Oocytes gene amplification

Brown, D. D., and LB. Dawid. 1968. Specific gene amplification in oocytes. Science, 160 272-280. 

There is one important observation that may provide a clue as to the mechanism of control of gene amplification. Oocytes from females heterozygous for the mutant nucleolusless contain the same number of nucleoli and nucleolar cores and the same amount of extra rDNA as wild-type oocytes (Brown and Dawid, 1968 Perkowska et al., 1968). Somatic cells, however, have lost half their rRNA genes and do not compensate for the missing rDNA complement (Wallace and Bimstiel,... 

Table III summarizes the species other than amphibians and insects for which extrachromosomal DNA in oocyte nuclei has been demonstrated. It is evident that this list is as yet too scanty to allow general conclusions on the phylogeny of the gene amplification process.

Amplification of DNA of chromosomes. During formation of oocytes parts of the DNA are "amplified" by repeated replication. This provides a way for the ovum to accumulate ribosomal RNA and various proteins in large amounts. Similarly, genes for two abundant proteins of the egg shell or chorion of insects are amplified. Bidirectional replication initiated at discrete positions yields an "onion skin" structure containing many copies of an 90-kb sequence containing the two genes. The polyploidy observed in some highly specialized cells such as the Purkinje cells... 

C. Amplification of rRNA Genes in the Oocytes of Other Animals. Ill... 

A. Amplification of Genes Coding for rRNA in the Amphibian Oocyte... 

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. 

In contrast to the high degree of rDNA amplification found in oocytes of amphibians, insects, and the teleost fish R. saxatalis (Vincent et al., 1969), a low level of amplification that amounts to a less than 10-fold increase of rDNA genes has been shown for the oocytes of S. soUdissima and U. caupo (Brown and Dawid. 1968 Dawid and Brown, 1970). 

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