olyploidy or whole-genome duplication provides genomic opportunities for evolutionary innovations in many animal groups and all flowering plants 1-5 , including most important crops such as wheat, cotton and canola or oilseed rape 6-8. The common occurrence of polyploidy may suggest its advantage and potential for selection and adaptation 2,3,9 , through rapid genetic and genomic changes as observed in newly formed Brassica napus 10 , Tragopogon miscellus 11 and polyploid wheat 12 , and/or largely epigenetic modifications as in Arabidopsis and cotton polyploids 5,13. Cotton is a powerful model for revealing genomic insights into polyploidy 3 , providing a phylogenetically defined framework of polyploidization (~1.5 million years ago (Ma)) 14 , followed by natural diversification and crop domestication 15. The evolutionary history of the polyploid cotton clade is longer than that of some other allopolyploids, such as hexaploid wheat (~8,000 years) 12 , tetraploid canola (~7,500 years) 16 and tetraploid Tragopogon (~150 years) 11. Polyploidization between an A-genome African species (Gossypium arboreum (Ga)-like) and a D-genome American species (G. raimondii (Gr)-like) in the New World created a new allotetraploid or amphidiploid (AD-genome) cotton clade (Fig. 1a) 14 , which has diversified into five polyploid lineages, G. hirsutum (Gh) (AD) 1 , G. barbadense (Gb) (AD) 2 , G. tomentosum (Gt) (AD) 3 , G. mustelinum (Gm) (AD) 4 and G. darwinii (Gd) (AD) 5. G. ekmanianum and G. stephensii are recently characterized and closely related to Gh 17. Gh and Gb were separately domesticated from perennial shrubs to become annualized Upland and Pima cottons 15. To date, global cotton production provides income for ~100 million families across ~150 countries, with an annual economic impact of ~US$500 billion worldwide 6. However, cotton supply is reduced due to aridification, climate change and pest emergence. Future improvements in cotton and sustainability will involve use of the genomic resources and gene-editing tools becoming available in many crops 9,18,19. Cotton genomes have been sequenced for the D-genome (Gr) 20 and A-genome (Ga) 21 diploids and two cultivated tetraploids 22-26. These analyses have shown structural, genetic and gene expression variation related to fiber traits and stress responses in cultivated

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.

The most widely cultivated species of cotton, Gossypium hirsutum, is a disomic tetraploid (2n=4x=52). It has been proposed previously that extant A- and D-genome species are most closely related to the diploid progenitors of the tetraploid. We used fluorescent in situ hybridization (FISH) to determine the distribution of 5S and 18S-28S rDNA loci in the A-genome species G. herbaceum and G. arboreum, the D-genome species G. raimondii and G. thurberi, and the AD tetraploid G. hirsutum. High signal-to-noise, single-label FISH was used to enumerate rDNA loci, and simultaneous, dual-label FISH was used to determine the syntenic relationships of 5S rDNA loci relative to 18S-28S rDNA loci. These techniques provided greater sensitivity than our previous methods and permitted detection of six new G. hirsutum 18S-28S rDNA loci, bringing the total number of observed loci to 11. Differences in the intensity of the hybridization signal at these loci allowed us to designate them as major, intermediate, or minor 18S-28S loci. Using genomic painting with labeled A-genome DNA, five 18S-28S loci were localized to the G. hirsutum A-subgenome and six to the D-subgenome. Four of the 11 18S-28S rDNA loci in G. hirsutum could not be accounted for in its presumed diploid progenitors, as both A-genome species had three loci and both D-genome species had four. G. hirsutum has two 5S rDNA loci, both of which are syntenic to major 18S-28S rDNA loci. All four of the diploid genomes we examined contained a single 5S locus. In g. herbaceum (A1) and G. thurberi (D1), the 5S locus is syntenic to a major 18S-28S locus, but in G. arboreum (A2) and G. raimondii (D5), the proposed D-genome progenitor of G. hirsutum, the 5S loci are syntenic to minor and intermediate 18S-28S loci, respectively. The multiplicity, variation in size and site number, and lack of additivity between the tetraploid species and its putative diploid ancestors indicate that the behavior of rDNA loci in cotton is nondogmatic, and considerably more complex and dynamic than previously envisioned. The relative variability of 18S-28S rDNA loci versus 5S rDNA loci suggests that the behavior of tandem repeats can differ widely.

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Genetically engineered crops have been grown for more than 20 years, resulting in widespread albeit variable benefits for farmers and consumers. We review current, likely, and potential genetic engineering (GE) applications for the development of disease-resistant crop cultivars. Gene editing, gene drives, and synthetic biology offer novel opportunities to control viral, bacterial, and fungal pathogens, parasitic weeds, and insect vectors of plant pathogens. We conclude that there will be no shortage of GE applications totackle disease resistance and other farmer and consumer priorities for agricultural crops. Beyond reviewing scientific prospects for genetically engineered crops, we address the social institutional forces that are commonly overlooked by biological scientists. Intellectual property regimes, technology regulatory frameworks, the balance of funding between public- and private-sector research, and advocacy by concerned civil society groups interact to define who uses which GE technologies, on which crops, and for the benefit of whom. Ensuring equitable access to the benefits of genetically engineered crops requires affirmative policies, targeted investments, and excellent science.

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