ABSTRACT

The West African yam collections held at the International Institute of Tropical Agriculture, Ibadan (IlTA) were characterized using the standard descriptor list for yam (Dioscorea spp.) IPGR1II1TA, 1997, to assess the extent of diversity and develop a core collection employing 77 out of 86 descriptors and the Shannon—Weaver diversity index. The core collection consists of 391 accessions (13% of entire collection). It represents all the six cultivated and two wild Dioscorea taxa. The appropriateness of the procedure was confirmed by comparing the mean and diversity distributions of 11 (Out of 13) quantitative traits. Genetic diversity as revealed by tree construction of the SSR data using unweighted Neighbour-joining method and PCA showed large number of interand intra-specific polymorphisms. Pairwise distance matrices using Jaccard coefficients resulted to similarities varying from 58% to 93%. A total of 97 alleles were detected for the D. alata germplasm with an aveIge allele number of 7.46 per locus. Polymorphism information content (PlC) mean value of 0.65 was obtained. Accessions from Nigeria showed highest gene diversity of 0.678 while those from Côte d'Ivoire had lowest diversity with 0.596. Observed mean heterozygosity value of 0.469 was obtained. Cluster and principal coordinate analysis showed 8 major cluster groups for the D. alata. A diversity of 0.677 was found among the accessions of Guinea yams. An allelic average of 8.06 and polymorphic information content (PlC) value of 0.65 was observed for the markers. Twenty distinct cluster groups generated by the radial phylogram shows that D. cayenensis and D. rohindata are distinct species with intermediate hybrid forms. There was no relationship between relatedness of the accessions and their geographical area of (Thh. ..4 1 .,4., (-\ (2,\ + (A\ l 1,1 uLIu1..Ju)C1 vcu iOtuy ivt O.I u-tflJ1U A), U1pflJIU IJA), LCuaptulu I,tA), ttapiu1u (6x) and octoploid (8x). Estimated nuclear DNA sizes ranged from 0.702 ± 0.004 pg for G1 nuclei of diploid D. dumetorum to 2.573 ± 0.020 pg for G1 nuclei of octoploid D. cayenensis. Results showed that there is a decrease in genome size with higher ploidy level. One mixoploid individual containing tetraploid and hexaploid nuclei was detected. No association was found between ploidy level and geographic origin of the germplasm evaluated. The most variable pmximate characteristic is sugar (coefficient of variation, CV= 38.32%), amylose (CV 16.31%), starch (CV 10.59%) and dry moisture content (CV 6.52%). Peak viscosity during heating ranged from 19.08 RVU to 442.59 RVU with a CV of 46.85%, the trough viscosity ranged from 15.55 RVU to 334.21 RVU with a CV of 44.72%, and the breakdown viscosity ranged from 2.71 RVU to 187.42 RVU with a CV of 46.19%. The final viscosity ranged from 25.00 RVU to 529.71 RVU with a CV of 44.13%. The set back ranged from 12.00 RVU to 423.46 RVU with a high CV of 86.35%. The pasting temperature ranged between 63.33 °C and 95.43 °C with a CV of 17.16%. Pastes from D. cayenensis, D. rotundaga and D. a/ala generally had higher values for peak viscosity, breakdown, final viscosity, and setback viscosity.