'Sexual conflict': Why do sex chromosomes exist? - study

Sex chromosomes, chromosomes responsible for determining if a mammal is male or female, may have evolved to reduce “sexual conflict,” a Japanese study published in July found. 

The peer-reviewed study, published in the academic journal Ecology and Evolution, looked at the evolution of sex chromosomes in the larval stage of fruit flies. 

Previous research led to the estimation that the Y chromosome has lost over 900 active genes over 166 million years. At this rate, genes linked to Y chromosomes are hypothesized to go extinct in 11 million years.

One theory explaining why the Y chromosome is shedding genes is the theory of “sexual conflict.” The theory states that some features that are helpful for one sex are harmful to the other, so the sex chromosome was developed to ensure the optimal survival of each sex. 

While the human Y chromosome is losing genes over time, data has likely been lost over several million years of evolution, making it particularly difficult for scientists to study human development. For this reason, scientists turned to the Drosophila fruit fly. 

Looking at larva

By looking at fruit flies with relatively recently evolved sex chromosomes (neo-sex chromosomes) and comparing them to species that have not developed one, researchers were able to contribute evidence to the theory of sexual conflict.

Researchers classified each gene in the nine genomes as male-biased, female-biased, or unbiased in each stage or tissue.

Flies at the larval stage developed sex-based genes. However, the researchers said this was unexpected as the larvae did not have pronounced sex-specific features.

The sex genes were found to have an associated link with metabolism and later impact the size the flies grow to, creating a distinction between sexes based on body size. The researchers explained that this would then reduce sexual conflict.

“At the larval and pupal stages, there was a general trend that the numbers of sex-biased genes are significantly greater for species with neo-sex chromosomes than species without [them],” the researchers explained. “The greater number of sex-biased genes in species with neo-sex chromosomes than in species without [them] was generally not confined to the neo-sex chromosomes (in comparison with the homologous autosome in close relatives) but was also observed for other chromosomes. 

“Yet, the trend was more conspicuous on the neo-sex chromosomes in many cases. By contrast, in adults and gonads, the differences in the number of sex-biased genes between species with and without neo-sex chromosomes were less clear, or there were fewer sex-biased genes in the former than the latter.”