Morphological and cytological diversity of some yams (Dioscorea spp.) in Sierra Leone.

Abstract

Yam (Dioscorea spp.) is a major source of income and a food security crop for many households in Sierra Leone. Despite the economic importance of the yam crop its improvement has suffered from the lack of knowledge of existing germplasm and the genetic potential within the yam gene-pool. As a consequence, many species of yam are being lost to changing tastes, industrialization and urbanization. All these lead to habitat destruction. This study assessed the extent of diversity in some yam germplasm from Sierra Leone using morphological and cytological descriptors, and ascertained the interrelationship between these two data sets. To this end, 52 genotypes comprising of forty three D. alata, two D. bulbifera, and seven D. rotundata sampled from the Sierra Leone germplasm were grown in a three replicate, in a randomized complete block design (RCBD) during 2010 planting season at the University of KwaZulu-Natal, Pietermaritzburg, South Africa. Principal component analysis (PCA) of 28 morphological characters indicated that the first 10 principal components (PCs) with eigen-values greater than 0.6 explained 86.61% of the total variation. The PCs that largely contributed to the variability included number of days to shoot emergence, leaf position, leaf shape, leaf size, density of leaf, leaf vein colour; colour of leaf, petiole, petiole wing and stem, shoot growth rate, tuber shape and flesh colour of central cross section of tuber. The two-dimensional plot of the first two PCs grouped the accessions according to their species, but did not separate them into the tuber shape groups of irregular, oblong, oval-oblong, round and cylindrical. Factor analysis (FA) grouped the morphological traits into six factors, which together explained 75% of the total phenotypic variation in the dependence structure. Factor 1 was strongly associated with absence or presence of wings, distance between lobes, leaf apex shape, leaf colour, leaf margin colour, leaf measurement length-2, leaf vein colour of upper surface, number of branches, number of stems, stem colour and tip length of mature leaf; factor 2 with leaf density, leaf measurement length-1, leaf vein colour of lower surface, petiole wing colour, tip colour, wing colour and flesh colour of central cross section of tuber; factor 3 with leaf measurement width-1; factor 4 with leaf measurement width-2; factor 5 with stem colour; and factor 6 with number of days to emergence. The dendrogram of the cluster analysis produced six major groups supporting the PCA and FA groupings. Clusters A, B, C, D, E and F were formed at the dissimilarity distance = 0.90; and they consisted of two, thirty eight, one, seven, two and two genotypes respectively. Genotypes of cluster A belong to D. bulbifera, while genotypes of clusters B, C, E and F belong to D. alata, and genotypes of cluster D belong to D. rotundata. The ploidy levels of the 52 genotypes were determined by flow cytometry. The various ploidy levels obtained included diploid (2x), triploid (3x), tetraploid (4x), pentaploid (5x) and hexaploid (6x). The estimated nuclear DNA content ranged from 1.634 pg for G1 nuclei of diploid Dioscorea alata to 2.118 pg for G1 nuclei of hexaploid Dioscorea rotundata. Genotypes NR 07/045 (4x) and NR 07/040 (5x), which belong to D. bulbifera had nuclear DNA content of 1.905 and 2.017 pg respectively. The nuclear DNA content per genome was higher in diploids compared to polyploids. The variations within the 4x accessions (p<0.005) and among the three species (p<0.037) were significant. Root tips of six genotypes were prepared for chromosome counting using the acetocarmine staining technique. Genotype ER 07/030 had 20 chromosomes, four genotypes including ER 07/036, NR 07/060, NR 07/071 and SR 07/072 had 40 chromosomes, and TDr 95/18544 had 60 chromosomes. The findings agree with the hypothesis that studied germplasm from Sierra Leone were morphologically different expressing inter- and intra-group variability. Duplicate accessions were observed supporting the hypothesis that some genotypes had different names in diverse cultural setting. Flow cytometric measurements and conventional chromosome counting showed the existence of inter- and intra-group diversity in ploidy level and nuclear DNA content. A correlation was established between agro-morphological and cytological traits used in the study. This study contributes to an understanding of yam diversity in Sierra Leone which will facilitate yam genetic resource management, conservation and utilization.