Estimating genetic diversity among selected cotton genotypes and the identificationof DNA markers associated with resistance to cotton leaf curl disease

To the extent of our knowledge, applications of DNA markers in marker-assisted breeding of cotton are handicapped due to low genetic diversity in cotton germplasm. Cotton leaf curl disease, a disease of viral origin, has substantially depressed cotton production in Pakistan, and this disease is also an emerging threat to the neighboring cotton-growing countries like China and India. The present study was designed to identify DNA markers, predominately simple sequence repeats (SSRs), associated with tolerance and/or resistance to the disease. Based upon 2 years of disease-screening field experiments, a total of 10 cotton genotypes (five highly tolerant, four highly susceptible, and one immune) of diverse origin were selected from the available cotton germplasm (~1200 accessions) of the National Institute for Biotechnology and Genetic Engineering, Faisalabad, Pakistan. In total, 322 SSRs derived from bacterial artificial chromosome end sequences of Gossypium raimondii (one of the progenitor species of cultivated tetraploid cotton) were screened. Out of these, 65 primer pairs were found polymorphic, and the extent of genetic similarity was in the range of 81.7% to 98.7%. A similarity matrix was used for studying their phylogenetic relationship using unweighted pair-group method with arithmetic means (UPGMA) analysis. The dendrogram showed the grouping of the genotypes into two distinct clusters comprising tolerant and susceptible genotypes, respectively. Out of the polymorphic markers, two SSR markers, PR-91 and CM-43, that were amplified only in tolerant genotypes showed significant association with resistance to the disease. These preliminary results set the stage for initiating in- depth marker-trait association studies, which will be instrumental for initiating marker-assisted breeding in cotton.