and quantity. Two BC7 lines, LONREN-1 and -2, were released by USDA in April of 2007 and additional lines are being developed and evaluated through cooperative research agreements between USDA and Bayer CropScience, or Delta and Pine Land Company, or Mississippi State University.
Other research in this project has focused on localization and mapping of the responsible gene(s), and identification of markers sufficiently close for marker-assisted selection (Dighe et al., 2007). Marker discovery initially emphasized representation of all A-subgenome linkage groups, a wide separation of loci, and more than 1,000 phenotyped plants spanning 7 backcross and 3 self generations. Three panels were used for marker screening and evaluation, i.e., polymorphism, trait-association and linkage-detection. SSRs identified as polymorphic were screened against the trait-association panel of DNA samples from 12 highly resistant and 12 highly susceptible plants. The results associated the resistance with BNL1066 and linkage group (LG) A03 (chromosome-11), which led to testing of 14 additional markers from public maps of A03 and its homeologue, D02. Association analysis and linkage estimation was extended to 88 classified selfed progeny (BC1-BC6) and 984 classified backcross hybrids (BC2-BC8) for BNL3279_114, BNL1066_156, BNL836_215, and green fuzz locus (Fzlon) respectively. The results indicated that BNL3279_114, BNL1066_156, BNL836_215 are mapped on one side within 1.4, 2.0, 4.4 cM, respectively, while, Fzlon is on the opposite side of the resistance locus with a linkage estimate of 4.5 cM. Release of the germplasm and marker information should facilitate incorporation of the trait into new cotton cultivars.
In a new collaborative project, A. A. Bell with USDA at College Station has made crosses between breeding lines carrying resistance from G. longicalyx and six elite Mississippi State University breeding lines, including PST006, PST246, Miscot 8824, Miscot 0141-15ne, Miscot 0110-1ne, and Miscot 0023-11ne. Miscot 8824 is considered root-knot nematode resistant. P. Thaxton and T. Wallace at Mississippi State University will plant F1 seed from these crosses in the greenhouse in 2007 to generate F2 seed, and in 2008 six F2 populations will be grown in 6-8 progeny rows at Stoneville, MS. B. Scheffler with USDA at Stoneville will utilize molecular markers to select resistant progeny. Plants also will be selected for the nectariless trait. Homozygous positive lines will be tested as F2:F3 progeny rows at Stoneville, MS in 2009 to confirm purity. Non-segregating progeny rows uniform in appearance will be harvested in bulk (F2:F4) to comprise a new reniform-resistant breeding line available for further testing. Heterozygous plants will be individually selfed in 2008, and plants within progeny rows will again be screened with codominant markers to determine homogeneity in 2009.
MECHANISMS, DURABILITY, AND GENETIC ENGINEERING
P. Agudelo at Clemson University has conducted several projects examining variability among reniform nematode populations, and the morphological aspects of the histopathological response characterizing resistance to the nematode in cotton (Agudelo et al., 2005a, 2005b, 2005c). Agudelo summarizes current and future research directions on the reniform nematode as: 1) characterization of virulence groups in R. reniformis; 2) histological characterization of resistance to R. reniformis on cotton genotypes, and characterization of post-penetration biochemical responses; and 3) assessment of host- induced selection on geographic isolates of the nematode.
R. Kantety and colleagues at the Center for Molecular Biology at Alabama A&M University are studying the morphometric as well as molecular variation of reniform nematode collections from several regions in Alabama. Their results indicate considerable variation