Phenotypic diversity based on morphology
There was considerable variation among the genotypes studied with regard to both the underground and aerial morphological parts. Variability among the genotypes in leaf, tuber, inflorescence, and stem morphological parts are presented in Fig 1 - 4, and classification based on geographical origin is presented in Table 3. Out of the 291 genotypes studied, 218 genotypes (74.9%) were classified as Dioscorea rotundata species, and 71 genotypes (24.4%) were observed to be Dioscorea alata. Two genotypes from Uganda were classified as unique based on trifoliate leaves observed. Generally, Dioscorea dumetorum is the species that has trifoliate leaves but this needs to be further confirmed through characterization with an additional number of genotypes having trifoliate leaves or through molecular markers. The majority (175 genotypes, 80.2%) of the Dioscorea rotundata genotypes were amongst materials sourced from West Africa (Ghana and Nigeria), with only 47 genotypes (21.6%) collected in Uganda (Table 3). On the other hand, the majority (39 genotypes, 54.9%) of the Dioscorea alata genotypes were observed to have been sourced within Uganda.
Table 3. Genotypes classified based on geographical origin and species
Four basic leaf forms were identified on yams assessed in this study; Ovate (egg-shaped), cordate (heart-shaped), sagittate (arrowhead-shaped), and hastate (spearhead shaped). The majority of the genotypes (120 genotypes; 41.2%) had cordate leaf form, followed by hastate (96 genotypes; 32.9%), and sagittate (73 genotypes; 25.1%) (Fig 1A). Only two genotypes collected from Uganda (0.7%) with ovate leaf form were trifoliate. It was observed that majority of the hastate and sagittate-shaped genotypes were Dioscorea alata from Uganda (39 genotypes; 61.3%). Dioscorea rotundata genotypes had cordate-shaped leaves (Fig 1). Leaf colour was generally green among all genotypes assessed, except one Dioscorea rotundata genotype (UGY20088) from Ghana, which was characterized by dark purple leaves. Three Dioscorea alata genotypes were purplish green in color and were all introduced from Ghana.
Fig 1. Phenotypic variability of plant morphology-based leaf shape and colour
In terms of tuber shape, majority of genotypes had either cylindrical (59.8%) or irregular (29.2%) tuber shapes. The remaining 13% of the genotype had either spherical/round (7%) or oval shapes (5%). Majority of the genotypes (174 genotypes; 59.8%) with cylindrical-shaped tubers are from West Africa (Ghana and Nigeria) while majority (85 genotypes; 29.2%) of genotypes with irregularly shaped tubers are from Uganda (Fig 2).
Fig 2. Phenotypic variability of plant morphology based on tuber shapes
Among the 291 genotypes studied, 192 genotypes (65.9%) did not flower or did not exhibit signs of budding (Fig 3). Nonetheless, of the 99 genotypes (34.0%) that flowered, 92 (92.9%) were Dioscorea rotundata genotypes, and 7 (7.0%) were Dioscorea alata genotypes. The intensity of flowering among the genotypes that flowered was assessed wherein 12.2% had low flowering intensity, 22.6% had moderate flowering intensity and the remaining had profuse flowering (Fig 3). The sex among flowering genotypes showed majorly male flowering with 67 genotypes (68.7%) while 31 genotypes (31.3%) were female. The Dioscorea alata genotypes (7%) from Uganda that flowered were primarily female. None of the Dioscorea alata genotypes sourced from Ghana or Nigeria flowered. Moreover, the Dioscorea rotundata genotypes that flowered were mainly from West Africa and the two unique trifoliate genotypes from Uganda were male flowering.
Fig 3. Phenotypic variability of plant morphology based on flowering intensity
The presence or absence of spines and the formation of wings were used to distinguish vine differences observed among the genotypes. Five main variations in vine forms were classified; i) vine without spines, ii) vines with many spines, iii) vines with few spines, iv) winged and v) unwinged veins. A total of 122 genotypes (41.9%) were without spines on the vines followed by 132 genotypes (45.4%) with few spines and 39 genotypes (12.7%) with many spines on the vines (Fig 4). Additionally, most of the thornless genotypes had winged smooth vines and were mainly Dioscorea alata. All the genotypes with spines on vines were unwinged and were mainly Dioscorea rotundata.
