02_ Sperm-Egg interactions during Fertilisation

As part of our investigations into sperm-egg interactions and fertilization in marsupials we are performing studies on (i) cytoplasmic and nuclear maturational changes that take place to the oocyte, and (ii) morphological changes that occur to the egg around the time of, and immediately following, sperm incorporation.

Over the last few years we have documented a variety of morphological changes that take palce at this time. Most of this work has been carried out with the dasyurid marsupial Sminthopsis crassicaudata, and these investigations have recently been extended to several other marsupial species so that some insight into the interspecific diversity and evolution of these processes within marsupials can be obtained.

Changes in the cytoskeleton of the marsupial egg following sperm incorporation have been determined with the use of antibodies to cytoskeletal proteins in collaboration with Dr G Schatten. In this study we demonstrated the paternal origin of the centrosome using the marsupial, Monodelphis domestica.

03_Dynamics of production of male germ cells and testis function

The testis is unusual in that it has two inter-related, but independent, functions, the production of spermatozoa and the secretion of the male sex hormones, androgens. It is also unique in that in most mammals, including humans, it originates in the abdominal cavity but migrates during fetal development to the scrotum where its temperature is considerably cooler than that of the abdominal cavity. If this process fails, and the testis remains cryptorchid, or if the testis is returned experimentally to the abdominal cavity, spermatogenesis is disrupted, although secretion of androgens is only marginally affected. Similar changes in the testis can be produced by exposing the whole animal to a hot environment or by locally heating the testes . As well as changes in the numbers of spermatozoa produced, and their motility, sperm produced from heated testes appear to be less efficient in fertilizing eggs and seem to produce embryos which grow less well, or are more likely to die, during development. We are investigating what changes take place in spermatogene sis if the testicular temperature is raised, and how these changes are produced.

Also a study on the dynamics, and control, of production of germ cells in Australian native rodents is being carried out. This study developed out of our earlier work in which we found huge differences in absolute and relative testis sizes between closely related species of Australian rodents. This group of endemic rodents is unique in that no other closely related group of mammals exhibits such diversity in testis size. This vast difference in testis size has been shown to be associated with very large interspecific differences in numbers of sperm stored in the male reproductive tract. In our early studies we attempted to determine the functional and evolutionary reasons for these differences. A series of investigation into the germ cell dynamics and efficiency of production of germ cells has shown that species with small testes, not only produce far fewer sperm, but also that the efficency of production is less. Such differences in germ cell production may relate to differences in breeding systems and intensity of intermale open competition.