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Drosophila Oogenesis: Generating an Axis of Polarity
Current Biology, Vol. 12, R835–R837, December 23, 2002, ©2002 Elsevier Science Ltd. All rights reserved. PII S0960-9822(02)01346-5
Drosophila Oogenesis: Generating an Axis of Polarity Buzz Baum
New studies of moesin function during Drosophila oogenesis show that Dmoesin tethers actin to the oocyte membrane. Interestingly, Dmoesin and an intact cortex are also required to stabilise ooycte polarity.
In eukaryotic cells, a global axis of polarity is established using two general mechanisms. In one, an oriented cytoskeleton is assembled which directs polar intracellular transport. In the other, cells establish stable marks within the actin-rich cortex which serve as reference points to capture and concentrate specific polar cell markers as they pass by. Often these systems work in tandem [1], so that cells polarize their cytoskeleton with respect to an existing cortical cue. Determinants of polarity are then transported along cytoskeletal filaments to this site, where they become anchored, amplifying the polar signal. To establish anterior–posterior polarity during Drosophila oogenesis, microtubules direct the transport of oskar mRNA to one end of the oocyte, which then defines the future posterior pole of the embryo [2–4]. Two new studies [5,6] of the function of the sole Drosophila member of the ezrin–rodexin–moesin (ERM) protein family [7], Dmoesin, have now shown that an actin-based cortical anchor is required to maintain this polarised distribution of oskar mRNA. The large cells of the Drosophila egg chamber make it an excellent system for identifying genes involved in the generation of cell polarity. Early on during oogenesis, a microtubule organising centre (MTOC) forms within the oocyte, which directs the transport of new synthesized materials along microtubules from the nurse cells into the developing oocyte [8]. Then, at stages 7–8, the oocyte microtubule cytoskeleton undergoes a dramatic re-organisation in response to a signal from the overlying posterior follicle cells [8]. The MTOC is disassembled and microtubules become nucleated at the anterior cortex of the oocyte, generating a gradient of microtubules with their ‘plus’ ends tightly focused at the posterior pole [9] (Figure 1). As microtubule inhibitors [10] and mutations which disrupt microtubule organisation [11] or microtubule motor function [10] block the proper localization of polar markers such as oskar mRNA, microtubules are thought to play a critical role in the establishment of oocyte polarity. Although the evidence that microtubules contribute to the establishment of oocyte polarity is compelling, the role of the actin cytoskeleton in this process is Ludwig Institute for Cancer Research, University College London branch, 91 Riding House Street, London W1W 7BS, UK. E-mail: [email protected]
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less clear. Rather surprisingly, however, several genes identified by their mRNA mislocalisation phenotype were found, upon cloning, to code for conserved actin-binding proteins. Several of these mutations, such as those affecting profilin [12], disrupt oocyte polarity, probably in part as an indirect consequence of changes in microtubule organisation. The new functional analysis of Dmoesin [5,6], however, convincingly shows that the actin-rich cortex plays a second role in
Figure 1. Actin filaments and microtubules cooperate to generate a stable axis of polarity during Drosophila oogenesis. The figure shows wild-type (top) and Dmoesin-deficient (bottom) egg chambers at stage 9–10. An uncharacterised signal from the posterior follicle cells (purple) has generated a polarised microtubule array aligned along the anterior–posterior axis of the oocyte. Once microtubule plus ends predominate at the posterior pole the oocyte, at stage 9–10, oskar mRNA is guided to the pole by a plus-end-directed microtubule motor (kinesin I). On reaching the microtubule plus ends, oskar mRNA complexes are captured by a cortical actin-filament based anchor which requires tropomyosin II, Dmoesin and Cap for its proper construction. As Oskar protein helps to recruit oskar mRNA, this initial asymmetry is rapidly amplified. At stage 10B, the polar microtubule array is then disassembled as nurse cells begin to dump their contents into the oocyte. But oskar mRNA remains tethered within the cortical actin mesh at the posterior of the oocyte, capturing newly synthesized components of the pole plasm as they pass by. During embryogenesis, these localised determinants then specify germ cell fate and direct the formation of the embryo’s abdomen. In the dmoesin mutant, oskar mRNA is frequently seen binding to the detached actin cortex at the posterior pole, or appears diffuse, while microtubules are unaffected. Microtubules are shown in green; microtubule plus ends are denoted +; kinesin is shown in yellow; actin is shown in red; complexes containing oskar mRNA are in black; anterior is to the left, posterior to the right.
