PATTERN OF VITELLOGENESIS AND FOLLICLE MATURATIONAL COMPETENCE DURINGTHE OVARIAN FOLLICULAR CYCLE OF FUNDULUS-HETEROCLITUS

The patterns of vitellogenesis and follicle maturational competence we re examined across the semilunar spawning cycle of Fundulus heteroclit us. Daily egg collection showed spawning cyclicity in six experimental groups, with a mean period between spawnings of 14.9 +/- 0.3 days, in dicated by the nonlinear regression sine-curve matching analysis. Each cycle cs as then dated from Day -7 to Day 7, with Day 0 as the peak e gg-collection day. Females from each group were sampled on selected da ys during two to three consecutive spawning cycles, and these days wer e 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 greater than or equal to 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 c ontinuous presence of vitellogenin (Vtg) 1 mRNA in the liver of the fe males. 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 popul ation of follicles up to 1.7 mm in diameter. A second and more selecti ve recruitment of full-grown follicles (1.3- to 1.4-mm diameter) towar d maturation was noted at Days -5, -4, which appeared associated with high plasma levels of estradiol-17 beta. However, the responsiveness o f those follicles undergoing oocyte maturation in vitro after gonadotr opin and maturation-inducing steroid (MIS), 17,20 beta-dihydroxy-4-pre gnen-3-one, stimulation dramatically declined from Days -1, 0, 1 to Da ys 4, 5, concomitantly with an increase of the population of the large st follicles (1.8- to 2.1-mm diameter) in the ovary. These findings ex tend previous observations on the process of follicular recruitment in F. heteroclitus and indicate that full-grown follicles may be recruit ed into maturation by a mechanism that modulates the oocyte sensitivit y to the MIS. (C) 1996 Academic Press, Inc.