Towards a ZP-based contraceptive for marsupials: comparative analysis and developmental expression of marsupial ZP genes.

Fertility control in the form of a zona pellucida (ZP)-based immunocontraceptive has shown potential as a humane form of control for overabundant marsupials including the brushtail possum and macropods. Further refinement and development of a ZP-based vaccine requires detailed knowledge of the protein structure and expression in order to ensure maximum efficacy and specificity. Sequencing and comparative analysis of the ZP3 protein from three marsupial orders in this study found a high overall level of conservation; within order Diprotodontia, the ZP3 protein is 86.9-98.9% identical. ZP3 identity falls to 56.6-57.2%, when the grey, short-tailed opossum (a Didelphimorphian) is compared to dasyurid and diprotodontan marsupials. This is similar to its amino acid identity with ZP3 from eutherian species (50.7-52.8%). Comparison of a 21 amino acid epitope in marsupial ZP3 that has shown contraceptive effects, reveals 95-100% identity between the four macropodid species, 81-86% amino acid identity between brushtail possum and the macropods and 67-71% identity between the diprotodontans and the fat-tailed dunnart (a dasyurid). This is comparable to the level of identity between related eutherian mammals. The expression pattern of three ZP genes during brushtail possum and tammar wallaby pouch young development was examined by RT-PCR. This analysis of ZP gene expression has confirmed that ZP mRNA transcription begins in the ovary during pouch young development by about 51 days of age. The presence of ZP transcripts at this stage in pouch young development suggests that marsupial ZP gene transcription begins before the onset of follicular development.