Plant Genome I Conference
Town & Country Conference Center, San Diego, CA, November, 1992.
PG-I: 41pg1
MEIOTIC MAPPING OF rDNA LOCI IN COTTON (GOSSYPIUM HIRSUTUM L.)
BY IN SITU DNA HYBRIDIZATION
C.F. Crane 1, D.G. Czeschin 1, H.J. Price 1, D.M. Stelly 1, T.D.
McKnight 2 and R. Wing 1, 1 Dep. Soil & Crop Sciences, and 2 Dep.
Biology, Texas A&M University, College Station, TX 77843-2474
USA.
Molecular cytogenetics offers considerable analytical
potential, by uniting molecular resolution and cytogenetic
perspective. The techniques have been well developed and are
being extensively exploited in human genomic mapping. Our team
is interested in furthering analogous technical advancement and
usage for plants. In contrast to all or virtually all other
laboratories, we are emphasizing development and usage of
nonradioactive molecular cytogenetic techniques as they relate to
meiotic chromosomes, rather than somatic chromosomes. In situ
hybridization with biotinylated probes was used to detect the
sites of the 5S and 18S-28S ribosomal RNA genes in the meiotic
chromosomes of cotton. Hybridization sites were detected by
streptavidin- peroxidase, with or without a preceding
amplification of signal by avidin and biotinylated anti-avidin.
Hybridization to quadrivalents of two or more chromosomal
translocations involving the same chromosome was sufficient to
map each probed locus to a specific chromosome. Interpretive
methods were developed so that the position of signals on the
quadrivalents permitted sites to be further localized to one of
three subchromosomal domains: the region proximal to the
translocation breakpoint, the region distal to the breakpoint, or
the whole arm opposite the breakpoint. Two 5S sites were
detected, the larger one near the centromere in the long arm of
chromosome 9 and the smaller one near the centromere in the short
arm of chromosome 23. Four 18S-28S sites were mapped, near the
telomeres of the short arms of chromosomes 9, 16, 23, and 7.
Signals at the last site were much smaller than at the other
three loci, perhaps due to lower copy number or reduced homology
to the probe. All four 18S-28S sites associated with the
nucleolus in prophase microsporocytes. Synteny of 5S and 18S
loci in chromosomes 9 and 23, which respectively belong to the A
and D genomes of this disomic tetraploid species, provides the
first evidence that chromosomes 9 and 23 are homeologues. The
difference in arm location of the 5S loci in chromosomes 9 and 23
suggests that a pericentric inversion arose after the A and D
genomes diverged. The 5S and 18S-28S rDNA sites are the first
genetic loci to be reported for chromosome 23. This research was
partially supported by USDA Grants 88-37262-3920 and
90-37140-5262.
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