Development of utricular otoliths, but not saccular otoliths, is necessaryfor vestibular function and survival in zebrafish

We have been studying the consequences of embryonic vestibular dysfunction caused by the monolith (mnl) mutation in zebrafish. mnl is a dominant mutat ion that specifically inhibits formation of utricular otoliths. However, br iefly immobilizing mnl/ mnl embryos in agarose with the otic vesicle orient ated at certain angles selectively induces or prevents formation of utricul ar and/or saccular otoliths. With this noninvasive technique, we generated six phenotypic classes of mnl/mnl mutants, designated S-S, U-U, U-S, S-US, U-US, and US-US, depending on which otoliths are present on each side (U, u tricular otolith; S, saccular otolith). All mnl/mnl larvae survived through day 10 of development. Thereafter, S-S larvae showed a rapid decline, prob ably because of starvation, and none survived to adulthood. Survival rates in all other classes of mnl/mnl larvae (those having at least one utricular otolith) were close to normal. The presence or absence of utricular otolit hs also correlated with vestibular function during early larval development , as measured by three criteria: First, unlike wild-type larvae, S-S mutant larvae showed almost no detectable counter-rotation of the eyes when tilte d tail up or tail down. Second, 95% of S-S mutant larvae never acquired the ability to maintain a balanced dorsal-up posture. Third, although most wil d-type larvae responded to gentle prodding by swimming in a straight line, S-S larvae responded by swimming in rapid circles, showing sudden and frequ ent changes in direction ("zigzagging"), and/or rolling and spiraling. All other phenotypic classes of mnl/mnl larvae behaved normally in these assays . These data demonstrate that bilateral loss of utricular otoliths disrupts the ability to sense gravity, severely impairs balance and motor coordinat ion, and is invariably lethal. The presence of a utricular otolith in at le ast one inner ear is necessary and sufficient for vestibular function and s urvival. In contrast, saccular otoliths are dispensable for these functions . (C) 2000 John Wiley & Sons, Inc.