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The evolution of sex variations in physical size is aided by the male Y chromosome.

Males and females are very different in many aspects, although they both have the same genome. The male Y chromosome is the lone exception. Using beetles as a model, new research from Uppsala University indicates that, although having only a few genes, the Y chromosome may substantially alter male body size, facilitating the evolution of sex differences.

Males and females differ in anatomy, physiology, and behavior in a variety of ways. Because females and males share the same set of genes, an evolutionary change in one sex should generate a correlative change in the other sex, preventing sex differences from evolving, how such sex differences, known as sexual dimorphism, evolve is a mystery. According to the new research, even minor genetic differences between the sexes can speed up the evolution of sexual dimorphism, allowing it to emerge in just a few generations.

"Our findings reveal that the autosomes, as well as both sex chromosomes, the X and Y, can carry genetic variation critical for sexual dimorphism, but the Y chromosome alone can change the size of the sex difference by up to 30%." This is noteworthy since, like humans, the Y chromosome in these beetles includes only a few genes and makes up a small portion of the genome. Many people believe that the Y only influences males' most crucial reproductive activities, such as sperm production. According to Philipp Kaufmann, a PhD student at Uppsala University's Department of Ecology and Genetics and the study's first author, "Our findings show that the Y chromosome may have a broader role than previously thought."

However, the evolution of sexual dimorphism is influenced not just by where genetic diversity is found in the genome, but also by how natural and sexual selection might work on it. The research team used lab evolution to show that while choosing on male size, sexual size dimorphism may evolve, but when selecting just on females, the shared component of the genome induced a correlated evolutionary response in men, preventing dimorphism from emerging.

"When we used sexually antagonistic selection, favoring the opposite body size in both sexes, we saw the most dramatic rise in sexual dimorphism, up to 50% in only ten generations. This demonstrates that sex differences may obviously evolve quickly, perhaps more easily than previously anticipated, under the proper kind of selection  according to Elina Immonen, an assistant professor at Uppsala University's Department of Ecology and Genetics and the study's principal investigator.

"Combining information on the types of genetic variation available to selection with various forms of selection is an effective tool to test the causes of sex differences evolution. We were able to explicitly demonstrate how substantial the effect of Y chromosome variation is by isolating it from the rest of the genome, something we didn't expect to see when we started the work, and this has helped us understand how sexual dimorphism has developed in this species. Future research will reveal more about how the Y chromosome may have such a huge impact on males, as well as how widespread its function in the evolution of sex differences across species "Immonen comes to a conclusion.

More information on the experiments

The researchers used a huge pedigree of over 8,000 beetles to analyze the genetic architecture of body size in males and females for their study (the seed beetle Callosobruchus maculatus). The autosomal and sex chromosome associated genetic variance in body size was quantified using this multi-generational family tree. Artificial selection was used to see how different types of selection effect the evolution of size dimorphism, such as selection acting exclusively on males, only on females, or selection operating sexually antagonistically (opposite directions) in both sexes. The sexual size dimorphism between the selection lines and the ancestral pedigree population was compared after ten generations of selection. These two tests demonstrated that the Y chromosome is critical in determining male selection response. A third experiment was conducted to further investigate the effect of Y related variation in isolation from variation in the rest of the genome. By inserting various Y chromosomes into a genetically identical background, they were able to isolate the effect of the Y chromosome on sexual size dimorphism in these beetles. To put it another way, making beetles that are identical twins except the Y chromosome.

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