Pujol-Martí, J., Zecca, A., Baudoin, J.P., Faucherre, A., Asakawa, K., Kawakami, K., and Lopez-Schier, H. (2012) Neuronal birth order identifies a dimorphic sensorineural map. The Journal of neuroscience : the official journal of the Society for Neuroscience. 32(9):2976-2987.
Spatially distributed sensory information is topographically mapped in the brain by point-to-point correspondence of connections
between peripheral receptors and central target neurons. In fishes, for example, the axonal projections from the mechanosensory
lateral line organize a somatotopic neural map. The lateral line provides hydrodynamic information for intricate behaviors
such as navigation and prey detection. It also mediates fast startle reactions triggered by the Mauthner cell. However, it
is not known how the lateralis neural map is built to subserve these contrasting behaviors. Here we reveal that birth order
diversifies lateralis afferent neurons in the zebrafish. We demonstrate that early- and late-born lateralis afferents diverge
along the main axes of the hindbrain to synapse with hundreds of second-order targets. However, early-born afferents projecting
from primary neuromasts also assemble a separate map by converging on the lateral dendrite of the Mauthner cell, whereas projections
from secondary neuromasts never make physical contact with the Mauthner cell. We also show that neuronal diversity and map
topology occur normally in animals permanently deprived of mechanosensory activity. We conclude that neuronal birth order
correlates with the assembly of neural submaps, whose combination is likely to govern appropriate behavioral reactions to
the sensory context.