A new study conducted on beetles shows that the Y chromosome contains very little genetic material, but still significantly influences the size of males.
- Even though the Y sex chromosome contains only a small amount of genome, it harbors significant genetic variations, which can significantly alter the size difference between the sexes.
- Experiments carried out on beetles, on the other hand, show that sexual dimorphism (the genetic differences between the sexes) could only evolve in males, and not in females, and vice versa.
In every species, including the human species, females and males most often show many morphological, physiological and behavioral differences. But the evolution of these sexual differences, known as sexual dimorphism, still remains a puzzle for scientists. While they share the same genome, how to explain that females and males are so different? Moreover, an evolutionary change in one sex should lead to a correlated change even in the other sex, thus preventing the evolution of sex differences.
In a new study published in the journal Nature Ecology & Evolutionresearchers from Uppsala University (Sweden) show that even small amounts of genetic differences between the sexes can facilitate the evolution of sexual dimorphism, so that it can evolve in just a few generations.
“Our experiments show that the autosomes (non-sex chromosomes, editor’s note) as well as the two sex chromosomes, the X and the Y, can harbor genetic variations important for sexual dimorphism, explains Philipp Kaufmann, first author of the work. But the Y chromosome alone can alter the height difference between the sexes by up to 30 percent.”
Genetic height variations linked to the Y chromosome
To reach this conclusion, the team conducted their research on beetles, the family of insects to which beetles, cockchafers and ladybugs belong. Researchers characterized the genetic architecture of body size in males and females by creating a large family tree of over 8,000 scarab beetles Callosobruchus maculatus. This multi-generational family tree was used to quantify genetic variation in body size linked to autosomal and sex chromosomes, and allowed to see how different forms of selection affect the evolution of size dimorphism. The experiments were conducted either on males only, females only, or sexually antagonistically in both sexes.
After ten generations of selection, the sexual size dimorphism was compared between the selection lines and the ancestral pedigree population. These experiments clearly indicated that the Y chromosome plays an important role in determining the response of males to selection.
According to Philipp Kaufmann, the fact that the Y chromosome is responsible for the size of the sexes up to 30% is remarkable because in these insects, “the Y chromosome contains only a handful of genes and is a very small fraction of the genome, just like in humans”. “Many have thought that the Y chromosome only affects the most important reproductive processes in males, namely sperm production. Our results suggest that the Y chromosome may have a broader role than previously thought”he continues.
Gender differences that evolve in both males and females
The researchers found, however, that the evolution of sex differences between males and females does not only depend on where the genetic variation is in the genome, but also on how natural and sexual selection can act on it. The team thus demonstrated in the laboratory that sexual dimorphism could evolve in the event of selection on the size of males, but that when selection acted only on females, the shared part of the genome provoked a correlated evolutionary response in males. , preventing the evolution of dimorphism.
“The most drastic change in sexual dimorphism, a 50% increase in just ten generations, occurred when we applied selection in a sexually antagonistic way – favoring the opposite body size in both sexes. This shows that with the right kind of selection, sex differences can evolve quickly, perhaps even more easily than previously thought”says Elina Immonen, assistant professor in the department of ecology and genetics at Uppsala University. “Future work will allow us to learn more about how the Y chromosome can have such a strong effect on males, and the general role it plays in the evolution of sex differences between taxa.”concludes the researcher.
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