93 Oestrous Synchronisation Studies in a Marsupial

Abstract

There is no strategy to precisely synchronise and predict oestrus and ovulation in marsupials. This is the major limitation in applying assisted reproduction to marsupial conservation. Routine methods of female synchronization that target the ovary (effective in eutherians) are ineffective in marsupials because they do not disrupt the corpus luteum. Gondaotropin-releasing hormone (GnRH) agonists have been used to suppress female cycling in several marsupials and work by targeting GnRH action in the pituitary. After the GnRH agonist reaches its active endpoint, female cycling returns. This study investigated whether Lucrin Depot (Abbott Laboratories, Lake Bluff, IL, USA), a tailored 1-month microsphere GnRH agonist preparation, has the potential to first suppress female cycling, and then facilitate a synchronous return to female cycling in a model marsupial, the tammar wallaby (Macropus eugenii). Preliminary studies indicated that Lucrin could be used to regulate ovarian activity, but a dose >0.20 mg kg⁻¹ was required. In this study, nonpregnant tammars (n = 18, 6 per group) had their pouch young removed (RPY) on Day 0; RPY suspends lactation and reactivates cycling, allowing for controlled assessment of oestrus and mating in treated and untreated females. On Day 0, an intramuscular (IM) injection of water (Control) or 1 mg kg⁻¹ of Lucrin Depot (group A) was given after RPY. Group B also received 1 mg kg⁻¹ of Lucrin Depot but this was not injected until Day 10 after RPY (a strategy to assess whether timing of Lucrin yields different results). Females were placed in breeding pens of 1 male to 3 females. Reproductive suppression and synchrony of return to female cycling was assessed by pouch and urogenital opening checks to detect newborn young and copulatory plugs. All data (time of mating/conception) were analysed using ANOVA (significance, P < 0.05). There was a significant difference between the first mating response in the Control compared with Groups A and B (P = 0.0003), but the time from Lucrin injection to mating, Day 0 or Day 10, was not significant (P = 0.1431). All control females mated before Day 32 RPY, whereas all Lucrin-treated females mated from Day 33 to 66. All Lucrin-treated females conceived as evidenced by pouch young. Further analysis revealed 2 Lucrin-injection to mating response intervals, those females delayed by ~10 days that conceived between 33 and 50 days (n = 7: 41 ± 2.1 days; mean ± SEM), and those females delayed by ~30 days that conceived between 61 and 66 days (n = 5: 62 ± 1.0 days; mean ± SEM) following injection. Thus, an IM injection of 1 mg kg⁻¹ of Lucrin Depot is sufficient to delay oestrus in breeding tammar wallabies whether given at the time of RPY or 10 days later. This study confirmed that Lucrin Depot can be used to regulate ovarian activity in macropod marsupials, and has potential to form the basis of a GnRH agonist-based synchronisation strategy for use in assisted breeding for marsupial conservation.

This research was supported by Holsworth Wildlife Research Endowment.