Gender selection in breeding

Foto: Karin Sevink

Photo: Karin Sevink

Wouldn´t it be wonderful if breeders could choose the gender of their foals? It can be done…But are we really looking at a future where we can ask for a filly or colt?
In recent decades progress in horse breeding techniques has been fast and furious. AI, ET, OPU and ICSI, the range of possibilities keeps growing. Most of these techniques have already been used in cattle breeding and the same applies to manipulating the gender of embryos. It´s used on horses too. But what are the ramifications of all the possibilities? And is everything in place to start using it in the Netherlands?

Not new

‘This has been common practice in cattle breeding for twenty years and is widely commercialised. So the technique in itself isn´t new’, Dr Tom Stout of the university clinic in Utrecht explains. ‘There are actually two methods to pre-determine the gender of the embryo. The first is by separating the semen into ‘male’ and ‘female’ sperm cells. The second is by establishing the gender of the embryo before transferring it to a surrogate mare.’
When semen- or sperm cells are produced all genetic material is divided, which means half of the genetic material will be present in the reproductive cell. All egg cells will therefore carry an X chromosome, but this is different for sperm cells. Here one half carries an X chromosome and the other half a Y chromosome. Melting together of all chromosome material during impregnation is the moment when the gender of an individual is determined, XX is female and XY is male.
Stout: ‘The nucleus of a sperm cell with an X chromosome is a lot bigger than one with a Y chromosome. A ‘female’ sperm cell has around three to four percent more DNA than a ‘male’ one. So that´s how they can be told apart and subsequently can be separated.’

Flow Cytometer

Stout: ‘The most-commonly used method is using flow cytometry. The first step is to colour the DNA in the nucleus of the sperm cell with an agent that does not affect the DNA. Next, a laser measures the fluorescence and amount of DNA material. Then the device must place every sperm cell in a drop of fluid and give it an electric signal, following which they can be separated. The sperm cells with more DNA are retrieved in a dish labelled ‘X’, the lighter cells in one labelled ‘Y’. The cells that cannot be distinguished by the device are kept apart from the others. The reliability rate of this method is around 95%.’

Pregnancy percentages

‘When the mare is inseminated with this semen within a short time frame and at the right time of her cycle, then the pregnancy percentage is about ten percent lower than normal. After all, the semen has to some extent, been manipulated and is always slightly affected by it. It´s also possible that the colouring agent triggers a reaction when entering the egg cell together with the sperm cell. Naturally, the egg cell will break down the colouring agent, but the question may rise if this happens at the expense of the impregnation or early development. Then there is the electric signal injected into the semen, and the number of available sperm cells is also a factor.’

A costly affair

A downside of this technique is its cost efficiency. Stout: ‘It´s expensive because it´s a time-consuming technique. Before the device has been properly calibrated, the ejaculate has been processed and enough sperm cells have been separated for one insemination sample we´re talking about a two- to three-hour job. And all this time somebody must monitor the device. In the cattle sector there is a whole string of devices that operate non-stop. In the equine sector this is not the case. Moreover, purchase costs of the device are extremely high.’

Interested to learn more about gender selection in breeding and its availability in the Netherlands? Then read the full article in the July issue of Phryso which appears this weekend.