Seasonal variation in ovarian response to pregnant mares serum gonadotrophin in the brushtail possum (Trichosurus vulpecula).

This study examined the seasonal variation in ovarian response to the exogenous hormone pregnant mares serum gonadotrophin (PMSG) in the brushtail possum (Trichosurus vulpecula). Ovarian stimulation was achieved by administration of a single intramuscular injection of 15 IU PMSG. Animals (n = 165) responded to this treatment in a variable manner depending on the month of the year. The maximum response (measured by the total number of large (>2 mm) follicles) was 15.8 +/- 1.5 follicles per animal (n = 12) in May and the minimum response was 4.9 +/- 1.6 follicles per animal (n = 7) in January. The response throughout the rest of the year closely paralleled the birth seasonal distribution observed in wild possums in published reports. In the summer months (December, January and February) when few females carried pouch young, 55%, 42% and 17% of PMSG-primed animals, respectively, exhibited a nil response to PMSG (i.e. no follicles >2 mm were found on the ovaries of treated animals). In addition, when wild possums were in lactational anoestrus after the autumnal breeding peak, possums with a nil response to PMSG accounted for 17% (July) and 6% (August) of those treated, but the other animals gave near maximal numbers of large follicles (July, 5-21; August, 2-25). Whether the ovaries of non-responders are downregulated in some manner outside the breeding season or they become insensitive to gonadotrophin remains to be investigated. These observations pinpoint times of the year when the highest productivity can be expected following PMSG treatment, and together with information on the timing of ovulation provide critical data for the development of artificial breeding strategies for the possum.