et al., 1996), LA887 or NemX, and BC1F2 seed have been collected for nematode resistance testing to try to recover the resistance level observed in the F2 plants from the original cross.
At the USDA laboratory at Mississippi State, J. N. Jenkins, O. A. Gutierrez, and J. C. McCarty have begun the process of generating recombinant inbred lines (RILs) from F2 populations from two crosses, each of which involves a root-knot nematode-resistant and a reniform nematode-resistant genotype, in order to provide tools useful to distinguish between the genes involved in root-knot and reniform nematode resistance. The root-knot nematode species to which resistance has been developed in cotton is Meloidogyne incognita (Koifoid & White) Chitwood race 3). Both crosses utilize M-315 RNR as the root-knot nematode- resistant genotype and either a day-neutral converted (DN) selection developed from TX 2468 (PI 607774) or the primitive G. barbadense accession GB 713 (PI 608139) as the reniform nematode resistance source. Plans are to evaluate RILs for both root-knot and reniform nematode resistance as well as for linked markers. This laboratory has been highly successful with this approach in the identification of markers for root-knot nematode resistance in cotton on chromosomes 11 and 14 (Ynturi et al., 2006).
In a complementary project, J. C. McCarty crossed DN TX-2468 with Deltapine 61 and evaluated day-neutral F2 plants for reniform nematode resistance in pots. Several promising plants were selected and carried to the F4 generation, where additional selections were made based on phenotype. Ten or 20 F5 plants from each selection were evaluated for nematode resistance during 2006 and 2007 and on average supported only 18% to 22% of the level of nematode reproduction measured for susceptible controls. Additional selections were made at the F5 and further crosses are planned during the 2007 growing season to obtain a better measure of plant-to-plant variability in resistance.
G. barbadense. J. L. Starr and C. W. Smith at Texas A&M University are working with progeny from a cross between root-knot nematode-resistant G. hirsutum M-315 RNR and reniform nematode-resistant G. barbadense TX 110 (PI 163608), originally reported as moderately resistant by Yik and Birchfield (1984). Numerous F1 plants tested separately against either the reniform or the root-knot nematode consistently showed high resistance to the nematode used as inoculum, indicating that all F1 plants were resistant to both nematodes. Based on failure of an F2 population to fit either a one- or a two-gene model, resistance was assumed to be a polygenetic trait. Using a pedigree breeding system, lines were advanced to the F5 generation following selection for day-neutrality in the F2 generation, and for reniform nematode resistance in the F3 and F5 generations. Unfortunately, most F5 generation lines had very low fertility due apparently to nonviable pollen. Eleven F6 generation lines with apparently normal levels of fertility were selected and screened for reniform and root-knot nematode resistance. Three lines with resistance to both nematodes were then advanced to the F7 generation. In a single test for seed cotton yield, in replicated single-row plots, all of the nematode-resistant selections had yields that were slightly better than M-315 RNR but not equal to those of three high yielding cultivars.
In another collaborative project between USDA scientists at College Station and Mississippi State, efforts are being made to backcross resistance from G. barbadense GB 713 (PI 608139) into several root-knot nematode-resistant breeding lines, including Auburn A623, M-315 RNR, Paymaster La887, and Acala NemX, as well as the obsolete but once extensively planted cultivar Deltapine 16. GB-713 was by far the most resistant G. barbadense accession identified in the extensive evaluation of G. barbadense in the USDA Cotton Collection conducted by Robinson et al. (2004) and it has been used as a resistant