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the localisation of posterior determinants within the Drosophila oocyte. Polesello et al. [5] and Jankovics et al. [6] identified Dmoesin in screens for flies whose offspring lack abdomens and germ cells — a phenotype typical of mutations that disrupt the localization of oskar mRNA during oogenesis. In Dmoesin-deficient germline clones, the F-actin rich oocyte cortex is often found loose, far from the plasma membrane. So like its mammalian counterparts [7], Dmoesin is required to crosslink actin filaments to the cell periphery. Strikingly, in some instances, when actin filaments become detached from the membrane at the posterior end of the oocyte, oskar mRNA and other posterior determinants are found bound to this filamentous actin mesh, while microtubule plus ends remain tightly focused at the pole. This shows that oskar mRNA is anchored within the actin cortex at the posterior end of the oocyte. Interestingly, the activation of Dmoesin by phosphorylation of conserved residue threonine 559 may be a rate-limiting step in the localisation of oskar mRNA, because expression of a mutated, phospho-mimetic form of Dmoesin in the wild-type was found to increase the accumulation of posterior determinants at the pole of the oocyte [5]. This also suggests that the mechanisms regulating Dmoesin activity are likely to be conserved between Drosophila and mammalian cells [6]. Two other actin-binding proteins, tropomyosin II [13] and Cap [14], are also required to anchor oskar mRNA at the posterior end of the oocyte. In sum, the data suggest that, during oogenesis, oskar mRNA is first transported to the ends of microtubules by the plus-end directed motor kinesin I [15]. Then, upon arrival at the posterior pole of the oocyte, oskar mRNA becomes tethered at the cortex. In other systems, tropomyosins and activated ERM proteins [7] help assemble parallel bundles of actin filaments, for example in muscle and in microvilli. Therefore the precise architecture of the actin cortex is likely to be critical for oskar mRNA binding. Is there something special about the cortex at the posterior pole of the Drosophila oocyte that enables it to function as an anchor for oskar mRNA? At present, the only evidence for the existence of a distinct oocyte posterior domain is the localisation of the small GTPase Rab11 [16]. Rab11 is found at the posterior pole of the oocyte in the wild type and in egg chambers with a defective polar microtubule array. It seems unlikely that Dmoesin has a specific function at the posterior pole of the oocyte, however, because Polesello et al. [5] found that both Dmoesin and its activated derivative are uniformly distributed along the oocyte cortex. Moreover, both groups [5,6] found that germline Dmoesin is also needed to maintain the tight apposition of actin filaments and the lateral oocyte membrane. In a study [17] designed to test the existence of a posterior anchor, labeled oskar mRNA was injected into oocytes at different stages in their development. Surprisingly, a proportion of the injected mRNA pool became localised at the posterior pole, before and after the dissolution of the polar microtubule array at stage
10B, even in the absence of microtubules. Although this implies the presence of a differentiated posterior cortex, the data may simply reflect the existence of a ‘feedforward’ system. As Oskar protein binds oskar mRNA [2,3], newly synthesized or injected oskar mRNA will preferentially bind previously localised Oskar complexes at the pole. Similar mechanisms are likely to operate in many systems, helping to break cellular symmetry by ensuring that initial differences in the localisation of polar determinants are accentuated over time. Thus, it is not clear from existing data whether the oocyte cortex is subdivided into distinct posterior, lateral and anterior domains. The alternative is that the posterior pole of the ooycte is defined solely by the axis of microtubule polarity, and it remains a mystery how the polar microtubule array is established. The existence of a cortical anchor for posterior determinants is likely to serve several related functions. First, an anchor is absolutely required to maintain the polar localisation of posterior determinants during ‘cytoplasmic streaming’, when the ooplasm swirls in a washing-machine-like vortex [8]. In fact, oskar mRNA must remain tethered at the cortex until early embryogenesis [18,19]. Second, an