[Transcription of ribosomal genes in nuclei of different ploidy]

It has been shown previously that the production of "mature" (27S and 18S) ribosomal RNA's (rRNA) at late stages of embryonic development of the loach (Misgurnus fossilis) does not depend on the degree of ploidy, the haploid (1n) embryos making, per cell, as much rRNA as do the diploid (2n) ones. In order to investigate the mechanims of the compensation of rRNA production in 1n embryos, incorporation of labelled nucleosides in different RNA fractions was compared in two genetical variants. The kinetic and sedimentation analysis showed that in 1n cells a compensatory increase in the production of 36S precursor of rRNA (pre-rRNA) takes place. Chase experiments showed that the rate of the processing and decay of the pre-rRNA was similar in the two genetic forms. Saturation hybridization of labelled, purified 27S rRNA with DNA isolated from the nuclei of 1n and 2n embryos failed to reveal any significant differential increase (amplification) in the proportion of ribosomal cistrons (rDNA) in the haploid genome. It is concluded that the compensation of the production of rRNA in the haploid embryos is accounted for by a greater rate of transcription of the totality of the ribosomal cistrons per single chromosome set.