The primary objective of the studies described in this thesis was to improve the cryopreservation success of koala spermatozoa for the purpose of establishing a genome resource bank for this species. A defining feature of the studies in this thesis was the implementation of an organelle-specific approach to better understand the causes of koala sperm cryo-injury. The functional attributes of spermatozoa, such as mitochondrial function, plasma membrane fluidity, membrane lipid asymmetry and DNA integrity were assessed as an indication of cryo-injury. Sperm mitochondrial function and plasma membrane integrity were examined by cryomicroscopy using the fluorescent probes JC-1 and propidium iodide (PI) respectively in a dual staining technique. Cooling and re-warming koala spermatozoa were more detrimental to mitochondrial function than to plasma membrane integrity. Mitochondrial membrane potential (MMP) was suppressed by freezing and thawing treatments; after thawing, MMP declined significantly during rewarming (from 5ºC to 35ºC). The distribution of GM1 ganglioside was examined using fluorescent-labelled cholera toxin B. No significant redistribution of GM1 was observed after chilling or cryotreatment. The externalisation of phosphatidylserine (PS) was examined using fluorescent-labelled annexin V. There was no significant increase in translocation of PS after chilling or cryopreservation. These observations imply that cryotreatment had little effect on plasma membrane lipid asymmetry. Koala spermatozoa were incubated in a range of anisotonic media to investigate whether nuclear swelling was caused by osmotic flux during the cryopreservation process. Although the most hypotonic solution tested (64 mOsm/kg) induced the highest incidence of nuclear relaxation (mean ± SEM; 12 ± 3%), this was not as severe as that previously documented following cryopreservation. Chromatin relaxation is a phenomenon observed in koala spermatozoa, where the sperm nucleus expands due to the result of structural changes in the natural conformation of the sperm DNA/protamine complex. DNA fragmentation was not a primary cause of cryopreservation-induced sperm chromatin relaxation, although in situ nick translation of putative DNA breaks indicated that these increased as the sperm head became progressively more relaxed. Using a Sperm Chromatin Dispersion test (SCDt) specifically developed and validated for koala spermatozoa, a continuum of nuclear morphotypes was observed, ranging from no apparent DNA fragmentation to spermatozoa with highly dispersed and degraded chromatin. A double comet assay was also developed to investigate DNA fragmentation in the koala spermatozoa. Conducted under neutral followed by alkaline conditions, this assay was able to differentiate between single- (SSB) and double-stranded (DSB) DNA damage in an effort to refine the interpretation of DNA damage in mature koala spermatozoa; the majority of the koala spermatozoa had nuclei with DNA abasic-like residues. The ubiquity of these residues suggested that constitutive alkali-labile sites are part of the structural configuration of the koala sperm nucleus. Spermatozoa with “true” DNA fragmentation exhibited a continuum of comet morphologies, ranging from a more severe form of alkaline-susceptible DNA, to nuclei that exhibited both SSB and DSB. Swelling of koala sperm chromatin following cryopreservation has largely been attributed to the absence of inter-molecular disulphide cross-linkages in the marsupial sperm nucleus. Fish spermatozoa also lack disulphide bonds within their chromatin, but nevertheless, have been successfully cryopreserved. To examine the hypothesis that the cryoprotectants used for fish sperm cryopreservation will confer a similar degree of protection on koala spermatozoa, various concentrations of five cryoprotectants (dimethyl sulphoxide, methanol, propylene glycol, ethylene glycol and dimethylacetamide) were evaluated. Each treatment was compared against an established koala sperm cryopreservation protocol that uses 14% glycerol. Dimethylacetamide at a concentration of 12.5% (v/v) was found to be comparable to glycerol in the successful cryopreservation of koala spermatozoa although high inter-male variability was observed. However, when the new protocol was subsequently validated for a larger population of captive koalas (n = 22), glycerol emerged the better cryoprotectant with respect to all sperm viability parameters assessed except for that of the incidence of chromatin relaxation, which was not affected by the cryoprotectant. Significant difference was also observed in the post-thaw survival of spermatozoa from different animals, which was independent of pre-freeze semen quality. Based on post-thaw semen viability parameters, the koalas could be divided into two distinct groups, where one group had significantly higher sperm viability compared to the other group, regardless of cryoprotectant used. Positive correlation between motility and MMP was observed before and after cryopreservation. However, cryopreservation significantly reduced the dependency between these variables (P < 0.001), suggesting that cryopreservation reduced the dependency between mitochondrial function and motility.
| Identifer | oai:union.ndltd.org:ADTP/280565 |
| Creators | Yeng Zee |
| Source Sets | Australiasian Digital Theses Program |
| Detected Language | English |
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