The ribosomal RNA genes of Locusta migratoria: copy number and evidence for underreplication in a polyploid tissue

Abstract

From libraries of Locusta migratoria genomic DNA in bacteriophage λ, clones have been isolated that hybridize with one or both of the 18S and 28S components of locust rRNA. Six clones studied show common patterns of restriction sites in the coding sequences and some include an intron in the 28S region. Subcloned sequences from within the 18S and 28S coding regions were used as probes in dot hybridization assays of the rRN A gene copy number in DNA prepared from several locust tissues and stages. The 18S and 28S probes gave similar results, showing about 4000 copies/haploid genome in testis, embryo, and fat body from larvae and immature adults of both sexes. This is by far the greatest rRNA gene copy number yet reported for any insect. DNA from reproductively mature adult female fat body showed a consistent reduction in the copy number by about one-third. Mature adult male fat body DNA showed no significant reduction in the copy number. It therefore appears that the final round of DNA replication in the adult female fat body, which produces 8-ploid and 16-ploid cells active in vitellogenin synthesis, involves underreplication of rDNA.