We report genetic maps for diploid (D) and tetraploid (AtDt) Gossypium genomes composed of sequence-tagged sites (STS) that foster structural, functional, and evolutionary genomic studies. The maps include, respectively, 2584 loci at 1.72-cM ‫006ف(‬ kb) intervals based on 2007 probes (AtDt) and 763 loci at 1.96-cM ‫005ف(‬ kb) intervals detected by 662 probes (D). Both diploid and tetraploid cottons exhibit negative crossover interference; i.e., double recombinants are unexpectedly abundant. We found no major structural changes between Dt and D chromosomes, but confirmed two reciprocal translocations between At chromosomes and several inversions. Concentrations of probes in corresponding regions of the various genomes may represent centromeres, while genome-specific concentrations may represent heterochromatin. Locus duplication patterns reveal all 13 expected homeologous chromosome sets and lend new support to the possibility that a more ancient polyploidization event may have predated the A-D divergence of 6-11 million years ago. Identification of SSRs within 312 RFLP sequences plus direct mapping of 124 SSRs and exploration for CAPS and SNPs illustrate the "portability" of these STS loci across populations and detection systems useful for marker-assisted improvement of the world's leading fiber crop. These data provide new insights into polyploid evolution and represent a foundation for assembly of a finished sequence of the cotton genome.

Tissue culture methods for improvement of cotton has lagged seriously compared to other major crops. A method for regeneration of cotton which includes a morphogenetically competent cell suspension was needed to facilitate selection of stress-resistant variants and gene manipulation. Preliminary screening of eight strains of Gossypium hirsutum L. for embryogenic potential resulted in the production of somatic embryos in all strains. Coker 312 was selected for use in the development of a model regeneration system for G. hirsutum. Calli were initiated from hypocotyl tissues of 3-day-old-seedlings. Globular embryos were present after six weeks in culture. Calli were subcultured to liquid suspension in growth regulator-free medium. After three to four weeks, suspensions were sieved to collect globular and heart stage embryos. Collected embryos developed further when plated onto semi-solid medium. To induce germination and plantlet growth, mature embryos were placed on sterile vermiculite saturated with medium. Upon development of roots and two true leaves, plantlets were potted in peat and sand, and hardened. Mature plants and progeny have been obtained with this procedure. A high percentage of infertile plants was observed among the regenerants.

Thirty eight cultivars, strains, and races ofGossypium were screened for somatic embryogenesis with the protocols developed as a model forG. hirsutum L. cv. Coker 312. Four classes of response were identified; high, moderate, low, and non-embryogenic. Four cultivars were further screened with 13 growth regulator regimes to determine if culture environment could change the classification or induce a higher level of response. The classification or level of response did not change. Screening of individual seedlings within a cultivar indicated that genotypic variation for embryogenesis existed. Highly embryogenic individuals were selected from cvs. Coker 312 and Paymaster 303 for use as germplasm sources for transfer of the embryogenic trait to other cultivars and genetic stocks. Only genetically responsive genotypes are amenable to the model developed for Coker 312.

Water needed for expansion is believed to enter plant tissue in response to a growth-induced water potential gradient that occurs as turgor is reduced during relaxation of cell walls or in response to increased solutes. Under water stress, the cotton flower petal continues to expand when all leaves on the plant are wilted and new leaf expansion has ceased in the shoot tips. This study was undertaken to determine if water for expansion entered the petal in response to a gradient or to increased solutes. Water potentials of cotton petal, leaf, bract and fruit were determined pre-dawn and midday in dryland and irrigated field plots. The mechanism by which petal expansion occurs appears not to be associated with a growth-induced water potential gradient or to increased solutes because the gradient is reversed from that needed to drive expansion. The water potential of the petal tissues was consistently higher than that of the subtending leaves and bracts both during and after anthesis, and under different water stress conditions. How this reversal in water potential gradient is established and maintained should provide insight into mechanisms involved in growth during water stress.

To reduce damage by drift-levels of the herbicide 2,4-dichlorophenoxyacetic acid, we have engineered the 2,4-D resistance trait into cotton (Gossypium hirsutum L.). The 2,4-D monooxygenase gene tfdA from Alcaligenes eutrophus plasmid pJP5 was isolated, modified and expressed in transgenic tobacco and cotton plants. Analyses of the transgenic progeny showed stable transmission of the chimeric tfdA gene and production of active 2,4-D monooxygenase. Cotton plants obtained were tolerant to 3 times the field level of 2,4-D used for wheat, corn, sorghum and pasture crops.