Individual variability in post-thaw sperm survival in a captive koala population
Yeng Peng Zee a,*, William V. Holt b, Vere Nicolson c, Michael Pyne d, Stephen D. Johnston a
a School of Animal Studies, The University of Queensland, Gatton, Qld. 4343, Australia
b Institute of Zoology, The Zoological Society of London, Regent’s Park, London, NW1 4RY, UK
c Dreamworld, Coomera, Qld. 4209, Australia
d Currumbin Wildlife Sanctuary, Currumbin, Qld. 4223, Australia
This study documented the extent of individual animal variation with respect to two proven methods of sperm cryopreservation in a captive population of 22 koalas. Semen samples were collected by electroejaculation, diluted in Tris–citrate glucose and equilibrated to 4°C before being further diluted and frozen in media containing a final concentration of either 14% (v/v) glycerol or 12.5% (v/v) dimethylacetamide (DMA). There were significant differences in post-thaw survival of spermatozoa from different animals that were independent of pre-freeze semen quality. Glycerol proved to be a better cryoprotectant than DMA in terms of maintaining motility, plasma membrane integrity and high mitochondrial membrane potential; however, there was no difference between cryoprotectants with regards to their ability to prevent chromatin relaxation. While a positive correlation was observed between motility and mitochondrial membrane potential, both before and after cryopreservation, the slopes of the pre-freeze regression lines of these relationships were significantly altered following cryopreservation, suggesting that the efficiency of energy generation by the mitochondria was lowered by the freeze–thaw process. Based on a cluster analysis of the post-thaw semen viability parameters, the koalas in this study could be divided into two distinct groups; Cluster 1 had significantly higher sperm viability compared to the other cluster, regardless of the cryoprotectant used. The unpredictability of assessing post-thaw survival from pre-freeze koala semen parameters requires further investigation but is likely to be associated with variation in ejaculate composition or inherent genetic differences between animals.