Identification of RAPD markers linked to nut weight and plant stature in cashew

Cashew is an important edible nut crop of tropics. Bulk segregant analysis (BSA) was carried out on DNA bulks constituted from F₂ population and germplasm in order to link or associate molecular markers with economic characters. In all 458 RAPD, 31 ISSR and 21 pairs of SSR primers were used and identified polymorphic markers between parents. Though screening F₂ bulks with these markers identified markers polymorphic between the bulks but none could be validated with the individuals of their bulks. Hence screening with germplasm bulks was carried out and could identify four RAPD markers polymorphic between the bulks for nut weight and plant stature and also between the individuals of their bulks. Of the four, three markers were associated with nut weight amplifying at 775, 475, 275, bp region in primers OPN 14, UBC 184 and UBC 185 respectively. Out of these three, two markers were specific to low nut weight and one marker was specific to high nut weight and their bands were present in greater frequency (50–77.8% and 75%) of individuals constituting the respective bulks. Similarly, the another marker UBC 185₂₇₅ was detected which was specific to low plant stature and was present in 66.7% and 10% individuals constituting short and tall bulks respectively. Markers identified with bulks and with the individuals of bulks were validated further with more individuals of F₂ and germplasm.     

Assessment of genetic diversity in cashew germplasm using RAPD and ISSR markers

Diversity and genetic relationship in 100 cashew germplasm accessions were analyzed by using RAPD and ISSR markers. Using 10 selected RAPD primers 60 bands were generated, of which 51 bands were polymorphic (85%), and with 10 selected ISSR primers 67 amplified bands were observed with 58 polymorphic bands (86.6%). Though both kinds of markers discriminated the accessions effectively, analysis of combined data of markers (RAPD + ISSR) resulted in better distinction of accessions. By combining markers, a total of 127 bands were detected, of which 109 bands (85.8%) were polymorphic and produced on an average of 5.45 polymorphic bands per primer. Primers with high polymorphic information content and marker index were identified for discriminating accessions. High percentage of polymorphism (>85%) observed with different markers indicated high level of genetic variation existing among the accessions. Genetic relationship estimated using similarity co-efficient (Jaccard’s) values between different pair of accessions varied from 0.43 to 0.94 in RAPD, 0.38 to 0.89 in ISSR and 0.43 to 0.87 with combined markers suggested a diversity (dissimilarity) ranging from 6 to 57%, 11 to 62% and 13 to 57% respectively and the diversity skewed around 50% indicated moderate diversity. The cluster analysis with UPGMA method separated the accessions broadly into 13 clusters and in that three into smaller clusters. Some correspondence between the molecular groupings and the morphological clusters were observed. Among the accessions, NRC-142 and NRC-12 were highly divergent and NRC-231 and NRC-232 were genetically similar.