Heterogeneity of Sperm Nuclear Chromatin Structure and its Relationship to Bull Fertility1

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Abstract
The relationship between sperm nuclear chromatin structure and fertility was evaluated in two groups of Holstein bulls: Groups 1, 49 mature bulls, and Group 2, 18 young bulls. Fertility ratings had been estimated for Group 1 and nonreturn rates were known for Group 2. Semen samples were measured by the sperm chromatin structure assay (SCSA): sperm were treated to induce partial in situ DNA denaturation, stained with acridine orange, and evaluated by flow cytometry. Acridine orange intercalated into double-stranded DNA emits green fluorescence upon excitation with 488 nm light, and red fluorescence when associated with single-stranded DNA. An index of DNBA denaturation per cell is provided by alpha-t [.alpha.t = red/(red + green) fluorescence]. The standard deviation (SD.alpha.t), coefficient of variation (CV.alpha.t) and proportion of cells outside the main population (COMP.alpha.t) of the .alpha.t distribution quantify the extent of denaturation for a sample. Intraclass correlations of the .alpha.t values were high (.gtoreq. 0.70), based on four collections obtained over several years from Group 1 bulls. Negative correlations were obtained between fertility ratings and both SD.alpha.t (-0.58, p < 0.01) and COMP.alpha.t (-0.40, p< 0.01) in Group 1, and between nonreturn rates and both SD.alpha.t (-0.65, p < 0.01) and COMP.alpha.t (-0.53, p < 0.05) in Group 2. These data suggest that the SCSA will be of value for identification of low fertility sires and poor quality semen samples.