DNA content in the nurse cell nuclei of viviparous and oviparous females of Megoura viciae

Microfluorometric evaluations of DNA content carried out in both viviparous and oviparous females of Megoura viciae have shown that the increased nuclear volume of the nurse cell nuclei is due to repeated duplications of the whole genome. DNA overreplication begins, together with the first appearance of the ommatidia, 3 days before birth both in viviparous and oviparous female embryos, but it proceeds at different rates in the different reproductive categories. In oviparous females, 11 replication cycles take place (maximum DNA content 2048C), whereas in viviparous females only a few DNA replications occur (maximum DNA content 16C). The relationship we found between the increased DNA content in nurse cell nuclei and development of ommatidia suggests that the switch mechanism determining the beginning of DNA overreplication may be directly influenced by photoperiod effects on the developing embryos.