Biology subjects

McDougall, A.

Publications and source records attributed to McDougall, A..

10 recordsLinked to original sources

bioRxiv2026

Spatiotemporal regulation of LGN/NuMA and astral microtubules generate mirror symmetric spindle rotations

The positioning of the mitotic spindle controls the size, content, and position of daughter cells within embryos and tissues. A major spindle positioning mechanism is cortical pulling whereby the membrane-bound complex LGN/NuMA/Dynein captures astral microtubules and pulls centrosomes toward the cell cortex. Cytoplasmic dynein and astral growth tend to counteract cortical pulling and position spindles at the cell center. It remains unclear how these opposing forces cooperate. Here we examine the ascidian embryo, where spindles of two germ line cells rotate toward one another causing divisions which are both mirror symmetric and unequal. We find that this spindle behavior is governed by transient enrichment of LGN and NuMA and enhanced cortical pulling at the shared cell contact. Inhibition of the LGN/NuMA complex disrupts spindle alignment, unequal cleavage, and mirror symmetry. Temporal analysis shows that cortical pulling force initates at anaphase when there is a sharp increase in astral microtubule length. These results point towards two phases of spindle positioning forces, with cytoplasmic pulling and cortical pulling operating sequentially during early and late mitosis.

cell biology↗

bioRxiv2026

The extraordinary robustness of mitotic spindle assembly to microtubule dynamics plasticity revealed by end-binding proteins and tubulin tagging

Microtubules are dynamic, conserved cytoskeletal filaments that are essential for all eukaryotic cells. Microtubule dynamic properties are primarily characterized by measuring how fast they grow and shrink (growth and shrinkage rates) and how often they switch between assembly and disassembly (catastrophe and rescue frequencies). These four parameters measured for individual filaments can inform on the behavior of the entire microtubule network at the cell level. By comparing microtubule dynamics in Caenorhabditis elegans one-cell embryos using different genetically-encoded fluorescent probes, we observed an unexpected high variability in these parameters. Microtubule dynamics parameters were consistently higher in C. elegans strains expressing a fluorescently labelled microtubule end-binding protein than in strains relying on tubulin labelling, with microtubule growth rates differing by nearly a factor of two between the two conditions. This discrepancy was not limited to C. elegans, as we observed a similar effect in embryos of the tunicate Phallusia mammillata. Despite this, spindle size and assembly timing were only mildly affected. However, embryos expressing labelled end-binding protein exhibited higher frequency of mitotic defects and perturbed embryonic development upon exposure to various stresses such as elevated temperature or a compromised spindle assembly checkpoint. Thus, our work reveals both the remarkable robustness of mitotic spindle assembly in response to extreme microtubule dynamics plasticity, and the requirement for strict control of microtubule dynamics across successive early embryonic divisions. Our findings should also serve as a cautionary note when using tagged end-binding proteins to measure microtubule dynamics.

cell biology↗

bioRxiv2025

Tug-of-war between cortical and cytoplasmic forces shaping planar of 4-cell stage embryos

Early embryonic cleavages often follow conserved geometric rules, resulting in species-specific cleavage patterns. How these rules are mechanistically implemented, however, varies widely across species and remains poorly understood. Here, using quantitative 3D live imaging, mechanical and biochemical manipulations, we dissect the mechanisms governing centrosomal complex (CC) migration and spindle orientation in ascidian 2-cell stage embryos, which generate the characteristic planar, square 4-cell stage. We show that following the first mitotic division, the CCs in each blastomere form at variable orientations relative to the mother spindle and progressively achieve parallel and coplanar alignment during interphase of the 2-cell stage. Final CC orientation is established before nuclear envelope breakdown, contrasting with the dynamic spindle reorientation during mitosis reported in other organisms. Our analyses reveal that CC rotation is driven by forces acting on long astral microtubules and guided by an anisotropic, endoplasmic reticulum (ER)-rich domain that surrounds the CCs. This ER domain is associated with a dense astral microtubule network, enabling efficient length-dependent microtubule pulling that can orient the CCs even in the absence of cell shape cues. In parallel, dynein-mediated cortical pulling refines CCs tilt and maintains coplanarity. Together, these findings uncover a cell-cycle-regulated cooperation between ER-mediated cytoplasmic forces and anisotropic cortical forces that ensures robust planar cleavage in ascidian early embryos.

