During the final stages of sperm production (spermiogenesis) the chromatin (DNA) in the sperm nucleus undergoes profound changes. At the start of spermiogenesis the chromatin in sperm is found in association with histone nucleoproteins which gives it a nucleosome core particle structure similar to the structure of chromatin in all other cells of the body. During spermiogenesis the histone proteins are replaced by transition proteins which are then replaced by protamines. As protamines progressively bind to sperm chromatin, the chromatin undergoes conformational changes including a major condensation which is thought to be a mechanism that protects chromatin in sperm from damage until the sperm reaches the egg for fertilisation.
The integrity and stability of the protamine-chromatin association in sperm can be measured using the sperm chromatin structure assay (SCSA). Studies primarily in men have indicated that sperm chromatin instability, as determined by the SCSA, does not influence fertilisation but is associated with early pregnancy failure. One of the aims in the present study was to ascertain the prevalence of sperm chromatin instability in bulls in northern Australia. This would serve as a first step in evaluating whether sperm chromatin instability is a contributing factor to reproductive wastage in cattle.
The SCSA yields a value for the DNA fragmentation index (DFI) and in men a DFI > 27% is associated with early embryonic mortality. A relationship between DFI and embryonic mortality has yet to be established in bulls and in the current project a DFI > 27% was arbitrarily chosen as indicative of substantial chromatin instability for bulls. The SCSA was conducted on semen samples from a total of 565 bulls at 14 locations and representing 7 genotypes. When samples were exposed to the SCSA for 0.5 minutes 4.9% of bulls had a DFI > 27% and this increased to 11.5% of bulls with DFI > 27% when samples were exposed to the SCSA for 5 minutes. The vast majority of bulls (513/565, 91%) had a DFI < 15% at 0.5 min SCSA and it could be concluded that sperm chromatin is stable in most bulls.
Location appeared to have a greater influence on DFI than genotype although this information was based primarily on Brahman. For 149 bulls, most (136/149, 91%) had a DFI < 15% on 2 occasions and for 21 bulls, 17/21 (81%) had a DFI < 15% on 4 occasions. These results suggested that the measure of sperm chromatin stability is repeatable; however, there were very few bulls in the 15-27% and > 27% DFI categories. Seventeen bulls with DFI ranging from 0.8 to 8.6% all sired calves and 4 bulls with DFI > 27% had also sired calves in the preceding 1 to 6 years. For half-sib bulls, 82/87 (94%; comprised of smaller half-sib groups) had a DFI < 15% and the odd progeny, amongst different sires, had a 15-27% DFI (4/87, 5%) or > 27% DFI (1/87, 1%). Six sires had a DFI ranging from 0.8 to 3.1% and of their progeny 41/45 (91%) had < 15% DFI, 3/45 (7%; different sires) had 15-27% DFI and 1/45 (2%) had > 27% DFI.
In summary, based on the 3 categories of sperm chromatin status adopted in the project, < 15% DFI (stable), 15-27% DFI (moderately stable) and > 27% (unstable), it would appear that the majority of bulls in northern Australia have a stable sperm chromatin structure and no relationship with reproductive performance was identified. Whilst it would appear that sperm chromatin structure may have a genetic component, the majority of bulls had a DFI < 15% with only small numbers of bulls with a DFI of 15-27% or > 27%. Larger numbers of bulls in the latter 2 categories will need to be monitored before the heritability of sperm chromatin structure can be determined.