Young gene HP6/Umbrea is dispensable for viability and fertility

Studies of the young gene Heterochromatin Protein 6 (HP6) have challenged the dogma that essential functions are only seen in genes with a long evolutionary history. Based on its prominent expression in Drosophila germ cells, we asked if HP6 might play a role in germline development. Surprisingly, we found that CRISPR-generated HP6 null mutants are viable and fertile. We identified an independent lethal allele and an RNAi off-target effect that prevented accurate interpretation of HP6 essentiality in previous studies. We found that the vast majority of young essential genes were viable when tested with orthologous methods. Together our data call into question the frequency with which young genes gain essential functions.