Time course of final oocyte maturation and ovulation in chub mackerelScomber japonicusinduced by hCG and GnRHa

Abstract

The question of whether the ovulation and spawning time in chub mackerel Scomber japonicus is entrained by a circadian rhythm was raised by our previous experiments. Further questions were also raised about whether the time course of human chorionic gonadotropin (hCG)-induced final oocyte maturation (FOM) and ovulation reflected the natural time course induced by endogeneous pituitary gonadotropin (GtH). To address these questions, hCG and gonadotropinreleasing hormone analog (GnRHa) were administered at two 'opposite' times, 14:00 and 02:00 hours, and the time courses of FOM and ovulation were compared. When hCG was injected, ovulation occurred 33 h post-injection in both groups, regardless of the timing of the hCG injection. The timing of ovulation in chub mackerel depends on the timing of hCG injection, but apparently not on circadian rhythms. When GnRHa was injected, ovulation began at 36 h post-injection of GnRHa, regardless of the timing of injection. These results indicate that the time course of FOM and ovulation in the chub mackerel followed a similar pattern whether stimulated by hCG injection or spontaneous luteinizing hormone (LH) surge because GnRHa induces the secretion of endogenous GtH (primarily LH) from the fish pituitary. Thus, it is concluded that the time course of hCG-induced FOM and ovulation in chub mackerel follows the natural time course induced by endogenous pituitary LH.

Changes in the expression of pituitary gonadotropin subunits during reproductive cycle of multiple spawning female chub mackerel Scomber japonicus

“…Diurnal expression profiles for FSHb mRNA have been proposed in two daily spawning species: bambooleaf wrasse, where FSHb mRNA increased from the end of ovulation to the next initiation of FOM (Ohta et al 2008), and gilthead seabream, where FSHb mRNA levels increased 4 and 8 h prior to spawning during GVBD and were maintained until 8 h after spawning when FOM was initiated (Gothilf et al 1997). In chub mackerel, it may not be possible to clarify whether the diurnal regulation of GtH subunit transcription plays a role since they do not necessarily undergo daily spawning cycles in the wild (Yamada et al 1998;Shiraishi et al 2008). The second possible reason for transient increase in FSHb mRNA expression is that ovulation and spawning may be a signal to accelerate vitellogenesis.…”

“…Since the exogenous GnRHa was cleared 4 days post-injection (Scott et al 1999), the GnRHa-induced multiple spawning observed after 1 week was likely controlled by the endogenous GnRH-GtH system. In captive chub mackerel, the time course from the initiation of FOM to ovulation/ spawning has been determined (Shiraishi et al 2008). Therefore, fish at specific reproductive stages can be 884 Fish Physiol Biochem (2012) 38:883-897 obtained.…”

Stimulus-specific development of learning ability during habitat shift in pre to post-recruitment stage jack mackerel

“…Fertilized T. japonicus eggs were obtained from spawning wild-origin broodstock maintained at the Fishery Research Laboratory of Kyushu University and the Maizuru Fisheries Research Station (MFRS) of Kyoto University after injection of a gonadotropin-releasing hormone analogue (GnRHa, 200 lg per individual) (Shiraishi et al 2008) and human chorionic gonadotropin (HCG, 170 IU per individual), respectively. Eggs were stocked in four transparent, 500-l, polycarbonate tanks supplied with filtered sea water at the MFRS as described elsewhere (Masuda et al 2008).…”

Spawning frequency of the Tsushima Current subpopulation of chub mackerel Scomber japonicus off Kyushu, Japan

“…We recently developed an experimental system using chub mackerel in which the final oocyte maturation (FOM) and ovulation in captivity can be controlled and induced at any time for a period of 3 months during the spawning season [7,8]. Using this system, it is possible to obtain a series of ovary samples at regular intervals from the time of ovulation.…”

“…Thus, the acquisition of age-determined POFs and accurate information outlining the time course of oocyte maturation and ovulation allows the more accurate prediction and determination of the reproductive state of individual females in wild spawning stock. Our objective was to investigate the S of the spawning stock of the Tsushima Current subpopulation based on our previous findings [7,8] for the reproductive state of chub mackerel.…”