Individual and combined contributions of the Ren.sup.barb1, Ren.sup.barb2, and Ren.sup.barb3 quantitative trait loci to reniform nematode

The infection of Upland cotton (Gossypium hirsutum L.) by the root parasite Rotylenchulus reniformis Linford & Oliveira, the reniform nematode, results in massive annual yield losses throughout the southeastern United States and portions of Texas. Resistance to reniform nematode was identified in the photoperiodic G. barbadense L. accession GB713. Previous research identified three quantitative trait loci (QTLs) that mediate GB713-derived reniform nematode resistance. Two of the QTLs reside next to each other on chromosome 21 (Ren.sup.barb1 and Ren.sup.barb2) while Ren.sup.barb3 was mapped to chromosome 18. In the present study, reniform nematode reproduction and fecundity was measured for eight G. hirsutum near isolines, developed through marker-assisted-selection (MAS), that represented every possible combination of Ren.sup.barb QTL genotypes. Our data clearly show that the Ren.sup.barb1-Ren.sup.barb2 QTL interval can be resolved to a single QTL locus, i.e., Ren.sup.barb2, that can be selected for in MAS using the BNL3279_106 SSR marker. Reniform nematode resistance equivalent to the GB713 parent was only obtained through combination of Ren.sup.barb2 and Ren.sup.barb3; however, Ren.sup.barb2 alone conferred an approximately 71% reduction in eggs gram.sup.-1 root compared to the susceptible check. The addition of Ren.sup.barb1 to isolines having only Ren.sup.barb2 or Ren.sup.barb3 did not improve resistance phenotypes. Adult reniform nematode females produced the same number of eggs per egg mass across all lines tested indicating that GB713-derived resistance does not manifest as a reduction in nematode fecundity. These findings streamline cotton MAS breeding programs by reducing the number of reniform nematode resistance loci that require tracking in segregating populations.