1. Polymorphism and marker efficiency
The current study evaluated the genetic diversity of 93 rice landraces. Here, the 12 SSR markers screened to assess the genetic variability and relatedness were produced reproducible and polymorphic bands. A total of 77 alleles were detected among rice genotypes with an average of 6.416 alleles per locus. The number of alleles per locus ranged from 4 (RM232 & RM248) to 9 (RM 206 & RM242), which indicates the richness of the population (Table 1A). The PIC value for SSR markers was ranged from 0.459 (RM232) – 0.809 (RM 242) with an average of 0.658. Markers RM242, RM26, RM228, and RM206 were the most informative primers on the basis of highest PIC of 0.809, 0.764, 0.763, and 0.688 respectively. Among the 12 SSR markers used in this study, eleven markers had PIC value exceeding 0.5 (Table 1A).
From the entire microsatellite loci analyzed in the study, the mean value of Fixation index (F), Shannon’s information index (I), number of effective alleles (Ne), number of different alleles (Na) were 0.98, 1.23, 3.02, 5.5 respectively. The heterozygosity value ranged from 0.41 (RM 349) to 0.8 (RM 242) (Table 1B). The genotypes showed Shannon information index ranged from 0.8 to 1.7 and the highest ‘I’ value was exhibited by RM242 which was having the maximum PIC value also. From the SSR markers evaluated in this study, RM242, RM206, RM48 produced maximum Na and Ne.
2. Genetic relatedness using cluster analysis and Principal co ordinate analysis
Cluster analysis based on microsatellite allelic diversity clearly demarcated rice landraces into four groups based on genetic distance and dissimilarity matrix and from which ‘Jaiva’ and’ Mavilan’ showed the highest genetic differences by indicating genetic dissimilarity coefficient greater than 0.5. Cluster I, which is the largest cluster, comprised of Black jasmine, Veluthan, Choverian, Anamodan, Chenkayama, Karakayama etc, Cluster II encompassed Rajakayama, Allikkannan, Assam black, Palliyaral, Okkapuncha etc. The third cluster included Rakthasali, Kunjinellu, Chettuchomala, Rasagatham, Chennellu etc. whereas cluster IV consist of Urunikayama, Karuthanjavara, Manja njavara (Fig.1A).
PCoA with SSR markers showed that large diversity existed in the rice genotypes. The first three principal coordinates explained 26.56%, 14.79%, 12.17% individually and overall, rice genotypes possess 54% of cumulative variation in their genotypes (Fig. 1B). The genotypes viz. Chennellu, Chenkayama, Mavilan, Palliyaral,Kalajeera, Kooran, White Basmati, etc. were found placing far away from the centroid of the cluster. Whereas the genotypes viz. Poonaran, Kavunni, Rasagatham, Kannichennellu, Karutha allikkannan, etc. were placed more or less around the centroid.
3. Haplotype block construction
In our recent study, Genotyping by sequencing of germplasm used for the current study yielded an average of 20 million reads per sample with an average depth on the reference genome (without Ns) in 5.37X to 13.01X range [16]. Haploblocks were constructed by using SNPs obtained from GBS of 93 rice landraces. A total of 270 haplotype blocks and 775 haplotypes were identified from all the chromosomes (Table 2). Out of 79,953 filtered SNPs, 1402 tag SNPs were grouped into haplotype blocks. The maximum number of haplotype blocks was determined by combinations of SNPs located on chromosome 8 (n=31). A lesser number of blocks were constructed by SNPs located on chromosome 10 (n=16). From the total blocks formed by the chromosomes, 775 haplotypes were obtained, while each block had three haplotype variants on average. The number of SNPs in each haplotype block ranged from two to 28. The largest haplotypes had a length of 499 kb obtained from chromosome 12.
4. Analysis of haplotype linked genes
Several genes determining various traits have been reported in rice. But the haplotypes for developing supreme variety from the landraces collected from Kerala remain intangible. To elucidate the relationship between haplotypes and traits, selected SNPs related to different genes and their corresponding haplotypes were described in (Fig.2). The functions of each gene were explored from the Rice Annotation Project Database and FunRiceGenes database.
4.1 Haplotypes of abiotic stress tolerance genes
Plant, being sessile organisms cannot escape from the extremes in environmental factors which can results in stressful conditions. These inauspicious effects can inhibit plant growth and development, and limit crop productivity up to 70%. Abiotic stress tolerance is a multigenic trait and we identified the genes, OsRZFP34, OsNADP-ME2, OsDHSRP1, OsSIRH2-14, OsGRAS23, CK2alpha2, OsSWI3C/OsCHB705, OsAIR2 and corresponding haplotypes responsible for tolerance against different abiotic stresses (Table 3A, Fig.3A).
4.2 Haplotypes of biotic stress tolerance genes
Biotic stresses include pathogens, insects, pests and weeds, leads severe yield losses or crop failure during infestation. During the past few decades, an ample variety of genes concerned in rice defense response have been recognized. But most of these cloned genes effective against one or a few strains of the pathogen and is effective only for few years. Therefore, diverse rice landrace is an ideal option to broaden the rice gene pool. A few genes, OsPSKR1, OsLOX10, Pita/Pi-4a and their haplotypes involved in biotic stress tolerance in rice landraces are depicted here in Table 3B Fig.3B.
4.3 Haplotypes of genes related to rice growth and development
Plant growth and development comprise a continuous process which is highly compatible to the change in environmental conditions. By regulating cell proliferation and expansion, several phytohormones control plant growth and these phytohormones regulate meristem function and organogenesis which entails controlling cell division. Some of the genes involved in plant growth and development, RAD51C, HAZ1/HOX1a, TIG1, WEG1, OspTAC2, OsABA2, OsCCC2 and their haplotypes are listed in Table 3C, Fig.3C.
4.4 Haplotypes of yield enhancing genes
The ideal way to increase yield within the existing agricultural land is genetic improvement of yield related traits in rice. The major traits like grain length, grain width, number of well filled grains per panicle, panicle number per plant, weight of 1000 grains etc. are directly associated with rice grain productivity. Some of the genes, OsSTAP1/OsLG3, OsMFT1, OsABCG3 and their haplotypes that strongly depend on the yield potential of rice landraces are illustrated in Table 3D, Fig.3D.