The sexual preferences of microscopic worms have been manipulated in the laboratory so that they are attracted to the same sex, offering new evidence that sexuality may be hard-wired in the brain.
By activating a single gene in the brains of hermaphrodite nematode worms, scientists have induced them to attempt to mate with other hermaphrodites, instead of being attracted exclusively to males.
The findings, from a team in the United States, provide a fresh indication that homosexual or heterosexual preferences are not purely a matter of choice, but are deeply influenced by underlying biology.
While nematode worms are extremely simple organisms, and details of their behaviour are difficult to apply to people with any accuracy, the researchers said that the existence of a biological pathway to same-sex attraction offered a possible insight into human sexuality.
Erik Jorgensen, Professor of Biology at the University of Utah, who led the study, said: “Our conclusions are narrow in that they are about worms and how attraction behaviours are derived from the same brain circuit.
But an evolutionary biologist will consider this to be a potentially common mechanism for sexual attraction.
“We cannot say what this means for human sexual orientation, but it raises the possibility that sexual preference is wired in the brain. Humans are subject to evolutionary forces just like worms. It seems possible that if sexual orientation is genetically wired in worms, it would be in people too. Humans have free will, so the picture is more complicated in people.”
Nematode worms, of the species Caenorhabditis elegans, are one millimetre long and live in soil, where they feed on bacteria. The overwhelming majority – more than 99.9 per cent – are hermaphrodites, which produce both sperm and eggs and generally fertilise themselves before laying eggs.
About 0.05 per cent of nematodes are male, however, and these worms must seek out hermaphrodites to reproduce. Hermaphrodites will mate with an available male rather than fertilise themselves, and though they produce sperm they will not impregnate other hermaphrodites as they lack the required copulatory structure.
There are no true females and hermaphrodites were treated as female for the purposes of the study. C. elegans shares many of its genes with human beings and other animals, and is a standard organism used for early laboratory studies of genetics.
“A hermaphrodite makes both eggs and sperm,” Professor Jorgensen said. “She doesn’t need to mate [with a male] to have progeny. Most of the time, the hermaphrodites do not mate. But if they mate, instead of having 200 progeny, they can have 1,200 progeny.”
As the worms have no eyes – hermaphrodites have only 959 cells and males 1,031 cells – they detect one another’s sex using scent cues.
In the study, published in the journal Current Biology, the scientists activated a gene called fem-3 in hermaphrodites. This gene makes the nematode body develop as male, with neurons that appear only in male brains and copulatory structures such as tails.
In the experiment, fem-3 was activated only in the brain, so the worms developed male nerve cells but not other male body characteristics. Despite this, they behaved like males, attempting to seek out and fertilise other hermaphrodites.
“They look like girls, but act and think like boys,” said Jamie White, who conducted the key experiments. “The [same-sex attraction] behaviour is part of the nervous system.”
Professor Jorgensen said: “The conclusion is that sexual attraction is wired into brain circuits common to both sexes of worms, and is not caused solely by extra nerve cells added to the male or female brain. The reason males and females behave differently is that the same nerve cells have been rewired to alter sexual preference.”
In a second phase of the study, the scientists manipulated different kinds of nerve cell in the male brain to determine which were responsible for switching on male attraction to hermaphrodites. They found that, although switching off one of the eight sensory neurons impaired attraction in adults, young males developed normally if just one such nerve cell was intact.
This finding suggests that there is considerable redundancy built into the sexual development of males. Dr White said: “It must be that the behaviour is very important.”
Biological puzzle
–– In 1993, Dean Hamer, of the US National Cancer Institute, found a region of the X chromosome that was linked to male homosexuality
–– Simon LeVay, a biologist who is gay, found evidence that the brain structure of homosexual and heterosexual men is different
–– Research suggests that for each older brother a man has the chances that he will be homosexual increase 33 per cent Men who have shorter ring fingers are relatively more likely to be gay
–– A biological enigma of “gay genes” is that homosexuals should have fewer children, and should thus not pass them on
Source: The Biology of Homosexuality Genes
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