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Title:   Author  item
                                            LIU, S - NEW MEXICO STATE UNIVitem
                                            CANTRELL, R - NEW MEXICO STATE UNIVitem
                                            McCarty, Jackitem
                                            STEWART, J - UNIVERSITY OF ARKANSAS
  Submitted to: Crop Science
                                    Publication Type: Peer Reviewed Journal
                                    Publication Acceptance Date: 4/7/2000
                                    Publication  Date: N/A
                                    Citation: N/AInterpretive Summary: The efficient use of cotton germplasm resources depends on reliable characterization of genetic diversity to insure that desired novel alleles are identified for introgression into elite germplasm.  Day-neutral cotton race stocks have been developed by backcross conversion from primitive germplasm accessions of Gossypium hirsutum L.  Liu et al. genotyped DNA  samples from 97 converted cotton race stocks for 63 Simple Sequence Repeat (SSR) marker loci.  High-throughput DNA markers systems were optimized for the germplasm screening.  The genetic distances computed for the race stock genetic samples from typical G. hirsutum revealed a shortage of genetic diversity at the bulk-accession level.  The majority of the converted race-stocks exhibited linkage drag from the donor parent that delayed or prevented full recovery of the recurrent parent.  Heterogeneity for DNA genotype was detected within accessions, which permitted selection  of diverse plants within certain accessions that approached the genetic diversity of the primitive recurrent parent.  Extensive genetic variation was displayed by wild photoperiodic tetraploid accessions (G. tomentosum, G. darwinii, and G. mustelinum).  For DNA marker loci, these exhibited greater than 80 percent unique alleles compared to typical G. hirsutum. This study illustrates that extensive molecular marker diversity exists in cotton germplasm collections at the accession level and at the individual plant level within certain accessions.  DNA markers, such as SSRs, are valuable tools for surveying and mining the diversity that is present in the collection.
                                Technical Abstract: Effective utilization of converted day-neutral Gossypium hirsutum L. race stocks in cotton genetic improvement programs depends on the extent of genetic variation and the accurate characterization of the variability within and among germplasm accessions in collection.  This study was conducted to survey the molecular variation in the converted race stock  collection by using simple sequence repeat (SSR) DNA markers.  Fifty-seven fluorescently-labelled SSR primer pairs arranged in multiplex bins were used to genotype 97 day-neutral BC4F4 race stock accessions.  A genetic distance matrix was constructed based on the SSR genotypes of each accession.  The majority of the accessions had genetic distances computed to be less than 0.25 from the G. hirsutum standard TM1.  The dendogram of all accessions revealed that the most diverse accessions originated from Guatemala.  There was strong evidence that the accessions were  heterozygous or heterogeneous, so ten plants were genotyped within the most diverse nine accessions, those with genetic distance from TM1 greater than 0.25.  The genetic diversity and graphical genotypes of these plants indicated there was variability within accessions for amount of the recurrent parent recovered.  In some families, the primitive photoperiodic parent was recovered and in others there was extensive linkage drag from the donor day-neutral parent.  The recovery of the primitive recurrent parent could be improved by marker-assisted backcrossing with SSR markers reported in this experiment.  Careful genotyping with SSR markers prior to introgression into breeding programs is suggested to insure maximum genetic diversity and integrity of the exotic race-stock donor germplasm.


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