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Koala (Phascolarctos cinereus) sperm nuclei show a tendency to swell after cryopreservation, but it is uncertain whether this phenomenon is associated with DNA fragmentation. In this study, we validated a modified version of the sperm chromatin dispersion test (SCDt) for use with koala spermatozoa, which is the first use of the test for a marsupial. Cryopreserved spermatozoa (multiple straws) from a single koala were used to explore the relationship between sperm morphology, viability, chromatin dispersion, and DNA fragmentation. A SCDt prototype kit (Sperm Halomax) was specifically developed for koala spermatozoa with the use of a lysing solution that did not contain dithiothreitol. DNA fragmentation of lysed and nonlysed spermatozoa was examined in microgel slides and validated by means of in situ nick translation (ISNT). The SCDt was then applied to the analysis of extended and frozen-thawed semen samples of 3 different koalas. Spermatozoa were classified into 3 distinct koala sperm morphotypes (KSMs) after the SCDt: 1) KSM-1, rod-shaped cells with no halo of DNA; 2) KSM-2, rounded nuclei with various degrees of halo formation about a dense chromatin core; and 3) KSM-3, rod-shaped or rounded nuclei consisting of an inner chromatin core but with large dispersed halos of stellar chromatin. Although ISNT after the SCDt did not label KSM-1, both KSM-2 and KSM-3 stained positively for DNA fragmentation. ISNT was not able to differentiate between KSM-2 and KSM-3. Although application of the SCDt to the spermatozoa of another 3 koalas showed no difference in the percentage of the 3 sperm morphotypes found between extended and frozen-thawed semen, thawed spermatozoa incubated at 35uC for 2 hours showed an increase in the incidence of KSM-3 and a corresponding decrease in KSM-2. We propose that KSM-1 and KSM-2 represent nuclei that show either no, or only limited, sperm DNA fragmentation, respectively. It is likely that the halos formed around KSM-2 are from DNA that is damaged as part of the normal processing of the spermatozoa and is a consequence of the lack of cysteine residues and associated stabilizing disulfide bonds in marsupial sperm DNA. ''True'' sperm DNA damage is most likely associated with KSM-3, which shows a massive dispersion of chromatin similar to that described in other species. A model of koala sperm chromatin structure is proposed to explain the behavior of the sperm nuclei after the SCDt. Further studies are required to determine whether DNA damage found in KSM-2 is indicative of single-stranded DNA breakage associated with an inherent lack of cysteine residues in marsupial sperm chromatin. Conversely, it will also be important to establish whether KSM-3 is caused by an increased incidence of double-stranded DNA breakage and whether this abnormality is correlated with impaired fertility as it is in other species.

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Spermatozoal haplotypic DNA is prone to damage, leading to male fertility problems. So far, the assessment of sperm DNA breakage has been challenging because protamines render the nuclear chromatin highly compacted. Here, we report the application of a new test to quantify DNA fragmentation in spermatozoa of an externally fertilizing teleost fish. The sperm chromatin dispersion (SCD) test uses a species-specific lysing solution to generate controlled protein depletion that, followed by DNA-specific fluorescent labelling, allows an easy morphological discrimination between nuclei affected by DNA damage. Using tench (Tinca tinca) as our model, we first trialled the test against established, but more technically demanding, assays employing in situ nick translation (ISNT) and the comet assay. The SCD test showed high concordance with ISNT, comet assay measures and a chromatin-swelling test, confirming the application of this straightforward SCD technique to various aspects of reproductive biology. Second, we examined between-male variation in DNA damage, and measured changes through time following spermatozoal activation. Between-male variation in the basal levels of average DNA damage ranged from 0 to 20% of sperm showing damage, and all showed increases in DNA fragmentation through time (0-60 min). The rates of DNA damage increase are the fastest so far recorded in sperm for a living organism, and may relate to the external fertilization mode. Our findings have relevance for broodstock selection and optimizing IVF protocols routinely used in modern aquaculture.