QTL Analysis of Resistance to Reniform Nematode in Gossypium barbadense L. Accession GB713.

The reniform nematode (RN), Rotylenchulus reniformis Linford & Oliveria, is a major pathogen of Upland cotton (Gossypium hirsutum L.).  The identification of molecular markers closely linked to RN resistance gene(s) in the G. barbadense L. accession GB713 would be very useful in cotton breeding programs.  In the present study, our objectives were to:  (i) determine the mode of inheritance of RN resistance in GB713, (ii) identify simple sequence repeat (SSR) markers linked with GB713 RN resistance QTLs, and (iii) map linked SSR markers to specific chromosomes.  We grew and scored plants for RN reproduction in the P_1, P₂, F₁, F₂, BC₁P₁, and BC₁P₂ generations from the cross of GB713 × Acala Nem-X.  All populations were grown under controlled-environment chamber conditions, inoculated with RN, and scored for RN reproduction which was expressed as the percentage of the reproduction on Acala Nem-X, nematodes per gram soil (RN g^-1soil), and log₁₀(X+1) nematodes per gram soil (log RN g^-1soil).  The generation mean analysis using the six generations indicated one or more genes were involved in the RN resistance of GB713.  The interspecific F₂ population of 300 plants was genotyped with SSR markers that covered most of the chromosomes of cotton.  Results showed two QTLs on chromosome 21 and one QTL on chromosome 18.  One QTL on chromosome 21 was at map position 168.6 (LOD 28.0) flanked by SSR markers BNL 1551_162 at position 154.2 and GH 132_199 at position 177.3.  A second QTL on chromosome 21 was at map position182.7 (LOD 24.6) flanked by SSR markers BNL 4022_199 at position 180.6 and BNL 3279_106 at position 184.5.  Our chromosome 21 map had 61 SSR markers covering 219 cM.  One QTL with relatively small genetic effects was localized to chromosome 18 at map position 39.6 (LOD 4.0) flanked by SSR markers BNL 1721_178 at position 27.6 and BNL 569_131 at position 42.9.  The two QTLs on chromosome 21 had significant additive and dominance effects which were about equal for each QTL.  The QTL on chromosome 18 showed much larger additive than dominance effects.  The SSR alleles identified in this study should be useful for selecting plants with high levels of RN resistance in segregating populations.  Following the precedent set by the naming of the G. longicalyx and  G. aridum sources of resistance, we suggest the use of Ren^barb1, Ren^barb2, and Ren^barb3 to designate these QTLs on chromosome 21 at positions 168.6 and 182.7, and chromosome 18 at position 39.6, respectively.