Most arthropods harbour a variety of endosymbionts that shape the life-history of their hosts. The recognition that organisms are comprised of entities from different taxa has challenged how we view organismal evolution. Although we know that organelles (for example, mitochondria and plastids) can derive from the integration of multiple organisms, the extent to which organisms harbour sequences of diverse origin has only been appreciated in the past few decades. The rise of cost-effective sequencing has revealed that most genomes contain a diverse array of repetitive DNA elements and sequences of foreign origin, in part derived by lateral gene transfer (LGT), also known as horizontal gene transfer. This realization has revolutionized our understanding of genome evolution.

A landmark study by Dunning Hotopp et al., published almost two decades ago, catalysed broad discussion and evaluation of the extent and functional relevance of LGT-derived genes. The authors provided evidence for extensive LGT from the intracellular bacterium Wolbachia to the nuclear genomes of several insect and nematode hosts. This striking claim suggested that genes derived from endosymbionts can be co-opted to take on novel functions in their host, with clear evolutionary ramifications. At first glance, this idea seems intuitive — the close association and co-evolution of host and endosymbiont increase the probability of gene exchange, particularly as many endosymbionts reside in host reproductive tissues, potentially facilitating incorporation into the germline.