Dynamics of oocyte and embryonic development during ovarian cycle of the viviparous mosquitofish Gambusia affinis

Abstract

The present study investigates the dynamics of oocyte and embryonic development between two successive parturitions (about 22-day interval) under 16 Light:8 Dark photoperiod at 25°C, in order to develop criteria for the ovarian cycle in the viviparous mosquitofish Gambusia affinis. On day 0 after parturition, the ovaries contained oocytes of various stages. Vitellogenic oocytes belonged mostly to a basal group and prominent group, each with two peaks and ranging 0.2 to 1.4 mm in diameter. The prominent group of oocytes increased in diameter by day 5, and the hepatosomatic index reached a peak on day 3, indicating active vitellogenesis. The oocyte maturation and fertilization did not occur simultaneously in an individual, but in order of oocyte size (oocytes over 1.7 mm in diameter were fertilized) between days 2 and 5. Mature oocytes were fertilized without ovulation, and then continued through embryonic development in the follicle. Many vitellogenic oocytes that did not reach fertilization degenerated and disappeared by day 10. Oil-droplet stage oocytes seem to be recruited by this time. Yolk accumulation for the next gestation started from day 10, and advanced slowly after that.