The origin of animal multicellularity may be reconstructed by comparing animals with one of their closest living relatives, the choanoflagellate Salpingoeca rosetta. Just as animals develop from a single cell–the zygote–multicellular rosettes of S. rosetta develop from a founding cell. To investigate rosette development, we established forward genetics in S. rosetta. We find that the rosette defect of one mutant, named Rosetteless, maps to a predicted C-type lectin, a class of signaling and adhesion genes required for the development and innate immunity in animals. Rosetteless protein is essential for rosette development and forms an extracellular layer that coats and connects the basal poles of each cell in rosettes. This study provides the first link between genotype and phenotype in choanoflagellates and raises the possibility that a protein with C-type lectin-like domains regulated development in the last common ancestor of choanoflagellates and animals.DOI: http://dx.doi.org/10.7554/eLife.04070.001

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Within the mitotic spindle, there are multiple populations of microtubules with different turnover dynamics, but how these different dynamics are maintained is not fully understood. MCAK is a member of the kinesin-13 family of microtubule-destabilizing enzymes that is required for proper establishment and maintenance of the spindle. Using quantitative immunofluorescence and fluorescence recovery after photobleaching, we compared the differences in spindle organization caused by global suppression of microtubule dynamics, by treating cells with low levels of paclitaxel, versus specific perturbation of spindle microtubule subsets by MCAK inhibition. Paclitaxel treatment caused a disruption in spindle microtubule organization marked by a significant increase in microtubules near the poles and a reduction in K-fiber fluorescence intensity. This was correlated with a faster t 1/2 of both spindle and K-fiber microtubules. In contrast, MCAK inhibition caused a dramatic reorganization of spindle microtubules with a significant increase in astral microtubules and reduction in K-fiber fluorescence intensity, which correlated with a slower t 1/2 of K-fibers but no change in the t 1/2 of spindle microtubules. Our data support the model that MCAK perturbs spindle organization by acting preferentially on a subset of microtubules, and they support the overall hypothesis that microtubule dynamics is differentially regulated in the spindle. INTRODUCTIONThe cytoskeleton must undergo dramatic reorganization as cells transition from interphase to mitosis to assemble the mitotic spindle. The spindle is a macromolecular structure composed of a dynamic array of microtubules (MTs) and their associated proteins that is necessary for proper chromosome segregation (Compton, 2000;Walczak and Heald, 2008). At the onset of mitosis, there is an increase in MT turnover that allows for breakdown of the interphase array and assembly of the mitotic spindle (Saxton et al., 1984;. This change is correlated with an increased catastrophe frequency (transition from growth to shrinkage) and a decreased rescue frequency (transition from shrinkage to growth) (Belmont et al., 1990;Gliksman et al., 1992;Verde et al., 1992;Rusan et al., 2001). The changes in MT dynamics are also correlated with a change in MT polymer levels during mitosis (Zhai et al., 1996). At nuclear envelope breakdown, there is a dramatic decrease in MT polymer levels, which may allow the cell to rapidly assemble a mitotic spindle because the released tubulin dimers can polymerize into new spindle MTs. As the cell progresses from mitosis to the next interphase, there is no change in the levels of MT polymer (Zhai et al., 1996), suggesting that MT polymer gets reorganized to reform the interphase MT cytoskeleton, without a significant change in the dynamics of the MTs. These studies highlight the need for the cell to possess a mechanism to temporally regulate MT dynamics.In addition to the temporal changes in dynamics throughout the cell cycle, the dynamics of MTs within mitosis are also...

In a previous study we established forward genetics in the choanoflagellate Salpingoeca rosetta and found that a C-type lectin gene is required for rosette development (Levin et al., 2014). Here we report on critical improvements to genetic screens in S. rosetta while also investigating the genetic basis for rosette defect mutants in which single cells fail to develop into orderly rosettes and instead aggregate promiscuously into amorphous clumps of cells. Two of the mutants, Jumble and Couscous, mapped to lesions in genes encoding two different predicted glycosyltransferases and displayed aberrant glycosylation patterns in the basal extracellular matrix (ECM). In animals, glycosyltransferases sculpt the polysaccharide-rich ECM, regulate integrin and cadherin activity, and, when disrupted, contribute to tumorigenesis. The finding that predicted glycosyltransferases promote proper rosette development and prevent cell aggregation in S. rosetta suggests a pre-metazoan role for glycosyltransferases in regulating development and preventing abnormal tumor-like multicellularity.