What we know—and don't know—about the biology of homosexuality

by jeeg 20. December 2012 21:31

The media was abuzz this week after an international group of researchers proposed that scientists may have been looking for the biological underpinnings of homosexuality in the wrong place. Although scientists have spent the last few decades scouring our genome for a “gay gene,” William Rice, Urban Friberg, and Sergey Gavrilets suggest in The Quarterly Review of Biology that homosexuality may have its roots in epigenetics, rather than in genetics.

According to the authors, much of we know about homosexuality suggests that it is not simply a result of direct genetic inheritance. First, despite thorough genome-wide research, no study has been able to find a gene or genetic marker that is consistently associated with homosexuality. Second, although twenty to fifty percent of the variation in sexual orientation appears to be inherited in some way, identical twins don’t necessarily share a sexual orientation; if one twin is gay, there’s only a twenty percent probability that the other twin is, too. This low probability (or “concordance”) suggests that simple genetic inheritance might not drive sexual orientation. Finally, the authors argue that any purely genetic “fitness-reducing phenotypes” like homosexuality would be selected against and weeded out of the gene pool.

Instead, the researchers suggest, epigenetic inheritance via “epi-marks” might be responsible for sexual orientation. Epi-marks are physical changes in our genetic material (such as chemical modification or changes in DNA packaging proteins) that regulate gene activity without actually changing the sequence of bases.

Rice, Friberg, and Gavrilets argue that epigenetics could theoretically play a role in sexual behavior via sex-specific epi-marks that affect androgen sensitivity. Under their hypothesis, female embryos would normally develop female-specific epi-marks which would decrease sensitivity to circulating testosterone and feminize the fetus. Male embryos, meanwhile, would gain male-specific epi-marks that would increase testosterone sensitivity and masculinize the developing fetus.

Normally, epi-marks are “erased” each generation, so they aren't usually passed along to offspring; however, in some instances, epi-marks can persist and be inherited by the next generation. The researchers argue that cross-sex epigenetic inheritance—either from mother-to-son or father-to-daughter—could contribute to homosexuality. A female-specific epi-mark that persisted in male offspring that inherited it would trigger feminization—and possibly sexual attraction to males. A similar scenario could happen if a male-specific epi-mark was inherited by a daughter, causing masculinization and, potentially, sexual attraction to other females.

By feminization of males and masculinization of females, the researchers are referring to “gonad-trait discordance,” which indicates that a person’s genitals and sexual preference aren’t in line with what is generally considered “normal.” The authors also cite two other androgen-influenced traits (cryptorchidism and hypospadias) that may be related to sex-specific epi-marks. However, “may” is the key word, since none of these hypotheses have been tested with real data.

In the end, that’s where the paper leaves us: with an untested hypothesis that appears to fit the statistical frequencies and inheritance patterns of homosexuality. The researchers use a mathematical formula to illustrate that this type of epigenetic inheritance could occur, but the paper is purely theoretical rather than data-driven, and it refers to remarkably few human studies.

This lack of human-related studies is one of the major criticisms of epigenetics today. As John Timmer explains, there’s very little evidence that epigenetic inheritance across generations plays a significant role in human behavior, at least for now.

There’s also little discussion in the paper of the spectrum of human sexual behavior, which ranges from those who engage solely in same-sex sexual encounters to those who simply like to experiment once in a while. The authors do, however, state that for the paper, the definition of homosexuality includes “any same-sex partner preference… including bisexuality.” And it doesn't deal at all with the fact that sexual preference is just one part of a large suite of behaviors that can be some combination of "masculine" and "feminine."

It’s important to see this paper as what it is—a theoretical possibility with a lot of unanswered questions—rather than a tested scientific conclusion. It’s easy for media outlets to overhype this type of untested hypothesis; some are already touting epigenetics as the "answer to homosexuality." The paper is also an easy target for outspoken activists, such as Bryan Fischer of the American Family Association, who is using the paper as anti-gay political fodder.

Homosexuality helps fish get girls

Another recent study also takes issue with one of Rice, Friberg, and Gavrilets’ claims—that homosexuality is necessarily a “fitness-reducing phenotype.” A recent study in Biology Letters suggests that, in some species, homosexual behavior can confer advantages in terms of reproductive success.

The researchers behind this experiment studied mollies (Poecilia mexicana), freshwater fish found in Mexico. Female mollies engage in what is called “mate choice copying,” meaning that they prefer to mate with males they have watched copulate with other females. Not only does this strategy make it easier for females to find mates, it also gives them clues about male quality.

But homosexual behavior among males is also common among mollies. This behavior, called “nipping,” occurs when a male nips at the genitals of another male. The researchers wondered if homosexual behavior would affect females’ preferences in the same way as heterosexual behavior does. Would seeing a male engage in nipping increase a female’s attraction to him?

There are two kinds of male mollies: dominant males, which are large and brightly colored, and subordinate males, which are small and drab in color. In both observations of natural behavior and in video playback tests conducted by the researchers, female mollies preferred dominant males to subordinate ones by a large margin.

However, video playback tests showed that the perceived attractiveness of subordinate males skyrocketed after females watched them engage in sexual behavior. In fact, this made such a difference that subordinate males that had been observed in a sexual interaction actually became preferable to dominant males that hadn't. Moreover, it didn’t matter whether males nipped at other males or copulated with females; engaging in any sexual behavior at all increased female preference for the males.

In nature, subordinate males are generally the ones engaging in homosexual nipping behavior. The researchers suggest that this behavior might be an alternative strategy: if less desirable males can’t get a boost in sexual attractiveness by actually mating, homosexual nipping behavior may be another way to get the girl. In this system, it appears that occasional homosexuality may actually increase fitness, at least for some males.

What it all means

It’s important to note that male mollies do not engage exclusively in homosexual behavior; in other words, males that nip at other males also mate with females. Exclusive homosexuality has only been recorded in a few species, such as humans, sheep, and some birds. It’s possible that the factors that drive these two types of homosexuality are quite different.

There have been studies of exclusive homosexuality in these species, and they have turned up some interesting information. We now know that the incidence of male exclusive homosexuality is about the same in humans and sheep, and that male sheep that engage exclusively in intercourse with other rams have different brain morphology and activity than male sheep that seek out sex with ewes. It has also been suggested (and demonstrated in some populations) that the female relatives of homosexual men have more offspring than those of straight men.

As Rice, Friberg, and Gavrilets note, the inheritance of homosexuality doesn’t appear to be as simple as a “gay gene.” But for now, we simply don’t know what factors contribute to homosexuality in humans or in most other species. While epigenetics may help us understand homosexuality one day, we’re not there yet.

 Kate Shaw, Ars Technica

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