How cells integrate the input of multiple polarizing signals during division is poorly understood. We demonstrate that two distinct Caenorhabditis elegans Wnt pathways contribute to the polarization of the ABar blastomere by differentially regulating its duplicated centrosomes. Contact with the C blastomere orients the ABar spindle through a nontranscriptional Wnt spindle alignment pathway, while a Wnt/beta-catenin pathway controls the timing of ABar spindle rotation. The three C. elegans Dishevelled homologs contribute to these processes in different ways, suggesting that functional distinctions may exist among them. We also find that CKI (KIN-19) plays a role not only in the Wnt/beta-catenin pathway, but also in the Wnt spindle orientation pathway as well. Based on these findings, we establish a model for the coordination of cell-cell interactions and distinct Wnt signaling pathways that ensures the robust timing and orientation of spindle rotation during a developmentally regulated cell division event.

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In this study we report a relationship between the ultrastructure of the neuromuscular junctions of tibial muscles and the temperature-induced paralysis in shibire flies. There is a decrease in the number of synaptic vesicles of neuromuscular junctions in flies which are held at or above 29~Shortly after return to 22~ the synaptic vesicles are again present in large numbers. Prior treatment with tetrodotoxin or barbiturate protects the junctions from the temperature-induced change in morphology.KEY WORDS neuromuscular junction 9 temperature-sensitive synaptic vesicles shibire t" 9 mutantThe structure, function, and development of the nervous and muscular systems are strikingly similar across many diverse phyla. Thus, it is reasonable to exploit the particular advantage of one organism, Drosophila melanogaster, under the assumption that a genetic approach will facilitate the dissection of complex problems in physiology and development.The strategy of selecting flies that are paralyzed at a restrictive temperature (12) has yielded a number of mutations at several loci (1). One mutant in particular has been the subject of careful analysis in several laboratories. This mutant, shibire t~l (shitsl), is characterized by reversible paralysis of both adult flies and larvae at temperatures above 28~ (1). It also exhibits a number of diverse phenotypic responses to heat pulses administered at different developmental stages (7). A defect in some aspect of neural function was suggested by the reversible loss of on-and offtransients of electroretinograms in shi ''1 flies at the restrictive temperature (5). The paralysis has been localized by electrophysiological methods to transmission failure at the neuromuscular junction (11, 3), since at high temperature the presynaptic nerve was able to conduct impulses and the muscle responded to direct stimulation. Further examination of this system, where presumably the temperature sensitivity of a single gene product is responsible for the effects on nervous activity, may be useful in understanding both synaptic transmission and the turnover processes of the synaptic vesicles at a molecular and genetic level.In this study we report a relationship between the ultrastructure of the neuromuscular junction of tibial muscles and the temperature-induced paralysis in shibire flies. MATERIALS AND METHODSThe principal experimental strain used in this study was shi m. Oregon R served as the wild-type control. One experiment used shit~6/df(1)sd 72a26, in which shi t~6 is left 520 J. CELL BIOLOGY 9 The Rockefeller University Press 9

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