What is a parasitism gene?

Parasites evolved from free-living ancestors and in so doing their genomes changed, including through the evolution of new genes, commonly called parasitism genes. Accurately identifying these parasitism genes will help us to better understand parasites and their parasitism. There are a number of commonly used approaches to doing this, but they all have important caveats, which we consider. While not having a wholly new solution to the problem of identifying parasitism genes, we suggest that wider genomic features of parasitism genes and their expression might be relevant. We emphasise the importance of being cautious and circumspect when identifying a gene as a parasitism gene, and doing this will bring greater clarity to understanding the most fascinating of lifestyles.