Linkage disequilibrium (LD)-based marker-trait association (MTA) was used to identify markers for sucrose and yield contributing traits in a panel of 108 sugarcane genotypes from sub-tropical India. Population structure (Q), kinship (K), and MTA study exploited a set of 989 marker loci generated from 123 genomic-and expressed sequence tag-SSR primers. The mixed linear model (MLM) coupled with a modified algorithm for population structure (Q) analysis was able to control both type I and type II errors and provided a deeper understanding of the genetics, population stratification and its manifestations on LD in the sugarcane genome. Significant associations were identified for four markers with cane diameter, seven markers each with cane length and number of millable canes (NMCs), eleven markers with number of nodes, six with sucrose per cent, and five markers with average cane weight. A total of 15 markers stable for all the 3 years of study explained 57 % trait variation for NMCs, 34 % for cane width, 27 % for cane length, 20 % for sucrose content, and 19 % for number of nodes. The frequent deviation of structure-based profiles from pedigree-based grouping in this complex heterozygous system reinforced the importance of using genotypic data for selection and breeding. The results contribute to a deeper insight of the complex genome and the identified MTAs could be exploited to fine-tune marker-assisted breeding programmes in genetically complex sugarcane crop.

Red rot is a serious disease of sugarcane caused by the fungus Colletotrichum falcatum that has a colossal damage potential. The fungus, prevalent mainly in the Indian sub-continent, keeps on producing new pathogenic strains leading to breakdown of resistance in newly released varieties and hence the deployment of linked markers for marker-assisted selection for resistance to this disease can fine tune the breeding programme. This study based on a panel of 119 sugarcane genotypes fingerprinted for 944 SSR alleles was undertaken with an aim to identify marker-trait associations (MTAs) for resistance to red rot. Mixed linear model containing population structure and kinship as co-factor detected four MTAs that were able to explain 10-16 % of the trait variation, individually. Among the four MTAs, EST sequences diagnostic of three could be BLAST searched to the sorghum genome with significant sequence homology. Several genes encoding important plant defence related proteins, viz., cytochrome P450, Glycerol-3-phosphate transporter-1, MAP Kinase-4, Serine/threonine-protein kinase, Ring finger domain protein and others were localized to the vicinity of these MTAs. These positional candidate genes are worth of further investigation and possibly these could contribute directly to red rot resistance, and may find a potential application in marker-assisted sugarcane breeding.

An understanding of the level of genetic diversity is a prerequisite for designing efficient breeding programs. Fifty-one cultivars of four cotton species (Gossypium hirsutum, G. barbadense, G. herbaceum and G. arboreum) representing core collections at four major cotton research stations with a wide range of eco-geographical regions in India were examined for the level of genetic diversity, distinct subpopulations and the level of linkage disequilibrium (LD) using 1100 amplified fragment length polymorphism (AFLP) markers with 16 primer pairs combinations. The AFLP markers enabled a reliable assessment of inter- and intra-specific genetic variability with a heterogeneous genetic structure. Higher genetic diversity was noticed in G. herbaceum, followed by G. arboreum. The genetic diversity in tetraploid cotton species was found to be less than that in the diploid species. The genotypes VAGAD, RAHS14, IPS187, 221 557, Jayhellar of G. herbaceum and 551, DLSA17, 221 566 of G. arboreum were identified as the most diverse parents, useful for quantitative trait loci (QTL) analysis in diploid cotton. Similarly, LRA 5166, AS3 and MCU5 of G. hirsutum and B1, B3, Suvin of G. barbadense were most diverse to develop mapping populations for fibre quality. The internal transcribed spacer sequences were sufficient to resolve different species and subspecies of diploid cotton. Low level of genome-wide LD was detected in the entire collection (r2 = 0.07) as well as within the four species (r2 = 0.11–0.15). A strong agreement was noticed between the clusters constructed on the basis of morphological and genotyping data.

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Ninety two sugarcane varieties from sub-tropical India were subjected to molecular profiling with 174 simple sequence repeat markers and characterized for 23 qualitative (morphological descriptors) and nine quantitative traits that directly or indirectly contribute to yield and juice quality. Using STRUCTURE-based population stratification study and a mixed linear model for marker-trait association (MTA) analysis, a total of 60 MTAs were identified for 22 qualitative traits that were able to explain a significantly higher (up to 40%) proportion of the phenotypic variations compared to all the previous reports of MTA studies in sugarcane. In addition, 21 MTAs stable over the three years of study were also identified for nine quantitative traits that explained 16-37% of the total trait variation. It could be concluded that the qualitative traits that are governed mostly by one or a few genes are more responsive to MTA studies and hence have a better potential to be adopted in marker-assisted breeding programmes in sugarcane. The MTAs identified in this study could also find significant applications in upcoming more stringent IP regime, which may necessitate tracking of specific alleles integrated in breeding programmes.