Gerald O. Myers

RFLP mapping of an aphid resistance gene in cowpea (Vigna unguiculata L. Walp)

SummaryRestriction fragment length polymorphism (RFLP) analysis has several advantages over traditional methods of genetic linkage mapping, one of these being the starting point for map-based cloning. The recent development of an RFLP map of cowpea (Vigna unguiculata L. Walp) has allowed the investigation of associations between genes of interest and RFLP markers. A cross between an aphid (Aphis craccivora Koch) resistant cultivated cowpea, TT84S-2246-4, and an aphid susceptible wild cowpea, NI 963, was screened for both aphid phenotype and RFLP marker segregation. One RFLP marker, bg4D9b, was found to be tightly linked to the aphid resistance gene (Rac1) and several flanking markers in the same linkage group (linkage group 1) were also identified. The close association of Rac1 and RFLP bg4D9b presents a real potential for cloning this insect resistance gene.

Status of the USA cotton germplasm collection and crop vulnerability.

The National Plant Germplasm System (NPGS) is a cooperative effort among State, Federal and Private organizations aimed at preserving one of agriculture’s greatest assets: plant genetic diversity. The NPGS serves the scientific community by collecting, storing, and distributing germplasm as well as maintaining a searchable database of trait descriptors. Serving the NPGS, a Crop Germplasm Committee (CGC) is elected for each crop and is comprised of a group of scientists concerned with development, maintenance, characterization, and utilization of germplasm collections. Each CGC serves in an advisory role and provides a status report every seven years to determine scientific efforts, adequacy of germplasm base representation, and progress in breeding through utilization of germplasm. In addition, each committee can call attention to areas of concerns regarding facilities and staffing associated with the maintenance, collection, and taxonomic activities for a specific crop within the system. The following report was developed by the CGC for cotton and provides a record of collections, activities, concerns, crop vulnerabilities, and recommendations associated with the cotton collection for the period 1997–2005. Information provided within this document is a much expanded and detailed description of a report provided to the NPGS and includes the most exhaustive citation of germplasm depositions and research activity descriptions available anywhere in the USA for this time period. This documentation will be a valuable resource to breeders, geneticists, and taxonomists with an interest in this important food and fiber crop.

Sequencing and Utilization of the Gossypium Genomes

Revealing the genetic underpinnings of cotton productivity will require understanding both the prehistoric evolution of spinnable fibers, and the results of independent domestication processes in both the Old and New Worlds. Progress toward a reference sequence for the smallest Gossypium genome is a logical stepping-stone toward revealing diversity in the remaining seven genomes (A, B, C, E, F, G, K) that permitted Gossypium species to adapt to a wide range of ecosystems in warmer arid regions of the world, and toward identifying the emergent properties that account for the superior productivity and quality of tetraploid cottons. The greatest challenge facing the cotton community is not genome sequencing per se but the conversion of sequence to knowledge.

Molecular Diversity Revealed by AFLP Markers in Upland Cotton Genotypes

Cotton is an important fiber crop grown over tropical and subtropical regions worldwide. The narrow genetic base of the cotton germplasm being used in breeding programs is one of the factors in failing to achieve appreciable amount of progress in improving fiber traits. Molecular markers have been successfully applied to estimate genetic diversity, besides their use in advancing the breeding cycle. Sixty upland genotypes from a cooperative regional testing program were screened with AFLP markers. Louisiana genotypes possessed coarse fibers, medium staple, and strong fibers, while heat tolerant genotypes showed high variability for these traits. The pairwise kinship estimates ranged between 0.1–0.88, while the PIC values ranged from 0.15–0.30, accounting for most of the shared ancestral alleles. The estimates of genetic diversity were in the range of 0.1–0.34. PCoA, and correspondence and UPGMA clustering methods showed no obvious grouping of the genotypes in line with their geographical origin. Some of genotypes, such as AU 5367, Acala 1517-99, and LA 05307025, were highly diverse from rest of the population. The DICE distance estimates among all the 60 genotypes reached a maximum of less than 0.15, indicating the relative lack of genetic diversity in this group of genotypes based on the markers used in this study.