'Battle of sexes' gives clue to low birth weight

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The Independent Online

A study showing how the genes of a father attempt to gain supremacy within a mother's womb could explain why some babies are born smaller than others.

A study showing how the genes of a father attempt to gain supremacy within a mother's womb could explain why some babies are born smaller than others.

Scientists have shed light on an ancient "battle of the sexes" that is fought out in the uterus during pregnancy. The fight is over how much of a mother's vital resources should be invested in the growing foetus.

The findings might have important implications, given that low birth weight is linked with higher death rates in newborn babies, a greater chance of mental or physical defects, and an increased risk of heart disease or diabetes in later life.

Research on mice has revealed that when certain paternal genes are switched on, they cause the placenta to grow bigger than it otherwise would, thereby making the offspring larger at birth. When the scientists turned off these paternal genes, the placenta – which provides nutrients to the foetus – is markedly smaller and the offspring are at greater risk of low birth weights.

The findings support a basic evolutionary theory about competing interests between the sexes, which determine how big their babies should be at birth.

According to this theory, females who mate with a number of malesbenefit from not investing too much of the limited resources of their bodies in producing big babies. Males would benefit from ensuring that their babies are as big as possible, even if that is to the detriment of the mother's health.

Miguel Constancia and colleagues at the Babraham Institute near Cambridge found that the placenta was the "battleground" where the two sexes fight over how big offspring should be.

The study, published in Nature, concentrated on a gene called insulin-like growth factor 2 (IGF2). This is known to be involved in controlling the size of the placenta – whichshares genes from both mother and father. Although the placenta has two copies of the IGF2 gene – one maternal and one paternal – only one is switched on. Normally it is the paternal gene that is activated. When Dr Constancia removed the paternal IGF2 genes from just the placental cells of the mice, though, the placenta failed to grow to the size it otherwise would have become.

The research might lead to an early test for women at high risk of giving birth to dangerously small babies.

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