Development of oculomotor circuitry independent of hox3 genes.

Hox genes have been shown to be essential in vertebrate neural circuit formation and their depletion has resulted in homeotic transformations with neuron loss and miswiring. Here we quantifiy four eye movements in the zebrafish mutant valentino and hox3 knockdowns, and find that contrary to the classical model, oculomotor circuits in hindbrain rhombomeres 5-6 develop and function independently of hox3 genes. All subgroups of oculomotor neurons are present, as well as their input and output connections. Ectopic connections are also established, targeting two specific subsets of horizontal neurons, and the resultant novel eye movements coexists with baseline behaviours. We conclude that the high expression of hox3 genes in rhombomeres 5-6 serves to prevent aberrant neuronal identity and behaviours, but does not appear to be necessary for a comprehensive assembly of functional oculomotor circuits.