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Dev Biol
2002 Mar 15;2432:272-80. doi: 10.1006/dbio.2001.0564.
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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.
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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.
FIG. 2. Cortical rotation in control and Xklp1-depleted prick-activated oocytes. Some sample germ plasm islands have been colored red to compare their movements. Cortical rotation occurs normally in control (A�C) and Xklp1-depleted (D�F) oocytes, but germ plasm does not localize to the vegetal pole in Xklp1-depleted oocytes (F) as it does in the controls (C). Frames shown in (A, B) and (D, E) are 13 min apart. Scale bar, 74 um.
FIG. 3. Second-phase movements during germ plasm localization in control (A, B) and Xklp1 oligo-injected (C, D) oocytes. Several areas of germ plasm have been highlighted. The time between the first and second frames in each case is 40 min. Individual islands of germ plasm fuse together in the controls, but the area of the oocyte occupied by the germ plasm does not decrease. No fusion occurs in the Xklp1-depleted eggs. Scale bar, 74 um for (A, B, D, E), and double this figure for (C, F).
FIG. 4. Phase three (surface contraction waves) in control (A, B) and Xklp1-depleted (C, D) activated oocytes. In the control, frames are 15 min apart and are immediately before and after the passage of a single wave, which occurred 1.5 h after oocyte activation. The highlighted islands of germ plasm are pushed together by the wave, as are the groups surrounding them (not highlighted), and the total area of egg cortex occupied by germ plasm islands is reduced. In the Xklp1-depleted, the frames are 50 min apart, starting 1 h after oocyte activation. During this period, at least two waves normally occur in control eggs. The total area occupied by germ plasm islands remains the same. Scale bar, 74 um
FIG. 5. Surface contraction wave passes in control egg (blue) but not in Xklp1-depleted egg (red) at end of the first cell cycle. (A) 12 min before cleavage furrow. Animal cap pigment is evenly pigmented. (B) 8 min before cleavage furrow. Wave moves outwards across animal cap, locally distorting the animal cortex and making the pigment lighter in color. The wave is marked by a blue line. (C) 4 min before cleavage furrow. Wave (blue line) has moved further outwards towards equator, and is now towards edge of animal cap. Outward distension of the cortical cytoplasm makes the pigment lighter. (D) Time of first cleavage furrow appearance. Wave has now reached equator. Pigment is now uniform over most of animal cap after passage of wave. Xklp1-depleted egg (red) does not show any change in shape or pigment pattern during this sequence of events in the blue one.
