Amplifications of rDNA

Low level amplification of rDNA seems also to occur in spermatogonia of Xenopm as evidenced by the presence of extra nucleoli (Pardue, 1969 Wallace et al, 1971) and demonstrated directly by inhybridization (Pardue, 1973). 

Amplification of rDNA in oocytes is not confined to amphibians and some insects. In this section it is intended to show that the amplification process seems to be phylogenetically widespread though certainly not universal. Moreover, it seems that the amplification process does not necessarily involve a dramatic increase in the number of rRNA genes as is the case in the amphibians and some insects, but might occur on a low level of gene multiplication. 

Specific amplification of rDNA has been demonstrated not only in amphibians, but also in Roccus fish, Colymbetes beetle, cricket, and other animals (Brown and Dawid, 1968 Gall, 1969). 

Some authors (Crippa et al., 1972 Tocchini-Valentini et al., 1974) have proposed that amplification of rDNA genes in amphibians occurs, not through gene amplification of the chromosomal matrix, but on the rRNA template in the cytoplasm. These authors demonstrated that amplification is inhibited by the same ri-fampycine derivatives which differentially inhibited the action of the reverse transcriptases. However, this interesting hypothesis contradicts available experimental material and, without convincing proof, cannot be accepted (Brown and Blackler, 1972). 

In many cases, these nurse cells have polyploid nuclei and synthesize large amounts of RNA s. In oocytes with metabolically inactive nuclei, the amplification of rDNA does not occur and rRNA are synthesized by trophic cells (Bier, 1965, 1967). In some beetles, for instance Dytiscus marginalis, active RNA synthesis is going on in trophic cells as well as in oocytes. Oocytes contain extrachromosomal DNA that is represented by Djiardine bodies (Bier et al., 1967). 

Time course of rDNA amplification in the Xenopus laevis oocyte. The amount of rDNA per cell is plotted against time. Note that the ordinate is a logarithmic scale. The sharp drop in rDNA at fertilization is thought to be due to dilution of the amplified rDNA copies by cell division, rather than to their destruction. (From A. P. 

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