anchor may also be required at late stages to capture residual posterior determinants as they enter the oocyte from nurse cells [13]. Finally, the anchor may be an important element of the feedforward loop described above, helping to break oocyte symmetry by generating a molecular memory of prior mRNA and protein localisation. In conclusion, the data from the functional analysis of Dmoesin [5,6], together with previous work [13,18,19], make a convincing case for the existence of a stable population of actin filaments which bind oskar mRNA complexes, maintaining their polar distribution through oogenesis and early embryogenesis. The actin cortex appears to play a similar role as an anchor during the localization Ash1 mRNA in budding yeast [14] and during polar growth in fission yeast [1]. As many of the molecules involved appear not be conserved — for example, moesin is not present in yeast — this may be an example of convergent evolution, where cells from different lineages have invented a similar means of generating an end. References 1. Mata, J. and Nurse, P. (1998). Discovering the poles in yeast. Trends Cell Biol. 8, 163–167. 2. Kim-Ha, J., Smith, J.L. and Macdonald, P.M. (1991). oskar mRNA is localised to the posterior pole of the Drosophila oocyte. Cell 66, 23–35. 3. Ephrussi, A., Dickinson, L.K. and Lehmann, R. (1991). Oskar organizes the germ plasm and directs localization of the posterior determinant nanos. Cell 66, 37–50. 4. Riechmann, V. and Ephrussi, A. (2001). Axis formation during Drosophila oogenesis. Curr. Opin. Genet. Dev. 11, 374–383. 5. Polesello, C., Delon, I., Valenti, P., Ferrer, P. and Payre, F. (2002). Dmoesin controls actin-based cell shape and polarity during Drosophila melanogaster oogenesis. Nat. Cell Biol. 4, 782–789. 6. Jankovics, F., Sinka, R., Lukacsovich, T. and Erdelyi, M. (2002). Dmoesin crosslinks actin and cell membrane in the Drosophila oocyte and is required for OSKAR anchoring. Curr. Biol. 10th December issue. 7. Bretscher, A., Chambers, D., Nguyen, R. and Reczek, D. (2000). ERM-Merlin and EBP50 protein families in plasma membrane organization and function. Annu. Rev. Cell Dev. Biol. 16, 113–143.
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8. Theurkauf, W.E., Smiley, S., Wong, M.L. and Alberts, B.M. (1992). Reorganization of the cytoskeleton during Drosophila oogenesis: implications for axis specification and intercellular transport. Development 115, 923–936. 9. Gonzalez-Reyes, A., Elliott, H. and St Johnston, D. (1995). Polarization of both major body axes in Drosophila by gurken-torpedo signalling. Nature 375, 654–658. 10. Clark, I., Giniger, E., Ruohola-Baker, H., Jan, L.Y. and Jan, Y.N. (1994). Transient posterior localization of a kinesin fusion protein reflects anteroposterior polarity of the Drosophila oocyte. Curr. Biol. 4, 289–300. 11. Benton, R. and Johnston, D. (2002). Cell polarity: posterior par-1 prevents proteolysis. Curr. Biol. 12, R479. 12. Manseau, L., Calley, J. and Phan, H. (1996). Profilin is required for posterior patterning of the Drosophila oocyte. Development 122, 2109–2116. 13. Erdelyi, M., Michon, A.M., Guichet, A., Glotzer, J.B. and Ephrussi, A. (1995). Requirement for Drosophila cytoplasmic tropomyosin in oskar mRNA localization. Nature 377, 524–527. 14. Baum, B., Li, W. and Perrimon, N. (2000). A cyclase-associated protein regulates actin and cell polarity during Drosophila oogenesis and in yeast. Curr. Biol. 10, 964–973. 15. Brendza, R.P., Serbus, L.R., Duffy, J.B. and Saxton, W.M. (2000). A function for kinesin I in the posterior transport of oskar mRNA and Staufen protein. Science 289, 2120–2122. 16. Dollar, G., Struckhoff, E., Michaud, J. and Cohen, R.S. (2002). Rab11 polarization of the Drosophila oocyte: a novel link between membrane trafficking, microtubule organization and oskar mRNA localization and translation. Development 129, 517–526. 17. Glotzer, J.B., Saffrich, R., Glotzer, M. and Ephrussi, A. (1997). Cytoplasmic flows localize injected oskar RNA in Drosophila oocytes. Curr. Biol. 7, 326–337. 18. Tetzlaff, M.T., Jackle, H. and Pankratz, M.J. (1996). Lack of Drosophila cytoskeletal tropomyosin affects head morphogenesis and the accumulation of oskar mRNA required for germ cell formation. EMBO J. 15, 1247–1254. 19. Lantz, V.A., Clemens, S.E. and Miller, K.G. (1999). The actin cytoskeleton is required for maintenance of posterior pole plasm components in the Drosophila embryo. Mech. Dev. 85, 111–122.
