MSc

architecture (plant height/dwarfness) in white yam (Dioscorea rotundata Poir.).The study also aims at assessing variability and genetic diversity among white yam genotypes using molecular markers like RAPD, ISSR and SSR and also to standardize in vitro protocol for the rapid propagation of dwarf/bushy white yam.
In present study, DNA was isolated from 30 white yam (Dioscorea rotundata Poir.) genotypes having dwarf, semidwarf and tall plant type. Based on preliminary screening, 15 ISSR, 10 SSR and 8 RAPD primers with high polymorphism were selected for the molecular characterization of white yam genotypes.
The total number of bands per ISSR primer ranged from 1 ((ACC)6Y) to 10 (UBC 825). UBC 825 that recorded the highest number of polymorphic bands (10) followed by UBC 827 with UBC 864 with nine bands. In the present study, the ISSR primers produced an average of six polymorphic alleles with a mean Hobs and PIC values of 0.738 and 0.707 respectively. Among the ten SSR markers studied, YM15 recorded the maximum number of polymorphic alleles (5) followed by Dab2D06 and YM26 with four alleles. The percentage of polymorphism ranged from 50 (Dab2C05, Dab2E07). The polymorphism information content ranged from 0.3398 (Dab2C05) to 0.7388 (Dab2D06).YM15 also recorded high PIC value of 0.7377. On average the SSR markers recorded
Hobs value of 0.5715 and polymorphism information content of 0.4866. Among the eight RAPD primers studied in white yam, OPW -16 was found be the best that produced ten polymorphic bands followed by OPG-02 and OPG-13 with six polymorphic bands. All the primers recorded high PIC value of >0.6.
The comparison of the different type of markers (ISSR, SSR, RAPD) studied, showed that the number of polymorphic bands are higher for ISSR and RAPD, the average polymorphism information content was lesser for SSR as compared to RAPD and ISSR primers evaluated. The results implied that the all the different type of markers used were efficient in discriminating the genotypes studied.
Cluster analysis done based on Euclidean distance of 30 accessions for ISSR, SSR and RAPD. Dendrogram showed the partition of most of the dwarf and tall genotypes in to two different clusters. Semi dwarf comes under the cluster of dwarf genotypes. The correlation between different clusters was found to be high (>0.5). The cluster data from dendrogram was in agreement with data from cluster analysis after PCA suggesting that most of the tall and dwarf genotypes grouped separately.
A preliminary study was also carried out to standardize the micropropagation protocol for the regeneration of plants from nodal explants taken from field. Sodium dichloroisocyanurate solution of 2 per cent for 15 minutes treatment found optimum for surface sterilization and got 76.6% recovery without any microbial contamination, when nodal cuttings collected from field were used as explants. MS medium with 2mg kinetin enhanced rooting compared to other media. Major problem faced during micropropagation is the delay of sprouting and contamination due to systemic microbes. However initial establishment of the explants is very slow and took 82-115 days. Hence further research is needed for hastening the in vitro establishment in white yam.
The results indicated the association of three SSR markers (Dab2C05, Dpr3F04, Dab2E07) with dwarfness in white yam. One of the SSR markers Dab2C05 showed unique band of size 1.5Kbp for dwarf genotype. The ISSR primer UBC 836 was also identified as better marker that showed specific band for dwarf plant type.
In addition to identifying molecular markers linked to dwarfness, the present study helped to identify a high yielding (8 kg/plant), highly divergent white yam genotype, DR17 that could be used for the genetic improvement of white yam in future.