Ovarian function and its manipulation in the tammar wallaby, Macropus eugenii

Abstract

This study aimed to develop a superovulation protocol for the monovulatory tammar wallaby (Macropus eugenii), and examined the regulation of ovarian activity which leads to alternate ovulation in this marsupial. The most effective stimulatory treatment was 20 I.U. pregnant mare serum gonadotrophin (PMSG) given intramuscularly (i.m.) 20 days after the activation of an oestrous cycle by the removal of a sucking pouch young (RPY). Bromocriptine treatment was given at the time of RPY if the animal was in early seasonal quiescence. Mating had generally occurred when animals were examined 2 days after PMSG treatment on the morning of Day 22 RPY. Ovulation occurred only if the animal was treated on Day 22 or 23 (i.e. 2 or 3 days after PMSG) with gonadotrophin releasing hormone (GnRH) to induce a luteinizing hormone (LH) surge. Three 30-micrograms injections of GnRH (in 0.2 mL olive oil) were delivered as i.m. injections at 3-h intervals. Radioimmunoassay confirmed that the PMSG dose used did not elevate circulating steroid hormone concentrations beyond those found in normal cycles and that the GnRH protocol led to an LH surge of at least 6 h. Although multiple ovulation was achieved, the number of ovulations was low (2 or 3 per female). A major factor influencing the low ovulation rate was that generally only one ovary responded. Fertilized eggs and cleaving embryos were obtained. However, the fertility of induced ovulations has not yet been examined systematically. Laparoscopic examination through successive natural cycles confirmed that follicle growth and ovulation in the tammar wallaby alternates between the right and left ovary. Inhibition of follicle development in the corpus luteum (CL)-bearing ovary was also seen in females treated with the exogenous gonadotrophin (PMSG/GnRH) superovulation protocol. Follicle development was inhibited during the first half of the cycle in the non-CL-bearing ovary and during the entire cycle in the CL-bearing ovary. This inhibition seemed to occur at the follicular level because exogenous gonadotrophin was unable to initiate a response during periods of inhibition and the response to gonadotrophin differed in the two ovaries. The number of follicles growing in the non-CL-bearing ovary in response to an exogenous gonadotrophin stimulus was inversely related to the weight of the growing CL for the first 19 days after RPY.(ABSTRACT TRUNCATED AT 400 WORDS)