Sperm membrane fatty acid composition in the Eastern grey kangaroo (Macropus giganteus), koala (Phascolarctos cinereus), and common wombat (Vombatus ursinus) and its relationship to cold shock injury and cryopreservation success
Miller, RR Jr, Sheffer, CJ, Cornett, CL, McClean, R, MacCallum, C & Johnston, SD 2004, Cryobiology, vol. 49, no. 1, pp. 137-148.
Koala cauda epididymidal spermatozoa have low membrane cholesterol levels and a high ratio of unsaturated/saturated membrane fatty acids compared to other studied species, including other marsupials. In particular, docosahexaenoic acid (DHA), a long-chain polyunsaturated fatty acid and the most predominant membrane fatty acid in the studied marsupial species, was found in high levels in koala spermatozoa and is thought to be related to its cryogenic tolerance.
Koala spermatozoa have a higher ratio of unsaturated/saturated membrane fatty acids than those of the common wombat, and a higher ratio of long-chain/short-chain membrane fatty acids at 7.6 than both the common wombat at only 1.9 and the eastern grey kangaroo at 5.2. In addition, the koala spermatozoa had the highest levels of DHA of the species studied, at 59.2 ± 8.5%, compared to the eastern grey kangaroo at only 30.6 ± 3.7% and the common wombat at 42.0 ± 3.2%. The koala spermatozoal cholesterol levels were 2.53 ± 0.13 pmol cholesterol/nmol phospholipid; approximately three times less than membrane cholesterol levels in the spermatozoa of the common wombat or eastern grey kangaroo.
The ratio of unsaturated/saturated membrane fatty acids is generally held as an indicator of susceptibility to cold shock, with a higher ratio indicating greater susceptibility. The ratio found in this study for koalas is higher than any other mammalian species recorded, yet is not accompanied by the expected cold shock susceptibility, contradicting previous research. It is still possible, however, that such a high ratio may negatively impact cryopreservation tolerance, but not through cold shock susceptibility. Furthermore, previous studies have found that long-chain polyunsaturated membrane fatty acids such as DHA are important in spermatozoal motility and cryogenic tolerance, a theory supported by the results of this article. DHA levels have been linked to membrane fluidity and the ability to cryogenically preserve the spermatozoa, supporting the notion that cryopreservation success for koala spermatozoa may be linked to high DHA levels. In addition, previous studies have hypothesised that high cholesterol plays a significant role in cold shock resistance; however, the koala spermatozoa exhibited cold shock resistance despite very low cholesterol levels, demonstrating that this effect may not be as significant as previously thought.
These findings contribute a greater understanding of marsupial spermatozoa composition and cryopreservation tolerance. In combination with further research, these findings may inform future spermatozoa cryopreservation protocols to increase post-thaw viability and motility for artificial insemination programs.
Summarised by Laura Wait
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