Here, we explain the function of reproductive proteins GnRH and ZP. These proteins are targets for contraceptive vaccines, which are being developed in New Zealand to help reduce possum numbers.

Scientists in the National Research Centre for Possum Biocontrol (NRCPB) are developing contraceptive vaccines that will make possums less fertile. This biocontrol method is being developed to reduce possum numbers because possums are a major pest in New Zealand.

Possums are marsupials – their young are reared in a pouch. In common with other mammals, proteins are important for controlling the possum’s reproductive cycle and fertility. Vaccines that make a possum produce antibodies against these proteins will block fertilisation or shut down the possum’s reproductive cycle and prevent it from producing offspring.

Proteins targeted for contraceptive vaccines

The proteins being targeted are gonadotrophin-releasing hormone (GnRH), a small hormone crucial for fertility, and zona pellucida proteins, which form a protective coat around the female egg and are required for fertilisation.

Function of GnRH

GnRH is produced in an area of the brain called the hypothalamus.

GnRH stimulates the pituitary gland to produce two hormones – FSH (follicle-stimulating hormone) and LH (luteinising hormone).

Role of FSH (follicle-stimulating hormone)

In females, FSH stimulates the growth of follicles in the ovaries from which the egg develops.

In males, FSH stimulates sperm production.

Role of LH (luteinising hormone)

In females, LH triggers the release of an egg from a follicle in the ovary (ovulation). The collapsed follicle that remains after ovulation becomes the corpus luteum. The corpus luteum produces the hormone progesterone, which prepares the lining of the uterus to receive the fertilised egg.

In males, LH stimulates the production of testosterone from the testes.

Effect of a vaccine on GnRH

The vaccine destroys the GnRH hormone, and insufficient FSH and LH are produced. Therefore, no mature eggs are produced in females and no testosterone or sperm are produced in males. By targeting this hormone, the possums become infertile, as they cannot produce eggs or sperm.

Function of zona pellucida proteins

A glycoprotein coat called the zona pellucida surrounds mammalian eggs. The possum zona pellucida consists of three specific proteins – ZP1, ZP2 and ZP3. These proteins allow sperm to attach to the egg and help the sperm’s entry into the egg. After fertilisation, the zona pellucida proteins ensure early embryonic development occurs.

Effect of a vaccine on zona pellucida proteins

The vaccine blocks the binding of sperm to the zona pellucida which prevents fertilisation.

Find out more about this in the article, Delivering biocontrol agents to possums.

    Published 18 December 2008