MSc.

The experiment entitled "Molecular marker analysis for cassava mosaic disease resistance" was carried out at Division of Crop Improvement, ICAR-CTCRI, Sreekariyam, Thiruvananthapuram during 2013-14. The main objective of the investigation was to analyse the genetic variation among cassava mosaic disease resistant genotypes and to identify molecular markers linked to cassava mosaic disease resistance incidence.
Sixty cassava genotypes were screened for CMD incidence and visual scoring was done in a 0-5 range scale. Among the 60 accessions evaluated, 39 were resistant and 21 were found to be suscetible. Cassava samples collected from CTCRI were diagnosed for the presence of Indian Cassava Mosaic Virus (ICMV) and Sri Lankan Cassava Mosaic Virus (SLCMV) through multiplex PCR analysis and the results were in agreement with field screening results.
For molecular analysis, the SSR primers were selected based on review of literature and were screened by Bulk Segregant analysis. Primers screened include SSRY 28, SSRY 44, SSRY 45, SSRY 100, SSRY 105, MeESSRY 19 and SSRY 234. From the markers studied, it was found that SSRY28 amplified the expected product size of 180 bp in eighteen highly resistant genotypes. SSRYMe 19 amplified tha allele in eighty seven percent of the resistant genotypes.. None of te alleles of the markers studied could distinguish completely between CMD resistant and CMD susceptible genotyhpes. Among the markers tested,SSRY 44 amplified the allele 250bp in 20 hghly resistant genotypes and was amplified only in five susceptible genotypes. Hence SSRY 44 (250 bp) can be used in marker assisted selection for CMD resistance in cassava after further validation.
The expected heterozygosity of the SSR markers ranged from 0.2637 (SSRMe9) to 0.6297 (SSRY44). The maximum PIC value was recorded by the marker SSRY44 (0.5586) followed by SSRY45 (0.5218). The gene diversity index was found to be the lowest for the primer SSRY234 (0.0944). The maximum gene deversity was recorded for the marker SSRY44 (0.4419) followed by SSRY105 (0.4111), SSRY28 (0.3886) and SSR Y 45 (0.3578).
Sixty cassava genotypes were characterized based on seven quantitative traits, five qualitative traits and CMD resistance. The genotypes viz.8W5, CR 43-7, CR 21-10 and CI-273, 9S 172 recorded very high leaf retention ability (>1000 leaves/plant).The tuber weight ranged form 0.3 (CMR 12) to 11 (CR59-8R) kg per plant. All the high yieldling genotypes viz MN-1, CR43-7, CR54A-19, CR52A41, CR21-10, CE185, S1284, CR20A-2, 9S165, CI273 and CR59-8R that recorded very high weight of tubers per plant(>6 kg/plant) were found to be CMD resistant too. The maximum value for harvest index among the cassava genotypes was recorded in 9S165 (0.84) followed by IMSI-9 (0.80),MN-2 (0.78), CR63-3 (0.76 and CR59-8 (0.76).
In the present investigation, cluster analysis was done based on Euclidian distance of 60 genotypes of the SSR markers studied. Based on morphologival data seven clusters were formed. Six clusers were again divied into two sub clusters and seventh cluster formed three sub clusters.
The PCA analysis provides information about associations of accessions which are useful to formulate better strategies for breeding. The patterns of variation and the relative importance of each morphological trait in explaining the observed variability was assessed through principal component analysis (PCA). The first principal component (PC-1) accounted 34.18% of the total variation and was correlated positively with total biomass (0.4521), tuber weight/plant (0.4099), stem weight (0.3769) and leaf retention (0.3429)while CMD (-0.2091) contributed negatively as expected.
SYBR Green real time PCR assay for finding the virus load was conducted to quantify the viral DNA and the resistant varieties were found to be free from viruses causing cassava mosaic disease.
In the present investigation, 39 genotypes with complete resistant to cassava mosaic disease caused by ICMV/SLCMV were identified. These resistant lines can be further evaluated for developing a high yielding CMD resistant variety in future. Molecular marker analysis indicates the association of SSRY 28 and SSRY 44 with CMD resistancce. These makers can be used for marker assisted selection to facilitate speed breeding of new cassava varieties.