The frequent occurrence of same-sex behaviors in beetles of one sex could be explained by genes that are favored by natural selection when expressed in the opposite sex, according to a study published in the open access journal BMC Evolutionary Biology.

The study by researchers from Uppsala University, Sweden sheds new light on same-sex sexual behavior in the animal kingdom through examination of the seed beetle, Callosobruchus maculatus, a common beetle found in bean stores across the world.

Same-sex sexual behavior (SSB) is defined as a behavior usually displayed towards the opposite sex during mating and courtship, but which is instead displayed towards individuals of the same sex. Traditionally SSB has been explained in males as misidentification of their own sex for females, coupled with the low cost of promiscuity versus the high cost of missing an opportunity to mate. The same costs and benefits of mating with multiple partners do not apply to females which generally benefit less from this behavior. SSB in females has been suggested to be the mimicking of male behavior, or to be the result of the incomplete 'switching off' of genes that code for male mating behavior and which are shared by both sexes

In this study the scientists looked for evidence to support the theory that genetic links exist between SSB and other characteristics which carry benefits in one sex but not the other. Thus, SSB in one sex could occur because genetically linked traits are favored by natural selection in the opposite sex - the genetic tug of war.

The scientists based their hypothesis on the fact that most genes are expressed in both males and females and often code for more than one characteristic. For example, previous studies have reported that the same genes that code for SSB are also the genes that code for mobility. Mobility is known to be costly to female seed beetles as they do not need to range as far as males to mate.

To test their hypothesis, the team of scientists selectively bred male and female beetles to display increased SSB, studying how this affected their mobility and reproductive success compared to beetles that had been bred to display decreased SSB. The scientists showed that when a particular sex had been bred for increased SSB, siblings of the opposite sex enjoyed an increase in reproductive performance. They also showed changes in traits such as mobility and sex recognition after selective breeding on SSB, providing evidence for genetic links between SSB and these traits across the sexes, according to the researchers.

Dr David Berger, lead author of the research paper, said: "Our findings show that studying the genetic links between different characteristics in males and females can hold major clues to how genetic conflicts between the sexes shape the evolution of traits, and same-sex sexual behaviors are just one example of this. The genetic mechanism explaining the occurrence of SSB that we demonstrate in these beetles could apply equally well in very different animals."

Source: BioMed Central