developmental biology↗

bioRxiv2023

Assessing the global prevalence of wild birds in trade

Trade represents a significant threat to many wild species and is often clandestine and poorly monitored. Information on which species are most prevalent in trade, and potentially threatened by it, therefore remains fragmentary. We mobilised seven global datasets on birds in trade to identify the species or groups of species that might be at particular risk. These datasets sample different parts of the broad trade spectrum but we nevertheless find a statistically strong congruence between them in which species are recorded in trade. Furthermore, the frequency with which species are recorded within datasets is positively correlated with their occurrence across datasets. This allows us to propose a trade prevalence score that can be applied to all bird species globally. This score discriminates well between species known from semi-independent assessments to be heavily or unsustainably traded and all other species. Globally, 45.1% of all bird species, and 36.7% of globally threatened bird species, were recorded in at least one of the seven datasets. Species listed in Appendices I or II of CITES, species with large geographical distributions and non-songbirds had higher trade prevalence scores. Speciose orders with high mean trade prevalence scores include the Falconiformes, Psittaciformes, Accipitriformes, Anseriformes, Bucerotiformes and Strigiformes. Despite their low mean prevalence score, Passeriformes accounted for the highest overall number of traded species of any order but had low representation in CITES Appendices. Geographical hotspots where large numbers of traded species co-occur differed between songbirds (South-East Asia and Eurasia) and non-songbirds (central South America, sub-Saharan Africa and India). This first attempt to quantify and map the relative prevalence in trade of all bird species globally can be used to identify species and groups of species which may be at particular risk of harm from trade and can inform conservation and policy interventions to reduce its adverse impacts. Article impact statementThe first metric to estimate the prevalence in trade of all the worlds bird species is presented.

ecology↗

bioRxiv2023

Embryo mechanics cartography: inference of 3D force atlases from fluorescence microscopy

The morphogenesis of tissues and embryos results from a tight interplay between gene expression, biochemical signaling and mechanics. Although sequencing methods allow the generation of cell-resolved spatio-temporal maps of gene expression in developing tissues, creating similar maps of cell mechanics in 3D has remained a real challenge. Exploiting the foam-like geometry of cells in embryos, we propose a robust end-to-end computational method to infer spatiotemporal atlases of cellular forces from fluorescence microscopy images of cell membranes. Our method generates precise 3D meshes of cell geometry and successively predicts relative cell surface tensions and pressures in the tissue. We validate it with 3D foam simulations, study its noise sensitivity, and prove its biological relevance in mouse, ascidian and C. elegans embryos. 3D inference allows us to recover mechanical features identified previously, but also predicts new ones, unveiling potential new insights on the spatiotemporal regulation of cell mechanics in early embryos. Our code is freely available and paves the way for unraveling the unknown mechanochemical feedbacks that control embryo and tissue morphogenesis.

biophysics↗

bioRxiv2022

Modulation of Spindle Assembly Checkpoint efficiency by cell fate in a chordate embryo

