediction (P‐BLUP) was used. The results revealed significant nonadditive genetic variances and medium to high (.45–.79) broad‐sense heritability estimates for the traits studied. The pattern of associations among the genetic values of the traits suggests that selection based on a multiple‐trait selection index has potential for identifying superior breeding lines. Parental breeding values predicted using progeny performance identified 13 clones with high genetic potential for simultaneous improvement of the measured traits in the yam breeding program. Subsets of progeny were identified for intermating or further variety testing based on additive genetic and total genetic values. Selection of the top 5% progenies based on the multi‐trait index revealed positive genetic gains for fresh tuber yield (t ha−1), tuber yield (kg plant−1), and average tuber weight (kg). However, genetic gain was negative for tuber dry matter content and Yam mosaic virus resistance in comparison with standard varieties. Our results show the relevance of P‐BLUP for the selection of superior parental clones and progenies with higher breeding values for interbreeding and higher genotypic value for variety development in yam.

Seed Viability, Seedling Growth and Yield in White Guinea Yam

The yam is an economic tuber crop utilized for food, feed, and various industrial applications. Botanical seed viability, seedling growth, and development are among factors that influence plant population dynamics, development, structure, and sustainability. However, little is known about seed viability, growth, and yield potential of seed-progenies developed using different mating designs. This study assessed seed germination, seedling growth, and yield traits in seed-progenies developed using North Carolina I (NC-1) and polycross mating designs. For this, seed germination and seedling nursery trials established using seed-progenies from different yam crosses were used. Results revealed that days to first seed germination (DAYFG), days to 50% germination (DAYSG), coefficient of velocity of germination, seed emergence speed (SES), germination index, final germination percent, and seedling vigor index significantly (p &lt; 0.05) varied within and among NC-1 and PC-derived families. The mean days to first seed germination (DFSG) and DAYSG seed-progenies of NC-1 were significantly lower than the polycross progenies. Moreover, the seedling-progenies from the polycross produced a higher number of stems and more elongate tubers than those originated from the NC-1 mating. Progenies of family TDr1687 from a polycross mating were among the families that had the highest stem number (2.2), longest tuber (7.5 cm), and widest tuber (2.8 cm). The inter-family means of both NC-1 and polycross had a non-significant variation for mean tuber weight per plant. Our results suggest the relevance of seed germination and seedling attributes for selection of superior progenies at the early generation stage trials in yam breeding.

Genome analyses reveal the hybrid origin of the staple crop white Guinea yam (Dioscorea rotundata)

White Guinea yam (Dioscorea rotundata) is an important staple tuber crop in West Africa. However, its origin remains unclear. In this study, we resequenced 336 accessions of white Guinea yam and compared them with the sequences of wild Dioscorea species using an improved reference genome sequence of D. rotundata In contrast to a previous study suggesting that D. rotundata originated from a subgroup of Dioscorea praehensilis, our results suggest a hybrid origin of white Guinea yam from crosses between the wild rainforest species D. praehensilis and the savannah-adapted species Dioscorea abyssinica We identified a greater genomic contribution from D. abyssinica in the sex chromosome of Guinea yam and extensive introgression around the SWEETIE gene. Our findings point to a complex domestication scenario for Guinea yam and highlight the importance of wild species as gene donors for improving this crop through molecular breeding.

Comparative assessment of genetic diversity matrices and clustering methods in white Guinea yam (Dioscorea rotundata) based on morphological and molecular markers

Understanding the diversity and genetic relationships among and within crop germplasm is invaluable for genetic improvement. This study assessed genetic diversity in a panel of 173 D. rotundata accessions using joint analysis for 23 morphological traits and 136,429 SNP markers from the whole-genome resequencing platform. Various diversity matrices and clustering methods were evaluated for a comprehensive characterization of genetic diversity in white Guinea yam from West Africa at phenotypic and molecular levels. The translation of the different diversity matrices from the phenotypic and genomic information into distinct groups varied with the hierarchal clustering methods used. Gower distance matrix based on phenotypic data and identity by state (IBS) distance matrix based on SNP data with the UPGMA clustering method found the best fit to dissect the genetic relationship in current set materials. However, the grouping pattern was inconsistent (r = − 0.05) between the morphological and molecular distance matrices due to the non-overlapping information between the two data types. Joint analysis for the phenotypic and molecular information maximized a comprehensive estimate of the actual diversity in the evaluated materials. The results from our study provide valuable insights for measuring quantitative genetic variability for breeding and genetic studies in yam and other root and tuber crops.

