Comparative Assessment of Greenbug Resistance Loci in Sorghum.

Sorghum is an important crop in the great plains of the United States and its production poses a severe threat from greenbug. We adopted a comparative mapping approach to study the genomic regions affecting greenbug biotype I resistance in sorghum accession, PI 607900. Two mapping experiments were done to assess the genetic factors responsible for greenbug resistance. These two mapping experiments involved two different kind of mapping populations/progenies derived from two common parents BTx 623, susceptible to greenbug and PI 607900, resistant to greenbug. 102 microsatellite marker loci were used to construct linkage map in an F₂ study and 115 marker loci were anchored on the second map developed using an intercrossed F₃ progenies. Both the experiments together added 56 novel microsatellite markers to sorghum linkage map which included 30 EST SSR, 22 Nuclear SSR, and 4 gene sequence-based markers. The phenotypic analysis for greenbug resistance was done in greenhouse using RCBD design with 3 replications. The phenotyping results revealed significant differences in response to greenbug infestation among all the lines. The greenbug resistance trait was highly heritable with values ranging from 68-83 % in two experiments. The results from linkage and QTL studies in both experiments were corroborative and indicated a consistent region on sorghum chromosome 9 contributing for greenbug resistance to biotype I. This region contained several candidate genes including a gene sequence similar to Xa21-binding protein3 which is known to be involved in increased R gene-mediated resistance to diseases. The QTLs identified in second project had a reduced/narrow confidence interval compared to first project which reduced the number of genes required for candidate gene analysis. The QTLs identified in two projects were consistent and explained a high phenotypic variation for greenbug resistance from 65-81 %. PI 607900 proved to be a promising resistant accession for greenbug resistance breeding.