Thesis defense: Guillaume Giraud

On december 17th, Guillaume Giraud of the team of Samir Merabet will support his thesis entitled:

"Dose and cofactors of Hox proteins during the development of insect flight organs"

Abstract:

During evolution, animals have developed an astonishing morphological diversity, allowing them to adapt to a wide variety of environments. One of the most striking examples is the radiation of flight appendages in insects, starting from ancestors with two similar pairs of wings. Several studies showed the implication of a particular Hox gene, Ultrabithorax (Ubx), in the formation of posterior flight organs. These organs correspond to the "halteres" in Diptera. Conversely, the formation of anterior flight organs is considered to be Hox independent.

During my Ph.D work, we have questioned this model. We showed that the Hox protein Antennapedia (Antp) is produced in the wing primordium of the fruit fly Drosophila melanogaster and is required for the formation of forewings. Surprisingly, the expression level of Antp is much lower than the level of Ubx in the haltere primordium, and the artificial increase of Antp level can transform the wing into a haltere. Conversely, the artificial decrease of the Ubx level can induce a haltere to wing transformation. These results show that this is not the type of the Hox protein, here Antp or Ubx, but the specific dose, which is important for the formation of a wing or a haltere in Drosophila. We also identified the transcription factor Homothorax (Hth) as an upstream regulator of Antp and Ubx in the wing and haltere primordia respectively. The observation of a correlation between Hox dose variation and wing morphology in other insect lineages supports a new model in which morphological diversification of flight organs during insect evolution could arise from changes in the Antp and Ubx expression level.

How the Ubx protein could specify the haltere developmental program at the molecular level remains elusive. It is expected that the dose-specific activity of Ubx in the haltere could rely on the interaction with other transcriptional partners. I present two complementary approaches that I developed to capture such cofactors. While one approach is still ongoing, the other led to the identification of interesting putative Ubx-partners. This work also revealed a surprising robustness in the Ubx transcriptional program that may explain why halteres diverged very little during insect evolution.

The defense will be in french.