Individual variability in post-thaw sperm survival in a captive koala population
Zee, YP, Holt, WV, Nicolson, V, Pyne, M, Johnston, SD 2009, Cryobiology, vol. 59, no. 1, pp. 69-74.
The tolerance of koala sperm obtained using electroejaculation to cryopreservation exhibits considerable variation at an individual level, supporting the hypothesis that variation in post-thaw sperm viability exists in a population. Single koala spermatozoa samples obtained from 22 captive koalas revealed that sperm viability and quality differed less between individuals prior to the cryopreservation process than after. This suggests that cryo-tolerance and post-thaw viability are dependent upon individual biophysical sperm characteristics.
Koala spermatozoa samples were obtained via electroejaculation to assess seminal viability and quality. Samples were diluted and warmed before being chilled to 4oC using tris-citrate glucose (TCG) buffer and 28% glycerol or 25% dimethylacetamide (DMA). Results indicated that glycerol as a cryoprotectant (a constituent employed to protect against freezing damage) was more effective than DMA with respect to sperm viability parameters both prior to and after cryopreservation. A cluster analysis showed the separation of koalas into two distinct groups based on sperm viability. Compared to the group with lower sperm viability, the group with higher sperm viability had a higher proportion of motile and live spermatozoa with mitochondrial membrane potential, as well as a lower proportion of spermatozoa with relaxed chromatin, with no differences observed between the cryoprotectants used.
Cryopreservation for artificial insemination (AI) is a precursor to maintaining genetic diversity in captive scenarios and is likely to be essential for the conservation of wild koalas. Preliminary studies concerning other mammalian populations describe species-specific variation in cryobiological characteristics of spermatozoa in conjunction with tolerance differences to the processes associated with cryopreservation; a pressing issue in cryobiology. This study is the first to explore the cryobiological features of koala spermatozoa that determines whether they are ‘good’ or ‘bad’ freezers.
Future cryopreservation protocols for the collection of koala ejaculate require review to negate the impacts of individual variance in cryo-tolerance and seminal environments to be effectively applied to wild koala populations. Other unexplored variables like sperm-head morphology were highlighted by the authors as being possible contributors to the freezability of sperm and ultimately the viability of spermatozoa post-thawing.
Summarised by Lauren Mousley
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