Effects of IGF1 on in vitro ovarian follicle maturation in rainbow trout (Oncorhynchus mykiss)

Authors: Weber, Gregory - Greg

Submitted to: Fishes
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: 7/11/2023
Publication Date: 7/14/2023
Citation: Weber, G.M. 2023. Effects of IGF1 on in vitro ovarian follicle maturation in rainbow trout (Oncorhynchus mykiss). Fishes. 8(7), 367. https://doi.org/10.3390/fishes8070367.
DOI: https://doi.org/10.3390/fishes8070367

Interpretive Summary: After rainbow trout ovarian follicles, or eggs, in the ovary are fully grown they must go through a complex developmental process called ovarian follicle maturation to prepare them for ovulation and fertilization. Although ovarian follicle maturation is primarily regulated by a gonadotropin, other hormones including insulin-like growth factors (IGFs) have also been shown to participate in the regulation of this process. However, how IGFs regulate this process has been shown to vary among species of fish and among fish with different reproductive strategies. Knowledge of the actions of IGFs in rainbow trout on ovarian follicle maturation would be valuable for better understanding of reproduction in rainbow trout as well as the evolution of IGF actions. We therefore investigated the role of IGF1 in the ovarian follicle maturation process in rainbow trout. We discovered that IGFs do not induce ovarian follicle maturation in this species fish. This is the first report of IGFs not being able to induce key steps in ovarian follicle maturation process in any fish species, and thus the hormonal regulation of this complex process differs from that of all other fish investigated, at least in terms of IGF actions.

Technical Abstract: Insulin-like growth factors (IGF) have been shown to participate in the regulation of ovarian follicle maturation (OFM) with species specific actions in teleost fishes. In the present study, in vitro treatment of ovarian follicles with recombinant human insulin-like growth factor-1 (rhIGF1) was not able to induce germinal vesicle breakdown (GVBD), a marker for the resumption of meiosis, in oocytes of rainbow trout. Co-incubation of follicle-enclosed oocytes with rhIGF1 and 17a,20b-dihydroxy-4-pregnen-3-one (17,20bP), the maturation inducing steroid (MIS) in rainbow trout, was not able to induce GVBD in follicles from rainbow trout that were not able to respond to the MIS alone suggesting IGFs cannot induce OMC in MIS-incompetent follicles. Nevertheless, addition of rhIGF1 with the MIS increased the proportion of oocytes completing GVBD compared with MIS alone, although this potentiation was small and varied greatly among clutches of follicles from fish with eggs at different stages of germinal vesicle (GV) migration. Collectively, these observations suggest IGFs may have synergistic actions with the MIS but cannot induce resumption of meiosis directly at the oocyte and are not potent induces of OMC in rainbow trout. Rainbow trout are the first fish in which IGF has not exhibited potent activity for inducing either OMC or resumption of meiosis, or both in vitro.