Subterranean Fishes of the World

3206

2013

AIM 2013

The anatomical organization and the size of the prosencephalon are set up during embryogenesis by morphogen signals which are secreted by the organizer centers of the neural plate and neural tube. These molecules activate signaling pathways and developmental gene networks which specify the fate of neural progenitors by controlling proliferation, neurogenesis, migration and differentiation events. Two of these essential signaling centers are the ventral midline center secreting Shh which is expanded in space in cavefish (heterotopy), and the rostral midline center producing Fgf8 which is shifted in time in cavefish (heterochrony). In another hand, the formation of the olfactory system results from association of the olfactory epithelium of placodal origin with the olfactory bulbs which are part of the pallial telencephalon. The olfactory bulbs in vertebrates contain two main types of neurons: the mitral cells (neurons projecting to other forebrain regions) and the inhibitory GABAergic interneurons (born in the ventral telencephalon) and migrate through the RMS (the rostral migratory stream) to integrate the olfactory bulbs and contribute to olfactory learning and discrimination. Our preliminary observations have shown that the olfactory bulbs are bigger (larvae stages examined) in cavefish and that this difference may stem in part from a differential neurogenesis of the interneurons population, as suggested by the increased expression of the transcription factor Lhx6. Indeed, expression analysis using gene markers of the basal telencephalon, coupled to pharmacological treatments of cavefish embryos using cyclopamine (an inhibitor of the Shh signaling pathway) have shown increased migrations of GABAergic interneurons from the ventral telencephalon to the olfactory bulbs in cavefish embryos (Menuet et al, 2007). AIM We would like to test the possibility that the olfactory system is coordinately modulated in cavefish, due to modifications of organizer centers (see also Bibliowicz‘s talk). We quantified proliferation and neurogenesis of interneurons in cavefish and surface fish olfactory system using EdU incorporation methods. In parallel, we compared mRNA expression of different factors such as pcna, ngn1, neurod, ascl1 to analyze proliferation and neurogenesis events in the olfactory system. In a near future, we will test the Shh- or Fgfdependence of these events and we will test olfactory capabilities of fishes in these developmentally-modified contexts. Work supported by ANR grant [ASTYCO]

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