Introgression of reniform nematode resistance and other germplasm from G. longicalyx and G. armourianum into G. hirsutum

The Gossypium genus includes 45 diploid and 5 tetraploid cotton species of which only 2 diploids and 2 tetraploids are cultivated in different parts of the tropics and sub-tropics, leaving the remaining diploid and tetraploid species as potential genetic sources for novel trait introgression. The reniform nematode (Rotylenchulus reniformis Linford and Oliveira) poses significant problems to US cultivated Upland cottons (Gossypium hirsutum L., 2n=52), all of which lack high resistance. This work was in collaboration with the USDA-ARS team that focussed on introgressing reniform nematode-resistance from a diploid cotton species, Gossypium longicalyx Hutch. & Lee into G. hirsutum by creating a tri-species hybrid, HLA and backcross breeding (Bell et al., 2007; Robinson et al., 2007). The main objectives of this work were [1] to cytogenetically evaluate and help select superior types at each introgressed generation; [2] to identify molecular markers tightly linked to the reniform nematode-resistance gene and to map the resistance loci; [3] (A) to introgress germplasm on a genome-wide basis, (B) to evaluate introgressed germplasm for traits of economic importance other than reniform nematode resistance, and (C) to evaluate breeding methodologies in terms of this specialized breeding material. Reniform nematode resistant plants were of diverse cytogenetic constitution but individuals that modally formed 26II chromosomal configuration were identified at BC2F1, BC3F1, BC4F1, and BC5F1 generations. Three SSR markers, BNL3279_114, BNL1066_156, and BNL836_215 and one phenotypic marker, green-colored fuzz (Fzglon), were identified to be tightly-linked to the resistance locus. Extension of the association analysis and linkage estimation to 16 susceptible self progeny (BC1S1, BC3S1 and BC6S1) and 374 susceptible backcross hybrids (BC2F1-BC8F1) mapped the resistance locus to chromosome 11 of cotton with BNL3279_114 on one side and Fzglon on the other at 0.8 cM and 2.8 cM, respectively. Besides reniform nematode-resistance introgression, genome-wide introgression efforts were also conducted. Low micronaire and high fiber strength were the two most promising traits identified in the HLA-derived introgressed generations. Most of the introgressed generations had high variability for the fiber-quality traits than the commercial checks, thus providing more opportunities for selection and improvement