The spindle assembly checkpoint (SAC) is a surveillance system which preserves genome integrity by delaying anaphase onset until all chromosomes are correctly attached to spindle microtubules. Recruitment of SAC proteins to unattached kinetochores generates an inhibitory signal that prolongs mitotic duration. Chordate embryos are atypical in that spindle defects do not delay mitotic progression during early development, implying that either the SAC is inactive or the cell-cycle target machinery unresponsive. Here we show that in embryos of the chordate Phallusia mammillata the SAC delays mitotic progression from the 8th cleavage divisions. Unattached kinetochores are not recognized by the SAC machinery until the 7th cell cycle when the SAC is acquired. Following acquisition, SAC strength, manifest as the degree of mitotic lengthening induced by spindle perturbations, is specific to different cell types and is modulated by cell size showing similarity to SAC control in early Caenorhabditis elegans embryos. We conclude that SAC acquisition is a process likely specific to chordate embryos, while modulation of SAC efficiency in SAC proficient stages depends on cell fate and cell size similarly to non-chordate embryos.

cell biology↗

bioRxiv2021

Combined effect of cell geometry and polarity domains determines the orientation of unequal division

Cell division orientation is thought to result from a competition between cell geometry and polarity domains controlling the position of the mitotic spindle during mitosis. Depending on the level of cell shape anisotropy or the strength of the polarity domain, one dominates the other and determines the orientation of the spindle. Whether and how such competition is also at work to determine unequal cell division (UCD), producing daughter cells of different size, remains unclear. Here, we show that cell geometry and polarity domains cooperate, rather than compete, in positioning the cleavage plane during UCDs in early ascidian embryos. We found that the UCDs and their orientation at the ascidian third cleavage rely on the spindle tilting in an anisotropic cell shape, and cortical polarity domains exerting different effects on spindle astral microtubules. By systematically varying mitotic cell shape, we could modulate the effect of attractive and repulsive polarity domains and consequently generate predicted daughter cell size asymmetries and position. We therefore propose that the spindle position during UCD is set by the combined activities of cell geometry and polarity domains, where cell geometry modulates the effect of cortical polarity domain(s). Graphical abstract O_FIG O_LINKSMALLFIG WIDTH=200 HEIGHT=200 SRC="FIGDIR/small/442692v1_ufig1.gif" ALT="Figure 1"> View larger version (75K): org.highwire.dtl.DTLVardef@4b0e87org.highwire.dtl.DTLVardef@1a6fdbdorg.highwire.dtl.DTLVardef@5ca0ddorg.highwire.dtl.DTLVardef@1450e61_HPS_FORMAT_FIGEXP M_FIG C_FIG HighlightO_LISpindle tilting in anisotropic cell shape induces unequal cell division C_LIO_LICortical polarity domain can exert attractive or repulsive effect on spindle C_LIO_LICell geometry and polarity domain cooperate to position the spindle C_LIO_LICell geometry modulates the effect of polarity domain C_LI

cell biology↗

bioRxiv2019

The spindle assembly checkpoint functions during early development in non-chordate embryos.

In eukaryotic cells, a spindle assembly checkpoint (SAC) ensures accurate chromosome segregation. This control mechanism monitors proper attachment of chromosomes to spindle microtubules and delays mitotic progression if connections are erroneous or absent. The SAC operates in all eukaryotic cells tested so far, but is thought to be relaxed during early embryonic development in animals. Here, we evaluate the checkpoint response to lack of kinetochore-spindle microtubule interactions in early embryos of diverse animal species from the main metazoan groups. Our analysis shows that there are two classes of embryos, either proficient or deficient for SAC activation during cleavage. Sea urchins, mussels and jellyfish embryos show a prolonged mitotic block in the absence of spindle microtubules from the first cleavage division, while ascidian and amphioxus embryos, like those of Xenopus and zebrafish, continue mitotic cycling without delay. SAC competence during early development shows no correlation with cell size, chromosome number or kinetochore to cell volume ratio, ruling out the hypothesis that lack of checkpoint activity in early embryos is due to the large egg volume. Our results instead indicate that there is no inherent incompatibility between SAC activity and large fast-dividing embryonic cells. We suggest that SAC proficiency is the default situation of metazoan embryos, and that SAC activity is specifically silenced in chordate species with fast dividing embryos.