Drosophila Oogenesis: Generating an Axis of Polarity Buzz Baum
New studies of moesin function during Drosophila oogenesis show that Dmoesin tethers actin to the oocyte membrane. Interestingly, Dmoesin and an intact cortex are also required to stabilise ooycte polarity.
In eukaryotic cells, a global axis of polarity is established using two general mechanisms. In one, an oriented cytoskeleton is assembled which directs polar intracellular transport. In the other, cells establish stable marks within the actin-rich cortex which serve as reference points to capture and concentrate specific polar cell markers as they pass by. Often these systems work in tandem [1], so that cells polarize their cytoskeleton with respect to an existing cortical cue. Determinants of polarity are then transported along cytoskeletal filaments to this site, where they become anchored, amplifying the polar signal. To establish anterior–posterior polarity during Drosophila oogenesis, microtubules direct the transport of oskar mRNA to one end of the oocyte, which then defines the future posterior pole of the embryo [2–4]. Two new studies [5,6] of the function of the sole Drosophila member of the ezrin–rodexin–moesin (ERM) protein family [7], Dmoesin, have now shown that an actin-based cortical anchor is required to maintain this polarised distribution of oskar mRNA. The large cells of the Drosophila egg chamber make it an excellent system for identifying genes involved in the generation of cell polarity. Early on during oogenesis, a microtubule organising centre (MTOC) forms within the oocyte, which directs the transport of new synthesized materials along microtubules from the nurse cells into the developing oocyte [8]. Then, at stages 7–8, the oocyte microtubule cytoskeleton undergoes a dramatic re-organisation in response to a signal from the overlying posterior follicle cells [8]. The MTOC is disassembled and microtubules become nucleated at the anterior cortex of the oocyte, generating a gradient of microtubules with their ‘plus’ ends tightly focused at the posterior pole [9] (Figure 1). As microtubule inhibitors [10] and mutations which disrupt microtubule organisation [11] or microtubule motor function [10] block the proper localization of polar markers such as oskar mRNA, microtubules are thought to play a critical role in the establishment of oocyte polarity. Although the evidence that microtubules contribute to the establishment of oocyte polarity is compelling, the role of the actin cytoskeleton in this process is Ludwig Institute for Cancer Research, University College London branch, 91 Riding House Street, London W1W 7BS, UK. E-mail: [email protected]
Dispatch
less clear. Rather surprisingly, however, several genes identified by their mRNA mislocalisation phenotype were found, upon cloning, to code for conserved actin-binding proteins. Several of these mutations, such as those affecting profilin [12], disrupt oocyte polarity, probably in part as an indirect consequence of changes in microtubule organisation. The new functional analysis of Dmoesin [5,6], however, convincingly shows that the actin-rich cortex plays a second role in
Figure 1. Actin filaments and microtubules cooperate to generate a stable axis of polarity during Drosophila oogenesis. The figure shows wild-type (top) and Dmoesin-deficient (bottom) egg chambers at stage 9–10. An uncharacterised signal from the posterior follicle cells (purple) has generated a polarised microtubule array aligned along the anterior–posterior axis of the oocyte. Once microtubule plus ends predominate at the posterior pole the oocyte, at stage 9–10, oskar mRNA is guided to the pole by a plus-end-directed microtubule motor (kinesin I). On reaching the microtubule plus ends, oskar mRNA complexes are captured by a cortical actin-filament based anchor which requires tropomyosin II, Dmoesin and Cap for its proper construction. As Oskar protein helps to recruit oskar mRNA, this initial asymmetry is rapidly amplified. At stage 10B, the polar microtubule array is then disassembled as nurse cells begin to dump their contents into the oocyte. But oskar mRNA remains tethered within the cortical actin mesh at the posterior of the oocyte, capturing newly synthesized components of the pole plasm as they pass by. During embryogenesis, these localised determinants then specify germ cell fate and direct the formation of the embryo’s abdomen. In the dmoesin mutant, oskar mRNA is frequently seen binding to the detached actin cortex at the posterior pole, or appears diffuse, while microtubules are unaffected. Microtubules are shown in green; microtubule plus ends are denoted +; kinesin is shown in yellow; actin is shown in red; complexes containing oskar mRNA are in black; anterior is to the left, posterior to the right.
