The patterns of vitellogenesis and follicle maturational competence were examined across the semilunar spawning cycle of Fundulus heteroclitus. Daily egg collection showed spawning cyclicity in six experimental groups, with a mean period between spawnings of 14.9 ± 0.3 days, indicated by the nonlinear regression sine-curve matching analysis. Each cycle was then dated from Day - 7 to Day 7, with Day 0 as the peak egg-collection day. Females from each group were sampled on selected days during two to three consecutive spawning cycles, and these days were each chronologically given a temporal relation to Day 0 to pool the data into a composite. The analysis of the size-frequency distribution of ovarian follicles ≤0.5-mm diameter across the composite revealed a constant recruitment of small follicles (0.5- to 0.7-mm diameter) into vitellogenesis, which was supported by the continuous presence of vitellogenin (Vtg) 1 mRNA in the liver of the females. The plasma levels of Vtg were also essentially constant across the cycle, except for a progressive decrease from Day -7 through Day 3 that could be related to a more active Vtg uptake by a dominant population of follicles up to 1.7 mm in diameter. A second and more selective recruitment of full-grown follicles (1.3- to 1.4- mm diameter) toward maturation was noted at Days -5, -4, which appeared associated with high plasma levels of estradiol-17β. However, the responsiveness of those follicles undergoing oocyte maturation in vitro after gonadotropin and maturation-inducing steroid (MIS), 17,20β-dihydroxy-4- pregnen-3-one, stimulation dramatically declined from Days -1, 0, 1 to Days 4, 5, concomitantly with an increase of the population of the largest follicles (1.8- to 2.1-mm diameter) in the ovary. These findings extend previous observations on the process of follicular recruitment in F. heteroclitus and indicate that full-grown follicles may be recruited into maturation by a mechanism that modulates the oocyte sensitivity to the MIS.
ASJC Scopus subject areas
- Animal Science and Zoology