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The best and poorest bovine semen samples used commercially for artificial insemination in dairy cattle typically differ in pregnancy rates by 20 to 25% but are within a range that pregnancy rates cannot be predicted consistently by commonly used laboratory assays. Sperm motility and morphology are the characteristics most often evaluated. Laboratory assays that measure other functional traits of sperm may be useful as supplemental assays to increase the reliability of predicting fertility. One such functional trait is the ability of sperm to bind to the zona pellucida, a process mediated by complementary receptors on each gamete. On mouse sperm, beta1,4-galactosyltransferase acts as a receptor for the zona pellucida. Beta1,4-galactosyltransferase is expressed on sperm from many mammals, including bovine sperm, and is a candidate for a zona pellucida receptor. The ability of sperm to bind to the zona pellucida may be related to the amount of beta1,4-galactosyltransferase present on sperm. The aim of this work was to determine if bull sperm beta1,4-galactosyltransferase activity was related to fertility. Beta1,4-galactosyltransferase enzyme assays were performed on sperm from 24 bulls whose fertility was estimated by nonreturn rate and on sperm from a second group of seven bulls whose fertility was ranked by in vivo competitive fertilization. Beta1,4-galactosyltransferase activity varied between individual bulls but was not correlated to fertility as estimated by nonreturn rate or by competitive fertilization. These results demonstrate that beta1,4-galactosyltransferase activity on sperm varies between animals, but that beta1,4-galactosyltransferase activity alone is not an accurate indicator of fertility in dairy bulls.

In mice, initial gamete recognition is mediated by the binding of sperm surface beta1,4-galactosyltransferase (GalTase) to a glycoprotein of the zona pellucida, ZP3. When sperm bind to the zona pellucida, ZP3 induces the acrosome reaction by aggregating GalTase. The acrosome reaction releases acrosomal enzymes allowing sperm to pass through the zona pellucida, bind to the egg membrane, and activate development. In addition to GalTase, there is evidence that other sperm proteins may also bind ZP3. Although fertilization in the mouse is morphologically similar to fertilization in most other mammalian species, the degree of parallelism at the molecular level is not well defined. Less information is available about the molecular details of fertilization in other species. The aim of this work was to determine whether sperm from other mammalian species express GalTase on their surface. We performed GalTase enzyme assays on sperm from six species, and all six expressed GalTase on their surface. The amounts of GalTase varied between species. Guinea pig, mouse, and rat sperm had higher levels of GalTase than bovine, porcine, and rabbit sperm. GalTase was localized by immunofluorescence on live and fixed sperm to the anterior portion of the sperm head in all species examined. This is the expected location for a receptor that binds the zona pellucida. Biotinylation of sperm surface proteins confirmed that GalTase detected by immunofluorescence and enzyme assay was expressed on the sperm surface. These results demonstrate that various mammalian species express GalTase on their surface and that it is found in the proper location to bind to the zona pellucida.

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