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Surface contraction waves (SCWs) in the Xenopus egg are required for the localization of the germ plasm and are dependent upon maternal stores of the kinesin-like protein Xklp1.

During the first four cell cycles in Xenopus, islands of germ plasm, initially distributed throughout the vegetal half of the egg cortex, move to the vegetal pole of the egg, fusing with each other as they do so, and form four large cytoplasmic masses. These are inherited by the vegetal cells that will enter the germ line. It has previously been shown that germ plasm islands are embedded in a cortical network of microtubules and that the microtubule motor protein Xklp1 is required for their localization to the vegetal pole [Robb, D., Heasman, J., Raats, J., and Wylie, C. (1996). Cell 87, 823-831]. Here, we show that germ plasm islands fail to localize and fuse in Xklp1-depleted eggs due to the abrogation of the global cytoplasmic movements known as surface contraction waves (SCWs). Thus, SCWs are shown to require a microtubule-based transport system for which Xklp1 is absolutely required, and the SCWs themselves represent a cortical transport system in the egg required for the correct distribution of at least one cytoplasmic determinant of future pattern.