Essential biological functions are thought by many to be evolutionarily ancient. By scrutinizing the origin and evolution of a new essential gene in Drosophila melanogaster, Karr and colleagues show that evolution of novel and important functions is, in fact, a continuing process.

Evolution of novelty is poorly understood, but looking at recently evolved genes is likely to provide important insights. Male-specific genes are thought to be among the genes that evolve fastest, so Loppin et al. chose K81, a fly gene that affects male fertility, as a case study.

Intriguingly, database searches, genomic dot blots and PCR failed to identify any K81-related sequences outside the melanogaster subgroup, within which K81 sequence and expression pattern are conserved, indicating functional conservation. Clearly, K81 must have evolved less than ∼30 Mya ago, when the melanogaster subgroup diverged, but how? On the basis of DNA sequence analysis and its genomic location, it is most likely that K81 arose by random retroposition of mRNA from an ancestral locus. The second step towards evolving its new function involved K81 expression coming under the control of a promoter of an upstream gene that is expressed only in the male germline; the ancestral locus is ubiquitously expressed. The final step might have involved adaptive evolution at the K81 locus as the gene acquired new function.

As well as new insights into the evolution of novelty, Loppin et al. provided the first molecular identification of a Drosophila paternal effect gene — its wild-type function is required for zygote viability. These are fascinating topics, and for insights into both the authors recommend turning to essential male-specific genes.