Stimulation of spermatogonial DNA synthesis in slug gonad by a factor released from cerebral ganglia under the influence of long days.

Incorporation of [3H]thymidine into gonadal DNA was shown to increase 1 week after implantation into an immature slug (Limax maximus) of a "brain" (circumesophageal ring of ganglia) from a male-phase donor. Light microscope autoradiography revealed that in stimulated gonads labeling was localized primarily in the nuclei of spermatagonia. Implant-stimulated spermatogonial DNA synthesis was found to depend upon implantation of supraesophageal (cerebral) ganglia. Neither subesophageal ganglia implants nor immature supraesophageal implants had an effect. Thymidine incorporation could also be stimulated by exposure of slugs to long-day lightcycles (LD 16:8) for 3 to 4 weeks. Similar duration of long-day treatment was also adequate to trigger male-phase development even after animals were returned to short days (LD 8:16). The results are consistent with the view that 3 to 4 weeks of long-day lightcycles are required to promote irreversibly the release from slug cerebral ganglia of a male-phase gonadotropic factor which directly or indirectly promotes spermatogonial proliferation.