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the localisation of posterior determinants within the Drosophila oocyte. Polesello et al. [5] and Jankovics et al. [6] identified Dmoesin in screens for flies whose offspring lack abdomens and germ cells — a phenotype typical of mutations that disrupt the localization of oskar mRNA during oogenesis. In Dmoesin-deficient germline clones, the F-actin rich oocyte cortex is often found loose, far from the plasma membrane. So like its mammalian counterparts [7], Dmoesin is required to crosslink actin filaments to the cell periphery. Strikingly, in some instances, when actin filaments become detached from the membrane at the posterior end of the oocyte, oskar mRNA and other posterior determinants are found bound to this filamentous actin mesh, while microtubule plus ends remain tightly focused at the pole. This shows that oskar mRNA is anchored within the actin cortex at the posterior end of the oocyte. Interestingly, the activation of Dmoesin by phosphorylation of conserved residue threonine 559 may be a rate-limiting step in the localisation of oskar mRNA, because expression of a mutated, phospho-mimetic form of Dmoesin in the wild-type was found to increase the accumulation of posterior determinants at the pole of the oocyte [5]. This also suggests that the mechanisms regulating Dmoesin activity are likely to be conserved between Drosophila and mammalian cells [6]. Two other actin-binding proteins, tropomyosin II [13] and Cap [14], are also required to anchor oskar mRNA at the posterior end of the oocyte. In sum, the data suggest that, during oogenesis, oskar mRNA is first transported to the ends of microtubules by the plus-end directed motor kinesin I [15]. Then, upon arrival at the posterior pole of the oocyte, oskar mRNA becomes tethered at the cortex. In other systems, tropomyosins and activated ERM proteins [7] help assemble parallel bundles of actin filaments, for example in muscle and in microvilli. Therefore the precise architecture of the actin cortex is likely to be critical for oskar mRNA binding. Is there something special about the cortex at the posterior pole of the Drosophila oocyte that enables it to function as an anchor for oskar mRNA? At present, the only evidence for the existence of a distinct oocyte posterior domain is the localisation of the small GTPase Rab11 [16]. Rab11 is found at the posterior pole of the oocyte in the wild type and in egg chambers with a defective polar microtubule array. It seems unlikely that Dmoesin has a specific function at the posterior pole of the oocyte, however, because Polesello et al. [5] found that both Dmoesin and its activated derivative are uniformly distributed along the oocyte cortex. Moreover, both groups [5,6] found that germline Dmoesin is also needed to maintain the tight apposition of actin filaments and the lateral oocyte membrane. In a study [17] designed to test the existence of a posterior anchor, labeled oskar mRNA was injected into oocytes at different stages in their development. Surprisingly, a proportion of the injected mRNA pool became localised at the posterior pole, before and after the dissolution of the polar microtubule array at stage
10B, even in the absence of microtubules. Although this implies the presence of a differentiated posterior cortex, the data may simply reflect the existence of a ‘feedforward’ system. As Oskar protein binds oskar mRNA [2,3], newly synthesized or injected oskar mRNA will preferentially bind previously localised Oskar complexes at the pole. Similar mechanisms are likely to operate in many systems, helping to break cellular symmetry by ensuring that initial differences in the localisation of polar determinants are accentuated over time. Thus, it is not clear from existing data whether the oocyte cortex is subdivided into distinct posterior, lateral and anterior domains. The alternative is that the posterior pole of the ooycte is defined solely by the axis of microtubule polarity, and it remains a mystery how the polar microtubule array is established. The existence of a cortical anchor for posterior determinants is likely to serve several related functions. First, an anchor is absolutely required to maintain the polar localisation of posterior determinants during ‘cytoplasmic streaming’, when the ooplasm swirls in a washing-machine-like vortex [8]. In fact, oskar mRNA must remain tethered at the cortex until early embryogenesis [18,19]. Second, an