How Does the Pattern of Sperm Maturation in the Possum Compare with That of Eutheria?

It is necessary at this stage to point out that knowledge of sperm maturation in the possum is based purely on morphology: nothing is known, as yet, about the acquisition of fertilizing ability. Nevertheless, a comparison with the 'typical' eutherian pattern raises some fascinating questions about the evolution of the epididymis and scrotum. Firstly, there are some obvious basic similarities in the process; for example, in the progressive condensation of excess sperm cytoplasm and in the increasing disulphide bonding within structural proteins. There is also the fact that in both models the sperm mass undergoes progressive concentration as it passes through the epididymis, and this indicates that fluid resorption is a major feature of both the marsupial and eutherian epididymis. A minor difference here is that the fluid resorption seems to occur over a relatively greater proportion of the duct in the possum, but this impression requires further investigation. Finally, it is also clear that the epididymis of the possum, as in eutherian mammals, is under androgenic control, for castration and treatment with androgen antagonists such as diethylstilboestrol cause regression of the duct and disruption of the pattern of sperm maturation (Cummins 1977, and in preparation).

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Despite these similarities, it is clear that possum sperm maturation possesses some unique features. The complex acrosomal changes involving loss of plasma membrane vesicles, the pitting and vesiculation of the cytoplasmic droplet, and the elaboration of the midpiece fibrous sheath are all examples of morphological modification occurring within the epididymal lumen, which in comparable Eutheria, one would expect to have been completed by the time the spermatozoa had left the seminiferous epithelium. The overwhelming impression is that possum sperm are liberated in a much more immature state than are the sperm of comparable eutherian mammals, and that the epididymis has consequently assumed greater control over the development of the mature gamete.