The avian auditory brainstem nuclei nucleus magnocellularis (NM) and nucleus laminaris (NL) display highly precise patterns of neuronal connectivity. NM projects tonotopically to the dorsal dendrites of ipsilateral NL neurons and to the ventral dendrites of contralateral NL neurons. The precision of this binaural segregation is evident at the earliest developmental stage at which connections can be observed. We have begun to examine the possibility that Eph receptor tyrosine kinase signaling is involved in establishing these spatially segregated connections. The expression of the EphA4 tyrosine kinase was examined at several developmental stages. EphA4 is expressed in rhombomere 5, which contains progenitors for both NM and NL. In this rhombomere, the labeling becomes striped during the time that precursor cells migrate to the auditory anlage. At the precise time when NM-NL projections are forming, EphA4 expression in NL is asymmetric, with markedly higher expression in the dorsal NL neuropil than in the ventral neuropil, suggesting a possible role in guiding growing axons to the appropriate region. At later embryonic ages EphA4 expression is symmetric around NL, and is absent in NM. As auditory function matures, EphA4 expression decreases so that by 4 days after hatch no EphA4 antibody labeling is evident in the auditory brainstem nuclei.

We describe a mutant, maelstrom, that disrupts a previously unobserved step in mRNA localization within the early oocyte, distinct from nurse-cell-to-oocyte RNA transport. Mutations in maelstrom disturb the localization of mRNAs for Gurken (a ligand for the Drosophila Egf receptor), Oskar and Bicoid at the posterior of the developing (stage 3–6) oocyte. maelstrom mutants display phenotypes detected in gurken loss-of-function mutants: posterior follicle cells with anterior cell fates, bicoid mRNA localization at both poles of the stage 8 oocyte and ventralization of the eggshell. These data are consistent with the suggestion that early posterior localization of gurken mRNA is essential for activation of the Egf receptor pathway in posterior follicle cells. Posterior localization of mRNA in stage 3–6 oocytes could therefore be one of the earliest known steps in the establishment of oocyte polarity. The maelstrom gene encodes a novel protein that has a punctate distribution in the cytoplasm of the nurse cells and the oocyte until the protein disappears in stage 7 of oogenesis.