Molecular Cytogenetic Mapping of a Nucleolar Organizer Region in Cotton

A biotin-labeled cloned fragment of 18S-–28S ribosomal DNA from soybean was hybridized to DNA in meiotic chromosomes of cotton Gossypium hirsutum L. by in situ hybridization. Hybridization sites were detected enzymatically by the streptavi-din-peroxidase procedure. Analysis of in situ hybridization to metaphase I meiocytes from two translocation heterozygotes and monosomics involving chromosome 9 indicated that a cluster of ribosomal RNA cistrons is on chromosome arm 9L. This is the first molecular marker to be mapped in cotton, and, except for translocation markers and arm deficiencies, it is the first marker to be mapped to chromosome 9. In situ hybridization to cytogenetic tester lines in combination with meiotic analysis is a powerful technique for assigning cloned DNA sequences to chromosomes and chromosome arms.

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