Morinda tomentosa and Morinda citrifolia, the flowering plant species of the genus Morinda are known for their immense health benefits because of the presence of secondary metabolites of medicinal importance and nutritional value due to excellent source of minerals and vitamins. Other than this, these species also provide wood, dye, oil etc. and can be used as bio-sorbents. Molecular characterization of such valuable species is essential for making strategies for their collection and conservation followed by its utilization. Molecular markers (RFLPs, AFLPs, RAPD, ISSR, SCoT, SSRs etc.) are very well recognized for assessing the level of genetic diversity (Powell et al. 1996). RAPD, ISSR and SCoT markers were used earlier to find the level of genetic diversity in Morinda spp. also (Singh et al. 2011; Singh et al. 2012; Arya et al. 2013; Arya et al. 2014). These studies predicted the level of genetic diversity in the samples collected from Andaman and Nicobar Islands, Tamil Nadu, Karnataka, Gujarat and Madhya Pradesh.
The present study was carried out to assess a) the level of genetic diversity between and within Morinda tomentosa and Morinda citrifolia collected from diverse geographical locations of India using ISSR markers b) use of ISSR markers in species differentiation in Morinda. ISSRs were chosen for the present study as these are simple to perform, more reproducible, stable than RAPD, multi-locus dominant marker system and have been used for diversity analysis, population structure analysis, DNA finger printing, phylogenetic analysis etc. in different plant species (Ansari et al. 2012; Zhang et al. 2015; Kumar et al. 2016; Ana-Cruz et al. 2017).
In the present study, ISSR markers revealed 90.34% polymorphism among the 31 Morinda genotypes belonging to Morinda tomentosa from Gujarat, Rajasthan and Tamil Nadu and Morinda citrifolia from Kerala and Rajasthan. The mean Nei’s gene diversity and Shannon’s information index values also revealed considerable genetic diversity in our genotypes.
In an earlier study by Singh et al. 2011 in 22 accessions of M. citrifolia, M. tinctoria and M. pubescens based on ISSR markers, polymorphism level of 56.02% was reported, which is very less than the polymorphism level reported in the present study. Our results indicated higher genetic variability in the Morinda genotypes collected from Gujarat, Rajasthan, Tamil Nadu and Kerala. So these regions must be explored thoroughly to collect the maximum diversity available in these regions for conservation, characterization and utilization.
Genetic relatedness analysis based on Jaccard’s similarity coefficients values revealed maximum similarity between the two genotypes of M. citrifolia from Thalikkulam and Valappadu, Thrissur, Kerala and maximum dissimilarity was observed between genotypes from Tamil Nadu (Morinda tomentosa) and Rajasthan (Morinda citrifolia).
Coming to species level, the two species M. tomentosa and M. citrifolia were grouped separately based on UPGMA as well as Structure analysis and clearly distinguished based on ISSR markers thereby confirming the utility of ISSR markers in phylogenetic analysis of Morinda genus. A recent study by Kumar et al. 2016 found ISSR markers better than psbA-trnH sequences in phylogenetic studies of Ocimum L. genus and thereby supports our study confirming the role of ISSR markers in species differentiation. Further, M. tomentosa showed higher polymorphism and genetic variation as compared to M. citrifolia. So the advantage of higher diversity in M. tomentosa must be advanced for its utilization in nutraceutical, nutritional and other purposes.
Within respective species viz. Morinda tomentosa the genotypes from Gujarat, Rajasthan and Dharampuri (Tamil Nadu) were clustered as distinct ecotypes, with Dharampuri genotypes as the most diverse among the three regions. Similarly, within Morinda citrifolia most of the genotypes from Kerayamparambu, Thrissur were separately grouped, emphasizing the utility of ISSR markers in genetic relatedness, species differentiation and geographic patterning studies. So in order to conserve the overall diversity of both the species, genotypes from all the regions should be collected. In-situ farm conservation should be promoted in the respective regions, including a substantial number of genotypes from that region.