At least four projects are actively pursuing development of reniform nematode resistant breeding lines from primitive accessions that either are G. hirsutum, or are maintained in the USDA Cotton Collection as G. hirsutum.
S. Stetina, L. D. Young, and W. R. Meredith, Jr. (USDA at Stoneville, MS) , and J. T. Johnson (currently Bayer CropScience) are working with TX 19 (PI 549146), TX 1347 and TX 1348. Crosses of three adapted G. hirsutum lines, DES 119B, DES 119H and 55-3, with nine primitive day-neutral G. hirsutum lines from the TX 19, TX 1347 and TX 1348 series are being evaluated. Sampling from the field is done by digging up five randomly selected plants in the progeny rows and counting the number of mature females on the tap and secondary roots. Data are expressed as nematodes/gram of root. The sampling and counting procedures are destructive. Only those progeny rows with low counts on at least four of the five root systems examined are selected for advancement. In 2003, 889 F2:3 progeny rows and 150 parent rows were screened in the field. Of these, 117 progenies and 53 parental lines were selected and advanced for field evaluation in 2004 (Young et al., 2004).
Subsequent field and greenhouse evaluations at Stoneville in 2005 and 2006 resulted in just 24 of the original progenies and 14 of the original parental lines remaining for evaluation in 2007. Two additional sets of crosses were made to characterize the moderate level of resistance in these lines. Crosses among selected day-neutral TX 19, TX 1347 and TX 1348 lines (diallel crossing scheme) will determine if genes from all three lines are unique. Crosses between these lines and three adapted G. hirsutum lines (MD 9, Tamcot 98-99 Ne, and FiberMax 966: North Carolina design II) will allow estimates of heritability and gene action. In anticipation of the development of molecular markers for reniform nematode resistance by other researchers, DNA is being collected from all parents and progeny for future evaluation.
D. B. Weaver, K. S. Lawrence and L. Mangineni at Auburn University are exploiting two of seven resistant accessions that they identified in an exhaustive, replicated evaluation of 1,973 G. hirsutum accessions in the USDA Cotton Collection. They are concentrating on TX 245 (PI 165358) and TX 1419, which stood out early in their screenings and thus these accessions were among the first available for breeding efforts. These accessions have been crossed with several adapted cultivars, such as Fibermax 966, Paymaster 1218, Delta Pearl, and Suregrow 747. At present they have F2:3 lines from these crosses and expect to begin evaluation of these materials soon. They also have made backcrosses to both parents, but have concentrated on backcrossing to the adapted parent. They have BC1F1 seed of these same populations, and will be growing these populations in the field the summer of 2007 to create BC1F2 populations for evaluations. They hope to shed light on heritability of the trait, and at the same time develop germplasm with reniform resistance that has better adaptability than the donor parents. A second focus of their work is to better characterize the resistance at the molecular level using gene-expression profiling. By exposing susceptible and resistant genotypes to nematode infestation, and extracting proteins from infected plants, they hope to identify proteins that are being differentially expressed in the resistant and susceptible plants.
In a collaborative project by USDA scientists at College Station, TX and Mississippi State, a half-diallel cross was made among the six most resistant primitive G. hirsutum accessions [TX 25 (PI 154035), TX 748 (PI 190112), TX 1586, TX 1828, TX 1860 (PI 530459) and TX 2469] identified by Robinson et al. (2004). No F1 plants were scored as resistant but 24 F2 plants had only 7.5 to 20% of the nematode population measured on susceptible controls. These F2 plants were backcrossed onto root-knot nematode-resistant M-315 RNR (Shepherd