Interlocking of co-opted developmental gene networks in Drosophila and the evolution of pre-adaptive novelty

The re-use of genes in new organs forms the base of many evolutionary novelties. A well-characterised case is the recruitment of the posterior spiracle gene network to the Drosophila male genitalia. Here we find that this network has also been co-opted to the testis mesoderm where is required for sperm liberation, providing an example of sequentially repeated developmental co-options. Associated to this co-option event, an evolutionary expression novelty appeared, the activation of the posterior segment determinant Engrailed to the anterior A8 segment controlled by common testis and spiracle regulatory elements. Enhancer deletion shows that A8 anterior Engrailed activation is not required for spiracle development but only necessary in the testis. Our study presents an example of pre-adaptive developmental novelty: the activation of the Engrailed transcription factor in the anterior compartment of the A8 segment where, despite having no specific function, opens the possibility of this developmental factor acquiring one. We propose that recently co-opted networks become interlocked, so that any change to the network because of its function in one organ, will be mirrored by other organs even if it provides no selective advantage to them.

Automated summary

The re-use of genes in new organs can lead to evolutionary novelties. This study found that the posterior spiracle gene network in fruit flies has been co-opted for use in the testis mesoderm, where it is required for sperm liberation. This co-option event also led to the activation of the Engrailed transcription factor in the anterior compartment of the A8 segment, controlled by common regulatory elements. This activation is necessary in the testis but not in the spiracle. The study demonstrates a pre-adaptive developmental novelty and suggests that recently co-opted networks become interlocked, influencing other organs even if it offers no selective advantage to them.