Exploiting cis-limited antigens in livestock

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The ability to pre-select the sex of offspring would be hugely important in animal husbandry. In many farm species such as pigs or dairy cattle, female offspring are much more economically important than males, while in other species such as racehorses or beef cattle, the opposite situation holds. In particular, meat from adult male boars can have an "off" flavour known as boar taint, meaning it is uneconomical to raise intact male pigs to high weights. Currently, boar taint is avoided by early slaughter or castration of male pigs, which is ethically undesirable as well as economically wasteful. We are trying to discover ways to sort sperm before fertilisation occurs, i.e. to distinguish between sperm bearing an X chromosome (which will give female offspring) and those bearing a Y chromosome (which will give males). Currently the only way to do this is by using a dye to stain the DNA in the sperm head, and separating the sperm that have more DNA from those that have less - this works because the X chromosome is slightly larger than the Y. This "flow-sorting" process is slow and cumbersome and not commercially viable for some species such as pig. Also, the flow-sorting process involves staining the DNA and shining an ultraviolet laser on it, both of which can potentially damage the DNA. There is thus a strong need for more efficient and safer ways of separating X and Y sperm. We are using two approaches to tackle the problem.Firstly, we are looking to see whether X and Y sperm naturally carry markers on their surface allowing them to be distinguished. We will do this by using flow-sorting to separate developing X- and Y-bearing sperm and looking to see which genes are active in each cell type. If we find genes which are only switched on in one of the two cell types, these are likely to be be useful in developing ways to separate the two. We will carry out this search in normal mice, in a strain of mice which naturally produce 60% female offspring, and in a transgenic strain which has altered levels of a gene we believe to be involved. From previous work, we know that in these strains with a skewed sex ratio, equal numbers of X- and Y-bearing sperm are produced, but that the Y sperm are less effective at fertilising the egg. This means that these strains will help us to home in on genes of interest - i.e. the ones which make X and Y sperm different from each other. Secondly, we want to test whether we can separate sperm using an artificial surface marker. To do this, we will breed transgenic mice which express a marker protein in their sperm cells. Ideally, sperm cells containg the transgene will show this "tag" on the cell surface, while those which do not contain the transgene will remain untagged. This will then allow us to separate the two types of sperm and only use untagged sperm to breed the next generation. In livestock, this would enable sperm selection while preventing genetically modified organisms entering the food chain.