Science is Golden: Hot Male on Male Action

The genetics of sexual orientation. Understanding the biological and genetic basis of complex phenotypes, especially phenotypes that include behavior, is very difficult given the complexity of the system and the inability to manipulate humans for genetic studies. For these reasons, simple model organisms, such as fruit files and worms, are used to understand the basics with the goal of gaining insights into the human condition.

Fruit fly males and females have distinct innate sexual behaviors. These behaviors are essential for reproductive success and are hardwired into the brain. Mature males court only females by tapping on the females with their forelegs, vibrating their wings, licking the females, and curling their abdomens. Females don’t court at all.

Video of standard courting behavior


In fruit flies, there is a gene called fruitless that takes on different forms in males and females. In males, a chunk in the middle of the gene is removed.


The researchers* put a female version of the gene into males and put a male version of the gene into females. If fruitless specifies sexual behavior, males with the female form should display little or no courtship activity and any residual courtship behavior should be directed towards males. Females with the male form should display courtship behavior directed towards other females and shouldn’t be receptive to normal males.

Females with the male form of fruitless showed standard male courting behaviors when paired with wildtype females. Males with the female version of fruitless paired with wildtype females showed little to no courting behavior.

Comparing courtship in male-male assays is more difficult than in male female assays. A more reliable way to test for male-male courtship in mutants is to monitor chaining behavior in groups of males. If groups of males with the female version of fruitless are left on plates, they begin to form courtship chains in which each male courts the one ahead of him. It is not clear how this chaining behavior relates to normal courtship, but it show that residual courtship behavior in altered males is directed towards other males.

Chaining: Hot male on male action


The researchers also noticed reduced fertility in females with the male version of fruitless. This could be due to altered sexual anatomy or lack of female behavior. Upon further investigation, they found that in altered females, that eggs were fertilized, but that these females lacked egg-laying behavior. The lack of egg-laying behavior resulted in the observation of fully developed embryos in the uterus of the female and occasionally even a “live birth” as the larva attempted to crawl out through the vagina. These observations suggest that the reduction in fertility is not due to altered anatomy caused by the male version of fruitless, but is due to a change in sexual behavior.

SUMMARY:
1. The female form of fruitless in males results in loss of male courtship behavior and orientation.
2. The male version of fruitless is essential for male behavior
3. Females with male splice forms behave as males
4. A complex innate behavior is specified by the action of a single gene!

Though fruit files and humans are distantly related and sexual orientation and sexual behavior in humans is much more complex, this study provides interesting insights into behavior and orientation and is certainly a leap forward in our understanding of these issues.


*Demir et al (2005) fruitless splicing specifies male courtship behavior in Drosophila. Cell: 121:785-794.