cell biology↗

bioRxiv2017

Role of midbody remnant in meiosis II creating tethered polar bodies

Polar body (PB) formation is an extreme form of unequal cell division that occurs in oocytes due to the eccentric position of the small meiotic spindle near the oocyte cortex. Prior to PB formation, a chromatin-centered process causes the cortex overlying the meiotic chromosomes to become polarized. This polarized cortical subdomain marks the site where a cortical protrusion or outpocket forms at the oocyte surface creating the future PBs. We observed that PB1 becomes tethered to the egg via PB2, indicating that the site of PB1 cytokinesis directed the precise site for PB2 emission. We therefore studied whether the midbody remnant left behind following PB1 emission was involved, together with the egg chromatin, in defining the precise cortical site for PB2 emission. During outpocketing of PB2 in ascidians, we discovered that a small corps around 1m in diameter protruded from the center of the cortical outpocket that will form the future PB2, which we call the \"polar corps\". During emission of PB2, this small polar corps became localized between PB2 and PB1 and appeared to link PB2 to PB1. We tested the hypothesis that this small polar corps on the surface of the forming PB2 was the midbody remnant from the previous round of PB1 cytokinesis. We had previously discovered that Plk1::Ven labeled midbody remnants in ascidian embryos. We therefore used Plk1::Ven to follow the dynamics of the PB1 midbody remnant during meiosis II. Plk1::Ven strongly labeled the small polar corps that formed on the surface of the cortical outpocket that created PB2. Following emission of PB2, this polar corps was rich in Plk1::Ven and linked PB2 to PB1. By labelling actin (with LifeAct::mCherry/GFP or TRITC-Phalloidin) we also demonstrated that actin accumulates at the midbody remnant and also forms a cortical cap around the midbody remnant in meiosis II that prefigured the precise site of cortical outpocketing during PB2 emission. Phalloidin staining of actin and immunolabelling of anti-phospho aPKC during meiosis II in eggs that had PB1 removed showed that the midbody remnant remained within the egg following emission of PB1. Dynamic imaging of microtubules labelled with Ens::3GFP, MAP7::GFP or EB3::3GFP showed that one pole of the second meiotic spindle was located near the midbody remnant while the other pole rotated away from the cortex during outpocketing. Finally, we report that failure of the second meiotic spindle to rotate can lead to the formation of two cortical outpockets at anaphase II, one above each set of chromatids. It is not known whether the midbody remnant of PB1 is involved in directing the precise location of PB2 in other species as in ascidians. However, a review of the literature indicates that PB1 is tethered to the egg surface via PB2 in a number of species including members of the cnidarians, lophotrochozoa and echinoids, suggesting that the midbody remnant formed during PB1 emission may be involved in directing the precise site of PB2 emission throughout the invertebrates.

developmental biology↗

bioRxiv2017

Kif2 microtubule depolymerase is required for unequal cell division and localizes to a subdomain of cortical endoplasmic reticulum

Unequal cell division (UCD) is a fundamental process responsible for creating sibling cell size asymmetry; however, how microtubules are specifically depolymerized on one aster of the mitotic spindle creating a smaller sibling cell during UCD has remained elusive. Using invertebrate chordate embryos (ascidian) that possess a large cortical structure (CAB) that causes UCD, we identified a microtubule depolymerase (Kif2) involved in creating cell size asymmetry. Kif2 localizes to the cortical subdomain of endoplasmic reticulum in the CAB. During three successive UCDs, Kif2 protein accumulates at the CAB during interphase and is delocalized from the CAB in mid mitosis. Rapid imaging of microtubule dynamics at the cortex revealed that microtubules do not penetrate the CAB during interphase. Inhibition of Kif2 function prevents the development of mitotic aster asymmetry and centrosome movement towards the CAB thereby blocking UCD, whereas locally increasing microtubule depolymerization causes exaggerated asymmetric spindle positioning. This study provides insights into the fundamental process of UCD and for the first time shows that a microtubule depolymerase is localized to a cortical site controlling UCD.

cell biology↗