Homosexuality

Gay worms get down and dirty with their mates

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.

Times Online
October 26, 2007

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 doesnt 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 anothers 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
 

CNN Online
10 May 2006

That's in line with an earlier study that had indicated gay men's brain responses were different from straight men -- though the difference for men was more pronounced than has now been found in women.

Lesbians' brains reacted somewhat, though not completely, like those of heterosexual men, a team of Swedish researchers said in Tuesday's edition of Proceedings of the National Academy of Sciences.

A year ago, the same group reported findings for gay men that showed their brain response to hormones was similar to that of heterosexual women.

In both cases the findings add weight to the idea that homosexuality has a physical basis and is not learned behavior.

"It shows sexual orientation may very well have a different basis between men and women ... this is not just a mirror image situation," said Sandra Witelson, an expert on brain anatomy and sexual orientation at the Michael G. DeGroote School of Medicine at McMaster University in Hamilton, Ontario.

"The important thing is to be open to the likely situation that there are biological factors that contribute to sexual orientation," added Witelson, who was not part of the research team.

The research team led by Ivanka Savic at the Stockholm Brain Institute had volunteers sniff chemicals derived from male and female sex hormones. These chemicals are thought to be pheromones -- molecules known to trigger responses such as defense and sex in many animals.

Whether humans respond to pheromones has been debated, although in 2000 American researchers reported finding a gene that they believe directs a human pheromone receptor in the nose.

The same team reported last year on a comparison of the response of male homosexuals to heterosexual men and women. They found that the brains of gay men reacted more like those of women than of straight men.

The new study shows a similar, but weaker, relationship between the response of lesbians and straight men.

Heterosexual women found the male and female pheromones about equally pleasant, while straight men and lesbians liked the female pheromone more than the male one. Men and lesbians also found the male hormone more irritating than the female one, while straight women were more likely to be irritated by the female hormone than the male one.

All three groups rated the male hormone more familiar than the female one. Straight women found both hormones about equal in intensity, while lesbians and straight men found the male hormone more intense than the female one.

The brains of all three groups were scanned when sniffing male and female hormones and a set of four ordinary odors. Ordinary odors were processed in the brain circuits associated with smell in all the volunteers.

In heterosexual males the male hormone was processed in the scent area but the female hormone was processed in the hypothalamus, which is related to sexual stimulation. In straight women the sexual area of the brain responded to the male hormone while the female hormone was perceived by the scent area.

In lesbians, both male and female hormones were processed the same, in the basic odor processing circuits, Savic and her team reported.

Each of the three groups of subjects included 12 healthy, unmedicated, right-handed and HIV-negative individuals.

The research was funded by the Swedish Medical Research Council, Karolinska Institute and the Wallenberg Foundation.