experimental control in more than 30 replicated growth chamber experiments at College Station where it consistently suppresses reniform nematode reproduction, usually by 90 to 98% compared to susceptible Deltapine 16.
As a first step in this study, a reciprocal cross was made at College Station between GB 713 and Acala NemX, followed by generation of F2 seed and seed of the BC1 with each parent. Reniform nematode resistance was then measured in the same experiment for 30 individuals of the GB 713 parent, 30 of the Acala NemX parent, 60 F1, 300 F2, and 150 BC1 with each parent. DNA was extracted from selected individual F2 plants that showed either high or low levels of reniform reproduction. These DNAs were then screened with more than 700 SSR markers using a bulked segregant analysis methodology. Preliminary results indicated three SSR markers for resistance. Nematode reproduction on F1 plants was uniform and intermediate between that on the two parents. Generation means analysis of nematode reproduction data from parents, F1s, F2s, and backcrosses onto both parents, indicated genetic control by a single partially dominant gene with additive effects. Thus inheritance indicated the trait was amenable to backcrossing into a root-knot nematode- resistant recurrent parent, by selecting for reniform but not root-knot nematode resistant backcrossed progeny at each generation. In practice, however, the level of reniform nematode resistance in the segregating progeny at each backcross has been low relative to environmental variation, throwing doubt on this direct approach. It may be necessary to self the plants after each backcross and select for reniform nematode resistance in BCnF2 progeny, where highly resistant homozygous plants are expected. So far, fourth and fifth backcross generation plants with nematode reproduction levels 3 to 19% of that on the susceptible control have been produced by making selections only from among F1's at each backcross. However, the mean level of nematode reproduction on putatively heterozygous resistant plants is so high that it is impossible to conclude a 1:1 segregation pattern. F2 progeny from the most advanced plants have not yet been tested against either nematode.
G. arboreum. In a project under the direction of E. Sacks with at the USDA at Stoneville, MS, introgression of resistance to the reniform nematode from G. arboreum into Upland cotton was initiated by crossing accession A2 190 (Burma C19) (Stewart and Robbins, 1995) with a 2[(AD1)D4] hexaploid bridging line named G 371. A single hybrid plant was obtained. The A2 190 × G 371 hybrid was subsequently crossed with Deltapine 16 and MD51ne to develop pseudo-backcross populations for nematode screening. In three resistance screens conducted in growth chambers using 500 ml pots, 174 backcross plants were evaluated along with 33 pots of susceptible control plants. The number of motile vermiform nematodes recovered per gram of soil confirmed that resistance in A2 190 was strong and similar to that of GB 713 (8% and 6% of the cultivar controls, respectively). Nematode reproduction in the backcross population, expressed as a percentage of the upland cultivar controls, had a bimodal distribution, suggesting the action of a dominant gene. The peak of the resistant class of the backcross population (heterozygous for resistance) was at about 15% of the cultivar controls. In addition to the A2 190 source of resistance, the hexaploid bridging line was subsequently used to obtain fertile hybrids with the resistant accessions A2 100 and A2 113. The new hybrids are currently being crossed with MD51ne to develop pseudo-backcross populations for nematode screening.
At the University of Arkansas, C. A. Avila, J. McD. Stewart, and R. T. Robbins have projects underway exploring at least four different aspects of reniform resistance introgression from G. arboreum. Initial inheritance studies evaluated plants in F1 and F2 generations from a cross between a highly resistant and a susceptible accession of G. arboreum to estimate the number of genes involved. The data indicated that a single gene confers reniform nematode resistance, because the F2 generation segregated in a ratio of 3 resistant individuals for