Zebrafish atoh1 genes: classic proneural activity in the inner ear and regulation by Fgf and Notch.
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Hair cells of the inner ear develop from an equivalence group marked by expression of the proneural gene Atoh1. In mouse, Atoh1 is necessary for hair cell differentiation, but its role in specifying the equivalence group (proneural function) has been questioned and little is known about its upstream activators. We have addressed these issues in zebrafish. Two zebrafish homologs, atoh1a and atoh1b, are together necessary for hair cell development. These genes crossregulate each other but are differentially required during distinct developmental periods, first in the preotic placode and later in the otic vesicle. Interactions with the Notch pathway confirm that atoh1 genes have early proneural function. Fgf3 and Fgf8 are upstream activators of atoh1 genes during both phases, and foxi1, pax8 and dlx genes regulate atoh1b in the preplacode. A model is presented in which zebrafish atoh1 genes operate in a complex network leading to hair cell development.