anchor may also be required at late stages to capture residual posterior determinants as they enter the oocyte from nurse cells [13]. Finally, the anchor may be an important element of the feedforward loop described above, helping to break oocyte symmetry by generating a molecular memory of prior mRNA and protein localisation. In conclusion, the data from the functional analysis of Dmoesin [5,6], together with previous work [13,18,19], make a convincing case for the existence of a stable population of actin filaments which bind oskar mRNA complexes, maintaining their polar distribution through oogenesis and early embryogenesis. The actin cortex appears to play a similar role as an anchor during the localization Ash1 mRNA in budding yeast [14] and during polar growth in fission yeast [1]. As many of the molecules involved appear not be conserved — for example, moesin is not present in yeast — this may be an example of convergent evolution, where cells from different lineages have invented a similar means of generating an end. References 1. Mata, J. and Nurse, P. (1998). Discovering the poles in yeast. Trends Cell Biol. 8, 163–167. 2. Kim-Ha, J., Smith, J.L. and Macdonald, P.M. (1991). oskar mRNA is localised to the posterior pole of the Drosophila oocyte. Cell 66, 23–35. 3. Ephrussi, A., Dickinson, L.K. and Lehmann, R. (1991). Oskar organizes the germ plasm and directs localization of the posterior determinant nanos. Cell 66, 37–50. 4. Riechmann, V. and Ephrussi, A. (2001). Axis formation during Drosophila oogenesis. Curr. Opin. Genet. Dev. 11, 374–383. 5. Polesello, C., Delon, I., Valenti, P., Ferrer, P. and Payre, F. (2002). Dmoesin controls actin-based cell shape and polarity during Drosophila melanogaster oogenesis. Nat. Cell Biol. 4, 782–789. 6. Jankovics, F., Sinka, R., Lukacsovich, T. and Erdelyi, M. (2002). Dmoesin crosslinks actin and cell membrane in the Drosophila oocyte and is required for OSKAR anchoring. Curr. Biol. 10th December issue. 7. Bretscher, A., Chambers, D., Nguyen, R. and Reczek, D. (2000). ERM-Merlin and EBP50 protein families in plasma membrane organization and function. Annu. Rev. Cell Dev. Biol. 16, 113–143.
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8. Theurkauf, W.E., Smiley, S., Wong, M.L. and Alberts, B.M. (1992). Reorganization of the cytoskeleton during Drosophila oogenesis: implications for axis specification and intercellular transport. Development 115, 923–936. 9. Gonzalez-Reyes, A., Elliott, H. and St Johnston, D. (1995). Polarization of both major body axes in Drosophila by gurken-torpedo signalling. Nature 375, 654–658. 10. Clark, I., Giniger, E., Ruohola-Baker, H., Jan, L.Y. and Jan, Y.N. (1994). Transient posterior localization of a kinesin fusion protein reflects anteroposterior polarity of the Drosophila oocyte. Curr. Biol. 4, 289–300. 11. Benton, R. and Johnston, D. (2002). Cell polarity: posterior par-1 prevents proteolysis. Curr. Biol. 12, R479. 12. Manseau, L., Calley, J. and Phan, H. (1996). Profilin is required for posterior patterning of the Drosophila oocyte. Development 122, 2109–2116. 13. Erdelyi, M., Michon, A.M., Guichet, A., Glotzer, J.B. and Ephrussi, A. (1995). Requirement for Drosophila cytoplasmic tropomyosin in oskar mRNA localization. Nature 377, 524–527. 14. Baum, B., Li, W. and Perrimon, N. (2000). A cyclase-associated protein regulates actin and cell polarity during Drosophila oogenesis and in yeast. Curr. Biol. 10, 964–973. 15. Brendza, R.P., Serbus, L.R., Duffy, J.B. and Saxton, W.M. (2000). A function for kinesin I in the posterior transport of oskar mRNA and Staufen protein. Science 289, 2120–2122. 16. Dollar, G., Struckhoff, E., Michaud, J. and Cohen, R.S. (2002). Rab11 polarization of the Drosophila oocyte: a novel link between membrane trafficking, microtubule organization and oskar mRNA localization and translation. Development 129, 517–526. 17. Glotzer, J.B., Saffrich, R., Glotzer, M. and Ephrussi, A. (1997). Cytoplasmic flows localize injected oskar RNA in Drosophila oocytes. Curr. Biol. 7, 326–337. 18. Tetzlaff, M.T., Jackle, H. and Pankratz, M.J. (1996). Lack of Drosophila cytoskeletal tropomyosin affects head morphogenesis and the accumulation of oskar mRNA required for germ cell formation. EMBO J. 15, 1247–1254. 19. Lantz, V.A., Clemens, S.E. and Miller, K.G. (1999). The actin cytoskeleton is required for maintenance of posterior pole plasm components in the Drosophila embryo. Mech. Dev. 85, 111–122.