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  1. Evaluation Summary:

    In their manuscript, Lu et al. use a combination of experimental approaches to determine how cellular components are transported from nurse cells into the growing oocyte during Drosophila egg development. The authors demonstrate that the minus-end directed microtubule motor, dynein, generates cortical flow by gliding microtubules along the cell cortex. This flow is capable of propelling cargoes through the ring canals into the growing oocyte via a bulk cytoplasmic transport mechanism. This action is distinct from dynein's cargo transport functions, as the authors are able to replace dynein with a minus-end directed kinesin linked to the cortex and observe the same phenomenon. Overall, this work had broad significance to cell biologists and developmental biologists interested in intracellular transport functions and oocyte development.

    (This preprint has been reviewed by eLife. We include the public reviews from the reviewers here; the authors also receive private feedback with suggested changes to the manuscript. The reviewers remained anonymous to the authors.)

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  2. Reviewer #1 (Public Review):

    This paper by Lu et al. is an exciting new contribution to our understanding of transport, especially in the context of oogenesis. Two stages of transport from nurse cells to oocytes, in the Drosophila egg chamber, have long been accepted: early selective transport and late dumping. The authors clearly establish that transport by dynein-driven advection is occurring before dumping begins. Using a combination of genetic knock downs, labeling and live imaging, the authors show that dynein and BicD are necessary for proper oocyte growth and focus on stage 8/9 of development. They proposal a model in which dynein-driven microtubule transport pulls cytoplasm along with it. Supporting this model, they have beautiful movies directly demonstrating movement of microtubules and several cellular components through the ring canals. The go on to show that passive flow of artificial cargo that does not interact with dynein or microtubules is transported as well. Importantly, they show that a minus end directed motor construct, with no dynein cargo interaction domains, when directed to the nurse cell cortex, is sufficient to drive this transport as well. Careful controls, such as eliminating a role for myosin are included.

    The claim is made that this movement of microtubules and associated cytoplasm is robust in stage 9 and maybe 8 but data are not presented regarding this timing. How early the streaming starts should be established, in order to determine whether or not there really is a slow selective stage first.

    The authors note that the cytoplasmic rings necessary for such transport between cell types, are observed in developing oocytes of many species. Cytoplasmic streaming is seen in other contexts, including later in Drosophila oogenesis, within the oocyte, and within the plant syncytium. Thus I wonder how many other cases we are still missing.

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  3. Reviewer #2 (Public Review):

    In this manuscript, Lu and colleagues demonstrate a novel role for Dynein in mediating bulk cytoplasmic transport into the oocyte. This function of Dynein is generated as a result of microtubule gliding rather than specific cargo transport. The conclusions are well-supported by the data. Appropriate controls have been included and the data have been quantified. In addition, the numerous live movies that have been included really help to visualize this process of bulk transport. The notion of bulk transport is best supported by the experiment utilizing GEMs (Figure 3). In addition, the ability of the chimeric cortically anchored plant Kinesin motor to restore oocyte growth in Dlic depleted egg chambers underscores the notion that cargo-independent bulk transport is critical for this process.

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  4. Reviewer #3 (Public Review):

    The authors address the role of dynein, a key minus-end directed microtubule motor in cytoplasmic polarized transport to the oocyte in the developing Drosophila egg chamber. Nurse cells to oocyte mediated transport is essential for oocyte growth and previous studies have shown that dynein mediated transport is important for this process. The authors with this present work shed a new light on this process.

    The dynein motor is required sequentially during oocyte and nurse cell development. To circumvent the requirement of the dynein motor complex in oocyte specification, the authors study oocyte growth by taking advantage of germline-specific Gal4 that is expressed after oocyte specification with different RNAi against dynein components. They thus illustrate that dynein core components and regulators are indeed essential for oocyte growth. By analyzing the putative requirement of BicD, Spindly and Hook, they show that BicD is the most important dynein activating adaptor for oocyte growth.

    By combining photoconversion of a-tubulin and fluorescently labeled microtubule-associated proteins, they interestingly show that microtubules are very dynamic both in the nurse cells and in the ring canals that separate nurse cells and oocyte. They provide evidence that the movement of microtubules through the ring channels is not part of myosin-II-induced nurse cells contraction, but requires the function of dynein, supporting the idea that dynein causes microtubules to slide into the nurse cells and through the ring canals to the oocyte.

    As previously shown, the authors report a dynein-dependent cytoplasmic flux and movement of organelles from nurse cells to the oocyte. However, by combining an ingenious assay with genetically encoded nanoparticles that display Brownian motion in tissue culture cells, they were able to show that direct dynein-cargo interaction is not necessary for this process and that neutral particles can be efficiently transported through the ring channels by dynein microtubule-associated movement of cytoplasmic flow.

    The authors next investigate how the dynein motor complex glides microtubules from the nurse cell to the oocyte. They show that in the nurse cells dynein presents a cortical localization.

    In order to study this possibility, the authors carried out two particularly ingenious types of construction (F-tractin-BicD with BicD-RNAi and F-Tractin-Kin14Vib motor domain with Dlic-RNAi), which enabled them to highlight that cortically-anchored microtubule minus-end motors that cannot directly transport cargoes drive oocyte growth. They further provide evidences that the C terminus part of Dlic subunit could localize the dynein complex to the cell cortex independently of the dynein activating adaptors BicD, dynactin and Lis1.

    Overall this work provides several interesting and new findings illustrating a novel mechanism for dynein-associated cytoplasmic transport. In the nurse cells, and in the ring canals, dynein motors anchored to the actin associated cortex glide microtubules that generate a cytoplasmic flow that move cargoes to the growing oocyte. This corresponds to a new mechanism of cargo transport required for Drosophila oocyte growth. The conclusions of this paper are well supported by the data. Several experiments have been carried out with original and innovative tools that reinforce the scope of this work. The quality of the results presented is very good in general. The figures and the movies are generally of good quality and well explained.

    However, the authors should also discuss that previous studies using fluorescent labelled RNA injection into nurse cells have shown that specific mRNAs are delivered in the oocyte through a dynein-mediated transport distinct from cytoplasmic flow (doi:10.1242/dev.02832).

    They should also discuss more about the velocity range of the various cargoes they analyzed as they passed through the ring canals.

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