Evidence for differential DNA endoreplication during the development of a molluscan brain

The DNA content of neurons in the cerebral ganglion of Achatina fulica was determined by the two-wavelength method of microspectrophotometry using Feulgen-stained sections. DNA measurements of mouse hepatocytes were used as a control of the methods. All sampled neurons with a nuclear diameter greater than 7 microns were polyploid. The shape of the frequency histogram of DNA contents is not compatible with an interpretation that assumes one or more simple duplications of the genome. It is suggested, instead, that the results are due to either the underreplication of some DNA sequences or the selective amplification of genes. Additional experiments using [3H]-thymidine autoradiography showed that the incidence of neuronal DNA synthesis is highest during the period of the animal's greatest growth and then declines rapidly near the onset of sexual maturity. The mesocerebrum incorporated greater amounts of [3H]-thymidine than the rest of the brain, whereas the procerebrum remained diploid throughout the course of the study.