THE MOLECULAR CORRELATES OF TOPOGRAPHICAL MAPS IN THE VERTEBRATE OLFACTORY BULB

The vertebrate olfactory bulb is topographically organized, where distinct odors are encoded in spatially distinct regions. This chemotopic map plays a crucial role in olfactory computations. However, the molecular correlates of this organization in projection neurons and interneurons are not fully understood. By generating and utilizing single-cell and spatial transcriptomic atlases for zebrafish and mouse, we investigated the molecular correlates of olfactory bulb topography in two vertebrates. We identified and matched basic cell types across species, revealing a continuum of gene expression diversity in classes of glutamatergic projection neurons and GABAergic interneurons. In zebrafish, projection neurons in spatially distinct olfactory bulb zones exhibit a high degree of molecular similarity. These molecularly identifiable groups of projection neurons are organized into non-overlapping regions across the dorsoventral and mediolateral axes of the olfactory bulbs, and overlap with zones encoding alarm, food, and social odors. In mouse, projection neurons exhibit lower degree of molecular topography, with slight but observable molecular differences for dorsal olfactory bulb projection neurons. In both species, interneurons exhibit a molecular topography, where the primary axis of transcriptomic diversity is the depth. While we observed no obvious zonal organization of interneuron molecular topography along the dorsoventral and mediolateral axes of the mouse olfactory bulb, the zebrafish olfactory bulb exhibits a prominent separation of molecularly different interneuron groups along these dimensions, similar to zebrafish projection neurons. Our results revealed a comparative atlas for the molecular logic of olfactory bulb neurons, that follows different but partially overlapping principles in teleost fish and mammals.