Molecular mapping of genes affecting pubescence of cotton

Upland cotton (Gossypium hirsutum L.) genotypes have varying densities of trichomes on the leaves and stems of mature plants, hence their species name. Most modern cotton cultivars are 'smooth' with few if any trichomes. Absence of trichomes reduces the attractiveness of the cotton plant to some major insect pests, reducing reliance on pesticides. A detailed RFLP map was used to map genes affecting density of leaf and stem trichomes. Based on quantitative measures of young and mature leaves, four QTLs were mapped. A 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 these traits may largely be controlled by different genes. A widely used qualitative classification system for scoring trichome density (DTL) detected only the chromosome 6 locus and was apparently not sensitive enough to detect alleles such as t₂ that had smaller phenotypic effects.