The brain of gastropod mollusks contains giant neurons whose nuclei are enlarged with a large amount of genomic DNA. Such DNA is produced by repeated endoreplication. We have previously demonstrated that the frequency of the neuronal DNA endoreplication is correlative to the body growth of the adult land slug and to the increase in the amount of transcripts within the neuron. However, it has long been controversial whether the neuronal DNA endoreplication entails whole genome amplification (polyploidy), or whether only the necessary genomic loci are amplified (polyteny, polysomy, or cis-amplification by unequal recombination). In the present study, we adopted two modern techniques - quantitative genomic PCR and 5'-bromodeoxyuridine labeling - to distinguish between these two possibilities. Our results demonstrated that multiple genomic loci were amplified to the same extent irrespective of the transcriptional activities at these loci. Moreover, the visceral giant cell, the biggest neuron in the slug's brain, was estimated to contain approximately 10 000-times as much genomic DNA as the haploid amount. The 5'-bromodeoxyuridine-labeling experiments also revealed a uniform DNA synthesis within the nucleus. These results strongly support the idea that the giant neurons contain a polyploid genome rather than a locus-specific amplified genome. Polyploid neurons in gastropods It has been unclear whether the whole genome is amplified (polyploidization) or only some parts of genome is amplified during developmental enlargement of neurons in gastropod mollusks. Approaches with modern techniques such as quantitative genomic PCR and BrdU labeling provided the data supporting the polyploidization in the giant neurons of the slug. The present study settled long controversial issue in neurobiology.
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