Sample Preservation and Plant Sex Prediction in White Guinea yam (Dioscorea rotundata Poir.)

Introduction: Yam (Dioscorea spp.) is an economically important staple food in tropical regions, especially for people in West Africa. Understanding of the flowering behavior of the crop to determine potential manipulation available to accomplish crop improvement at early stage remain key challenge in the yam breeding. The methods that reliably yield quality DNA and distinguishing sex type at the early stage of growth have been a challenge in yam genetics and breeding studies. This study assessed the effect of sample preservation methods on DNA quantity and quality during extraction and potential of DNA marker to diagnose plant sex at the early seedling stage in white Guinea yam. Materials and Methods: Five sample preservation methods were assessed for quality DNA extraction during field leaf tissue collection, namely liquid nitrogen, dry ice, silica gel, 95% ethanol, and oven drying. The predicted sex at the seedling stage using the molecular marker was further validated with the visual score for the sex phenotype at the flowering stage. Results: According to the findings of the present study, the DNA extracted from leaf samples preserved in liquid nitrogen, silica gel, dry ice, and oven drying methods were higher in molecular weights than samples stored in ethanol solution. Yam plant sex diagnosis with the DNA marker (sp16) identified a higher proportion of ZW genotypes (female or monoecious phenotypes) than the ZZ genotypes (male phenotypes) in the studied materials with 74% prediction accuracy. Conclusions: The results from this study provided valuable insights on suitable sample preservation methods for quality DNA extraction and the potential of DNA marker sp16 to predict sex in white Guinea yam.

Paternity Assignment in White Guinea Yam (Dioscorea Rotundata) Half-Sib Progenies from Polycross Mating Design Using SNP Markers.

White Guinea yam is mostly a dioecious outcrossing crop with male and female flowers produced on distinct plants. Fertile parents produce high fruit set in an open pollination polycross block, which is a cost-effective and convenient way of generating variability in yam breeding. However, the pollen parent of progeny from polycross mating is usually unknown. This study aimed to determine paternity in white Guinea yam half-sib progenies from polycross mating design. A total of 394 half-sib progenies from random open pollination involving nine female and three male parents was genotyped with 6602 SNP markers from DArTSeq platform to recover full pedigree. A higher proportion of expected heterozygosity, allelic richness, and evenness were observed in the half-sib progenies. A complete pedigree was established for all progenies from two families (TDr1685 and TDr1688) with 100% accuracy, while in the remaining families, paternity was assigned successfully only for 56 to 98% of the progenies. Our results indicated unequal paternal contribution under natural open pollination in yam, suggesting unequal pollen migrations or gene flow among the crossing parents. A total of 3.8% of progenies lacking paternal identity due to foreign pollen contamination outside the polycross block was observed. This study established the efficient determination of parental reconstruction and allelic contributions in the white Guinea yam half-sib progenies generated from open pollination polycross using SNP markers. Findings are useful for parental reconstruction, accurate dissection of the genetic effects, and selection in white Guinea yam breeding program utilizing polycross mating design.

Identification of candidate flowering and sex genes in white Guinea yam (D. rotundata Poir.) by SuperSAGE transcriptome profiling.

Dioecy (distinct male and female individuals) and scarce to non-flowering are common features of cultivated yam (Dioscorea spp.). However, the molecular mechanisms underlying flowering and sex determination in Dioscorea are largely unknown. We conducted SuperSAGE transcriptome profiling of male, female and monoecious individuals to identify flowering and sex-related genes in white Guinea yam (D. rotundata), generating 20,236 unique tags. Of these, 13,901 were represented by a minimum of 10 tags. A total 88 tags were significantly differentially expressed in male, female and monoecious plants, of which 18 corresponded to genes previously implicated in flower development and sex determination in multiple plant species. We validated the SuperSAGE data with quantitative real-time PCR (qRT-PCR)-based analysis of the expression of three candidate genes.We further investigated the flowering patterns of 1938 D. rotundata accessions representing diverse geographical origins over two consecutive years. Over 85% of accessions were either male or non-flowering, less than 15% were female, while monoecious plants were rare. Intensity of flowering varied between male and female plants, with the former flowering more abundantly than the latter. Candidate genes identified in this study can be targeted for further validation and to induce regular flowering in poor to non-flowering cultivars. Findings of the study provide important inputs for further studies aiming to overcome the challenge of flowering in yams and to improve efficiency of yam breeding.

