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Epistatic and environmental effects on foreign gene expression in cotton (Gossypium hirsutum L.) could influence the breeding, stability and, in the case of pest resistance, efficacy and durability of the foreign gene. This study was undertaken to characterize the expression and segregation of two foreign crylA genes in a range of insect-resistant cotton lines derived in three backgrounds. The crylA genes encoded insecticidal proteins from the bacterium Bacillus thuringiensis spp. kurstaki. The transformed coUon lines, MON 81 expressing the crylA(b) gene and MON 249 expressing the crylA(c) gene, were crossed to 14 cotton isolines with five different insect-resistance traits. CrylA gene expression and variation were e.xamined in terminal leaves of 2293 F2 progeny and subsequently in 12 F2:4 lines [crylA(b) only] in field experiments conducted at two locations in Texas. CrylA gene expression was variable and influenced by genetic and environmental factors. Site-of-gene-insertion and cotton-background effects were significan! sources of variation for the crylA gene. Significant epistatic and/or somadonal effects increased plant-to-plant variation and caused crylA gene expression to behave as a quantitative trait. Environmental effects, between and within locations and over time, decreased parent-offspring correlations of mean crylA gene expression between individuals from the F~ and F~:4 generations. Gene dosage at the crylA locus influenced insecticidal protein concentration in Fp opulations with the crylA (b) gene insert-homozygotes produced 14% more CryIA(b) protein than hemizygotes. The CryIA phenotype segregated as a simple, dominant Mendelian trait. However, non-Mendelian segregation occurred in some lines derived from MON 249. Expression of crylA genes in cotton lines was influenced by one or more of the following: site of gene insertion, gene construct, background genotype, epistasis, somaclonal mutations, and the physical environment. These results indicate that appropriate evaluation and selection procedures should be used in a breeding program to develop new cotton lines with pest-resistant traits conferred by foreign genes. Moreover, that a practical backcross breeding program could be used to develop cotton cultivars combining one or more pestresistant traits from foreign and native gene sources.

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