Structural organization of Acheta rDNA : Evidence for differential amplification of soma and germ-line-specific rDNA sequences

Abstract

Amplification is one of the mechanisms whereby the expression of genes can be specifically reinforced. Ribosomal gene amplification in amphibian and insect oocytes is a particularly well documented case. We studied heterogeneity, amplification and size of Acheta domesticus (insects; Orthoptera) ribosomal DNA and characteristics of male and female somatic or germ line rDNAs by analysis of genomic clones from a conventional and a microclone library. The length of the Acheta rDNA repeat unit (transcription unit and non-transcribed spacer (NTS) varied from 47 × 10 3 to 60 × 10 3 base-pairs, with highest variability within the NTS region. Deletions, fragment length heterogeneity and size variability in small steps of individual NTS segments are responsible for the observed size variation. The number of rDNA repeat units per haploid genome of Acheta was determined as 190(±10%). The rDNA is amplified 14(±10%)-fold in the oocyte, producing about 10,000 gene copies per cell. Our results show that the amplification mechanism does not favor individual fragments within the repeat unit. Thus, it can be concluded that amplification does not change the chromosomal characteristics of the rDNA pool. Two fragments specific for oocyte rDNA suggest that the rearrangements accompanying amplification are preferentially located in one distinct EcoRI fragment. Certain regions of Acheta rDNA contain cell-type-specific fragments: it was thus possible to characterize one purely male fragment and a second one specific for male and female soma but not for germ line rDNA. We show that Acheta rDNA reveals a combination of many features reported from different organisms and novel tissue-specific alterations on an extremely large repeat unit. The tissue-specific alterations indicate sexual and soma/germ line differentiation events that are derived by as yet unknown mechanisms.