An investigation into the similarities and differences governing the cryopreservation success of koala (Phascolarctos cinereus: goldfuss) and common wombat (Vombatus ursinus: shaw) spermatozoa
Johnston, SD, MacCalum, C, Blyde, D, McClean, R, Lisle, A & Holt, WV 2006, Cryobiology, vol. 53, no. 1, pp. 218-228.
Cryopreservation of spermatozoa is far less successful for koala sperm compared to the sperm of the closely-related common wombat. After thawing, the survival of spermatozoa was greatest when samples were frozen slowly in a 14% glycerol solution. Koala sperm also has low osmotic tolerance compared to that of the wombat. The authors aimed to determine the factors responsible for these differences in post-thaw survival in the two marsupials and predicted that the presence of the microfilamentous protein F-actin in koala spermatozoa might cause inflexibility in the sperm plasma membrane. As F-actin was not detected, however, it is suggested that some other factor may be responsible for the low success rates of koala sperm cryopreservation.
The post-thaw survival of wombat and koala spermatozoa was investigated by testing the sperm’s responses in different osmotic environments. Post-thaw survival, measured in terms of motility, plasma membrane integrity, and intact sperm heads, was found to be highest for koalas when the sample was frozen at the slower rate of 6OC per minute and in a solution with 14% glycerol concentration. Importantly, however, despite this method proving to be the most effective of those tested, sperm samples were only viable after thawing in 50% of trials. Hyperosmotic environments were also found to negatively affect sperm motility and percentage of sperm with intact plasma membranes. F-actin was predicted to be responsible for the reduced ability of koala sperm to tolerate osmotic flux and therefore withstand cryopreservation. Given its absence, however, authors instead suggest that the lack of F-actin in koala sperm may be partly responsible for their observed decondensation after cryopreservation. Decondensation is the process by which the genetic material within the sperm nucleus becomes activated for fertilisation, and the decondensation of koala sperm after cryopreservation makes the sample unusable for artificial insemination.
Compared to attempts in other marsupials, the cryopreservation of koala spermatozoa has been relatively unsuccessful. The ability to cryopreserve koala sperm will be an important development for the establishment of genetic resource banks and artificial insemination programs for the species, reducing reliance on fresh semen samples. The close phylogenetic relationship between the common wombat and the koala makes comparison between the success of their sperm cryopreservation appropriate, offering a unique opportunity to identify the potential factors that are responsible for the low success of sperm cryopreservation for koalas.
As interest in assisted breeding technologies and programs for koala conservation grows, it becomes increasingly important to refine each aspect of the cryopreservation process to improve the viability of frozen-thawed koala sperm. The findings of this study suggest that sperm nuclear instability and plasma membrane intolerance are important areas for future research.
Summarised by Joanna Horsfall
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