MOLECULAR MAPPING REVEALS BOTH CLASSICAL GENES, AND UNEXPECTED ADDITIONAL GENES, ASSOCIATED WITH DISEASE RESISTANCE AND PLANT PUBESCENCE IN COTTON

¹ Terra International, Inc., 117 Kennedy Flat Rd., Leland, MS 38756 USA

² Department of Soil and Crop Science, Texas A&M University, College Station, TX 77843-2474 USA

A detailed RFLP map was used to identify and map cotton (Gossypium spp.) genes/QTLs for two traits. Genetic mapping generally corroborated classic predictions regarding the number and dosage effects of genes conferring resistance to the bacterial blight pathogen, Xanthomonas campestris pv. malvacearum (Xcm). One noteworthy exception (b₆) appeared to uncover additional QTLs from both resistant and ostensibly susceptible genotypes, some of which corresponded in location to resistance genes effective against other Xcm races. One putatively defeated resistance allele (B₃) reduced severity of Xcm damage by virulent races. Among the six resistance gene derived from tetraploid cottons, five (83%) mapped to D-subgenome chromosomes. A significant D-subgenome bias of Xcm resistance genes may suggest that polyploid formation has offered novel avenues for phenotypic response to selection. Four QTLs affecting density of leaf trichomes (pubescence) were mapped. One QTL on chromosome 6 that imparts dense leaf pubescence is inferred to be the t₁ locus. A second QTL on chromosome 25, which is homoeologous to chromosome 6, fits the description of the t₂ locus. Two additional QTLs QLP₁ and QLP₂, explained significant phenotypic variation in leaf pubescence these may represent the t₃, t₄, or t₅ loci. Some QTLs appeared to be specific to particular developmental stages; for example, QLP₁ reduced hairiness only in young leaves while QLP₂ increased hairiness in mature leaves. A single locus associated with variation in trichome density on the stem did not correspond to the genes/QTLs affecting leaf trichomes, suggesting that different genes may largely control these traits.