Seasonal changes in oocyte maturational competence, ovulatory competence and gonadotropin sensitivity of ovarian follicles in hybrid sturgeon, bester (Huso huso × Acipenser ruthenus )

Abstract

Oocyte maturation and ovulation in sturgeon species are induced by injection of hormones such as the luteinizing hormone-releasing hormone (LHRH). Appropriate timing for the LHRH injection is determined by monitoring the maturational stage of the ovarian follicles. In the present study, ovarian follicles from a group of female bester sturgeon (Huso huso × Acipenser ruthenus) were sampled continuously from September to May and incubated in 100% L-15 medium (pH 8.2) containing 17α-hydroxyprogesterone or salmon pituitary extract (SPE) for 48 h at 15°C. After incubation, ratios of oocyte maturational competence (OMC; ability of oocytes to undergo final oocyte maturation in response to maturation-inducing steroid, MIS), ovulatory competence (OC; ability of ovarian follicles to ovulate under MIS stimulation), and GTH sensitivity (ability of ovarian follicles to respond to GTH) were examined. Almost all oocytes maintained OMC throughout the sampling period. However, OC was acquired in circa December and May. The ratio of follicles that acquired ovulatory competence showed wide variation among individuals. Most females first acquired a higher degree of ovulatory competence in May, although some only did so around December. SPE-induced oocyte maturation was relatively high throughout the entire experiment, and the ovulation rate showed a tendency similar to that of the ovulatory competence. LHRH-a injections in May successfully induced ovulation in all females. Higher hatching rates resulted in females whose ovarian follicles showed an increasing trend of ovulatory competence and GTH sensitivity for ovulation in May. These results indicate that ovulatory competence and GTH sensitivity can be used as indicators to determine the timing of LHRH injections for sturgeon.