Comparative Effect of Poultry and Swine Manures on the Performance of White Guinea Yam (Dioscorea rotundata Poir) in an Ultisols Environment

White Guinea yam (Dioscorea rotundata Poir) is an important staple food in a number of African countries, including Nigeria and Ghana. However, the tuber yield in these areas is below its potential yield. The most probably constraint to low tuber yield is the low soil nutrient status. A field trial was conducted at the Experimental Farm of the Department of Crop Science, Faculty of Agriculture, University of Benin, Benin City, Nigeria, between April and November 2016 and 2017, to evaluate the effect of poultry and swine manures on the growth and tuber yield of white Guinea yam (D. rotundata) in an ultisols environment. The trial was laid out in a 2 × 3 split plot arrangement fitted into randomized complete block design with three replicates. The main plots were organic fertilizers (poultry and swine manures) and the sub-plots were represented by three rates (0, 300 and 450 kg N ha-1) of application. Data collection on growth characters were on vine length (cm), vine girth (cm), number of nodes, number of leaves and leaf area index. At harvest, tuber length (cm), tuber girth (cm), number of tubers per plant, tuber weight and tuber yield (t ha-1) were also evaluated. The results showed that growth and yield variables varied significantly (p &lt; 0.05) with organic fertilizer types and application rates. Poultry manure treated plants produced higher (p &lt; 0.05) number of nodes and leaves than swine manure treated plants. Application rates of 300 and 450 kg N ha-1 of poultry manure significantly increased vine length, vine girth, number of nodes and leaves as well as leaf area index. Tuber weight and yield were higher in plants treated with poultry manure with the application rates of 300 and 450 kg N ha-1 performed better than control in 2016, while the application rate of 300 kg N ha-1 had the highest tuber weight and yield in 2017. Application rates of 300 and 450 kg N ha-1 performed better than the control (0 kg N ha-1) in 2016. However, in 2017, application rate of 300 kg Nha-1 had the highest tuber weight and yield. The present study suggests that poultry manure at application rate of 300 kg N ha-1 could be adopted by white Guinea yam farmers for high productivity of the crop under intense and continuous cropping in humid ultisols environment.

Carotenoid profiling of yams: Clarity, comparisons and diversity

Screening carotenoids of elite accessions of yam (Dioscorea spp.) used in the global yam breeding program has been conducted to quantitatively determine the carotenoid composition of the crop. Comparisons to previous data reporting cerotenoid levels in yam has been made, in order to deduce greater perspectives across multiple studies. Characterisation of complex species and accession -specific profiles have shown a rich base of diversity that can inform breeding strategies. Key findings include; (i) the identification of accessions rich in β-carotene which can aid provitamin A biofortification, (ii) Data disputing the commonly held belief that yellow Guinea yam (D. cayennensis) has higher β-carotene content than that of white Guinea yam (D. rotundata), and (iii) the tentative identification of C25-epoxy-apocarotenoid persicaxanthin with potential implications for tuber dormancy.

Occurrence and prevalence of nematodes in yam fields from four community-based farming scheme locations in Ogun State, Nigeria

