What led to trichome loss in Drosophila sechellia? Srijani Roy, Sujay Naik and Ella Preger Ben-Noon

Animals adapt to their ecological niche by evolving specialised morphologies, physiologies and behaviours. Various studies have revealed adaptive variations between species but very little is known about their genetic and molecular basis.

Drosophila sechellia is an endemic species to the Seychelles archipelago that belongs to the D. melanogaster group of species. D. sechellia feeds and reproduces exclusively on the ripe Morinda citrifolia fruit (noni fruit), a fruit that is toxic to other species in the group. Previous studies suggested that the resistance of D. sechellia to the noni fruit toxicity involves genes of the Osiris family. D. sechellia is also distinguished from its related species by its naked first instar larvae. While larvae of other species are covered with cuticular trichomes, D. sechellia larvae have lost many trichomes due to the loss of expression of the shavenbaby gene in D. sechellia embryos. Shavenbaby encodes a pleiotropic transcription factor that regulates trichome production as well as other developmental processes. While the genetic and molecular mechanisms underlying the loss of trichomes in D. sechellia are well understood, the selective pressure that led to this evolutionary transition remains a mystery.

Here, we hypothesize that Shavenbaby regulates the expression of the Osiris genes, and that this regulatory relationship serves as a strong selective pressure for the loss of shavenbaby expression in D. sechellia embryos. We combine genomic analyses, reporter gene assays and genetic studies to test this hypothesis and to reveal the adaptive benefits that prompt the loss of trichomes in D. sechellia larvae.