Characterization of Cell Division During Early Oogenesis in Copepod Females Emerging From Diapause

The sub-arctic copepod Neocalanus flemingeri females survive periods of low food abundance by initiating a post-embryonic diapause and relying on stored energy to fuel reproduction. This strategy allows females to increase their reproductive success by delaying egg production to synchronize their progeny with the spring phytoplankton bloom when food is again abundant. Gene expression patterns show that completion of the N. flemingeri reproductive program takes over two-months starting with early germline development after emergence from diapause. In the current project, we are examining cell division directly in order to determine where and which cells are dividing in females as diapause is terminated and oogenesis begins. In July 2019, diapausing females were collected from depth in Prince William Sound, Alaska. 5-Ethynyl-2´-deoxyuridine (EdU) was used to quantify and locate cell division within the body at nine time points post collection (0, 24, 36, 72 hours, and 1, 2, 3, 4, 4.5 weeks). EdU is incorporated into the cell during the S phase of cell division when DNA is being replicated. The movement of early germline oogonia and oocytes through the ovary was tracked with a pulse-chase experiment. Imaging of EdU-treated females using fluorescence and confocal microscopy shows that cell division is low initially but ramps up quickly in the second time point (24-48 hour incubation), with oogonia forming in the posterior end of the ovary. These results suggest a rapid termination of diapause following collection. Cell division data are being compared with other morphological changes that occur in post-diapause females during oogenesis. This study proposes to help bridge the gap between traditional histological work and newer molecular approaches, to date no papers have been published using EdU on copepods.