Nematodes is one of the major biotic constraints affecting profitable yam production throughout Nigeria. They affect yams both in the field and in storage thus threatening food security and economic deprivation to growers and their households. The research work identified the types, frequency and population of nematodes associated with Dioscorea rotundata grown by the Farm Practical Year Programme Trainees of the Federal University of Agriculture, Abeokuta in four Community-based Farming Scheme locations, namely Isaga-Orile, Iwoye-Ketu, Ode-Lemo and Odogbolu in Ogun State. Soil samples were taken and assayed for nematode populations. Results showed that 26 Nematodes genera were found in the four locations ranging from 11 to 3201 individuals/250 g soil. Genera of plant-parasitic nematodes recovered included Scutellonema, Meloidogyne, Pratylenchus, Paratylenchulus, Trichodorus, Helicotylenchus, Hoplolaimus, Tylenchorhynchus, Aphelenchus, Xiphinema, Ditylenchus, Rotylenchulus, Rotylenchus and Aphelenchoides while the free-living nematode genera found included Aulolaimoides, Cephalobus, Cylindrocorpus, Lotonchus, Monhystera, Oncholaimus, Paraplectonema, Prismatolaimus, Dorylaimus, Tylencholaimellus, Tylencholaimus, and Tyleptus. Out of the twenty-six nematodes encountered, Meloidogyne spp. Scutellonema spp. and Pratylenchus spp. were mostly encountered in the soil and the only three recovered from the tubers which mean that these three nematodes are of economic importance to yam production. Farmers are therefore advised to go into yam production with management measure of nematodes putting the three key nematodes in mind, in order to avoid yield loss.Keywords: White guinea yam; Meloidogyne spp.; Scutellonema spp.; Pratylenchus spp.

Genome sequencing of the staple food crop white Guinea yam enables the development of a molecular marker for sex determination

BackgroundRoot and tuber crops are a major food source in tropical Africa. Among these crops are several species in the monocotyledonous genus Dioscorea collectively known as yam, a staple tuber crop that contributes enormously to the subsistence and socio-cultural lives of millions of people, principally in West and Central Africa. Yam cultivation is constrained by several factors, and yam can be considered a neglected “orphan” crop that would benefit from crop improvement efforts. However, the lack of genetic and genomic tools has impeded the improvement of this staple crop.ResultsTo accelerate marker-assisted breeding of yam, we performed genome analysis of white Guinea yam (Dioscorea rotundata) and assembled a 594-Mb genome, 76.4% of which was distributed among 21 linkage groups. In total, we predicted 26,198 genes. Phylogenetic analyses with 2381 conserved genes revealed that Dioscorea is a unique lineage of monocotyledons distinct from the Poales (rice), Arecales (palm), and Zingiberales (banana). The entire Dioscorea genus is characterized by the occurrence of separate male and female plants (dioecy), a feature that has limited efficient yam breeding. To infer the genetics of sex determination, we performed whole-genome resequencing of bulked segregants (quantitative trait locus sequencing [QTL-seq]) in F1 progeny segregating for male and female plants and identified a genomic region associated with female heterogametic (male = ZZ, female = ZW) sex determination. We further delineated the W locus and used it to develop a molecular marker for sex identification of Guinea yam plants at the seedling stage.ConclusionsGuinea yam belongs to a unique and highly differentiated clade of monocotyledons. The genome analyses and sex-linked marker development performed in this study should greatly accelerate marker-assisted breeding of Guinea yam. In addition, our QTL-seq approach can be utilized in genetic studies of other outcrossing crops and organisms with highly heterozygous genomes. Genomic analysis of orphan crops such as yam promotes efforts to improve food security and the sustainability of tropical agriculture.

English

Genetic improvement of major food yams is constrained by a number of factors, such as the scarcity of flowers and lack of synchronization between male and female flowering. Consequently, somaclonal variation including somatic polyploidization has been considered as a useful tool in yam breeding. Somatic polyploidization and its effect on phenotypic traits of Dioscorea species such as D. alata, D. japonica and D. zingiberensis has been reported; however, optimization of this method in two major yam species, D. rotundata and D. cayenensis, is yet to be achieved and the effect of polyploidization on phenotypic traits of this species yet to be elucidated. In the present study, a high rate of somaclonal polyploid variation was successfully achieved by in vitro colchicine treatment of D. rotundata and D. cayenensis. In most cases, except TDc 3704, the highest rate of polyploid induction appeared after 0.1% colchicine treatment. However, in triploid yellow yam accessions the induction rate was relatively low. Tetraploid variants of D. rotundata tended to display somewhat rounder leaves than their diploid parents. The size and shape of D. rotundata stoma were also affected by levels of ploidy, with tetraploid variants exhibiting larger stomata at a lower density compared to their diploid parents. The efficient method of in vitro polyploidy induction reported here is therefore a highly useful tool for obtaining polyploid variants for use as genetic resources in D. rotundata breeding. Key words: Leaf, nodal segment culture, ploidy level, somaclonal variation, stoma, white guinea yam, yellow Guinea yam.