Exploiting cis-limited antigens in livestock

预先选择后代性别的能力在畜牧业中非常重要。在许多农场物种中，如猪或奶牛，雌性后代在经济上比雄性重要得多，而在其他物种中，如赛马或肉牛，情况正好相反。特别是，成年公猪的肉会有一种“难闻”的味道，这意味着把完整的公猪养到高体重是不经济的。目前，通过早期屠宰或阉割雄性猪来避免公猪污染，这在道德上是不可取的，在经济上也是浪费的。我们正试图发现在受精发生前对精子进行分类的方法，即区分携带X染色体的精子（这将产生女性后代）和携带Y染色体的精子（这将产生男性后代）。目前唯一的方法是用一种染料染色精子头部的DNA，然后把DNA多的精子和DNA少的精子分开——这是有效的，因为X染色体比y染色体略大。这种“流分类”的过程既缓慢又麻烦，而且对猪等一些物种来说在商业上是不可行的。此外，流式分选过程包括染色DNA和用紫外线激光照射DNA，这两种方法都有可能损坏DNA。因此，迫切需要更有效、更安全的方法来分离X和Y精子。我们正在用两种方法来解决这个问题。首先，我们正在观察X和Y精子是否在其表面自然携带标记，从而使它们能够被区分开来。我们将通过流式分选来分离发育中的携带X和y染色体的精子，并观察每种细胞类型中哪些基因是活跃的。如果我们发现基因只在两种细胞类型中的一种中开启，那么这些可能对开发分离这两种细胞的方法很有用。我们将在正常小鼠中进行这项研究，在自然产生60%雌性后代的小鼠品系中进行研究，在转基因品系中进行研究，我们认为转基因品系已经改变了一种基因的水平。根据之前的研究，我们知道在这些性别比例失调的品系中，携带X染色体和Y染色体的精子数量相等，但Y染色体的精子使卵子受精的效率较低。这意味着这些菌株将帮助我们锁定感兴趣的基因，即那些使X和Y精子彼此不同的基因。其次，我们想测试是否可以使用人工表面标记分离精子。为此，我们将培育在其精子细胞中表达标记蛋白的转基因小鼠。理想情况下，含有转基因的精子细胞将在细胞表面显示这种“标签”，而那些不含转基因的精子细胞将保持不标记。这将允许我们分离两种类型的精子，只使用未标记的精子来繁殖下一代。在牲畜中，这将使精子选择成为可能，同时防止转基因生物进入食物链。

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.