Fig 4. Phenotypic variability of plant morphology spines on stem
Contribution of individual traits to phenotypic variation
Patterns of variation and relative importance of each descriptor in explaining the observed variability were assessed through principal components analysis (PCA). Each eigenvalue for the first six principal components was greater than 1.0 and cumulatively contributed to 62% of total morphological variation. Scores of PC1, which accounted for 23% of the total variation, were correlated (r > 0.45) to leaf width, leaf length, leaf area, leaf density, petiole length, plant vigour tuber weight, tuber circumference, tuber surface texture, tuber branching, tuber length, tuber shape and tuber size (Table 4). Scores of PC2 explained 15% of the total variation. Vine characteristics such as sprout spines, vine spines, and vines per plant were contributed mainly to the variation in PC3 and PC4, respectively. Considering disease characteristics (Yam mosaic virus and Yam anthracnose disease), at PC4 only yam anthracnose disease was relevant in causing any variation among the genotypes studied. Scores of PC5 contributed to a variation of 5% correlated well with traits such as Tuber branching and Tuber shape. Based on the contribution of each of the measured traits to the most informative principal components, all 28 traits were found to be relevant in discriminating the yam genotypes.
Table 4: Proportion of morphological variation and traits contribution explained by first six PCs
To assess the clustering/grouping of individual genotypes, the scores of each genotype on the first two principal components, (PC1 and PC2) set were plotted (Fig 5). The genotypes were coloured based on geographical origin. While most of the genotypes clustered around the center of the graph, others were widely scattered along the PC axes. Despite a large amount of overlap between Ghana and Nigeria groups, the dispersion pattern generally separated the species based on the measured morphological traits (Fig 5). The genotypes originating from Uganda clustered at different quadrants with few overlaps with genotypes from West Africa. Most traits such as leaf characteristics and disease-related traits were the main variables associated with the cluster of genotypes with Uganda origin (Fig 5).
Fig 5. Two-dimensional plot of the first two principal components (PC1 and PC2)
Genetic relationship among yam genotypes
There was considerable variation among the genotypes with regards to the morphological traits used in the study. The analysis of morphological variability based on “ward” hierarchical clustering resulted in two major clusters (Cluster A and Cluster B (Fig 6). Cluster A consisted of 194 genotypes (66.7%) with a distance ranging from 28.1 to 188.8 and an average of 52.8. The majority of the genotypes in this cluster were from Nigeria (69 genotypes; 35.6%) followed by Ghana (67 genotypes; 34.5%) and remaining from Uganda (58 genotypes; 29.9%) (Fig 6). Genotypes within this cluster are mainly characterized by cylindrical/oval tubers, hairy tubers, sagittate/cordate-shaped leaves, green with a few purple-colored leaves, and non-winged vines. The flowering of the genotypes was classified as highly profuse and predominantly female. Two subclusters (A1 and A2) were identified within cluster A of which subcluster A1 had 74 genotypes (38.1%) and subcluster A2 had 120 genotypes (61.9%).
In cluster B, the distance ranged from 27.6 to 159.3 with an average distance of 53.0. The cluster was made up of 97 genotypes (33.3%). The highest number of genotypes in this cluster were from Ghana with 40 genotypes (41.3%) while the number of genotypes from Nigeria and Uganda was 37 (27.8%) and 30 (30.9%), respectively (Fig 6). Genotypes in this cluster were characterized by thornless stems, hastate-shaped leaves, green leaf color, dark green leaves, and a few purple-colored petioles, amorphous shaped and rough surface tubers.
Fig 6. Dendrogram showing diversity among 291 yam genotypes based on phenotypic traits