Precocious activation of APC/C-Cdh1 at pre-anaphase causes genome instability

Faithful chromosome segregation and thereby accurate gene transmission are crucial for all organisms. Until proper attachment of the mitotic spindle to the kinetochore is established, the ubiquitin ligase (E3) Cdc20-activated APC/C (anaphase promoting complex/cyclosome) is repressed by the spindle assembly checkpoint (SAC) and sister chromatin cohesion is protected. Mutants defective in SAC fail to arrest at metaphase even in the presence of damaged microtubules. Interestingly, a similar phenomenon occurs in yeast cells defective in Bub2, a negative factor of the mitotic exit network (MEN), which is required for telophase onset, although its precise molecular mechanism is unknown. Here, we show that chromosome missegregation occurs frequently in bub2∆ cells in the presence of damaged microtubules. The loss of Bub2 caused precocious activation of APC/C-Cdh1/Hct1 at pre-anaphase, leading to securin degradation and then separase-mediated cohesin cleavage. Overexpression of CDH1 and CDC14, encoding Cdc14 phosphatase, at pre-anaphase similarly caused chromosome missegregation. Thus, sequential activation of APC/C-Cdc20 and then APC/C-Cdh1 is critical for precise chromosome segregation and precocious activation of APC/C-Cdh1 at pre-anaphase causes genomic instability. Since degradation of human securin is also mediated by APC/C-Cdc20 and APC/C-Cdh1, this study predicts that precocious activation APC/C-Cdh1 in human cells similarly causes genomic instability, and thereby cell death or tumorigenesis

The eco‐evolutionary dynamics of prior selfing rates promote coexistence without niche partitioning under conditions of reproductive interference

1. Pollinator-mediated reproductive interference can occur when two or more plant species share the same pollinators. Recent studies have suggested that prior autonomous selfing mitigates reproductive interference, potentially facilitating coexistence even in the absence of pollination niche partitioning (i.e. the pre-emptive selfing hypothesis). However, whether the evolution of prior selfing promotes coexistence, in the context of the eco-evolutionary dynamics of population size, selfing rates and inbreeding depression, remains poorly understood. 2. We constructed an individual-based model to examine the conditions under which the evolution of prior selfing promotes coexistence in the context of mutual reproductive interference. In the model, two plant species compete by way of mutual reproductive interference, and both have the potential to evolve the capacity for prior autonomous selfing. We expected that purging of deleterious mutations might result in evolutionary rescue, assuming that the strength of inbreeding depression declines as the population selfing rate increases; this would enable inferior competitors to maintain population density through the evolution of prior selfing. 3. Our simulation demonstrated that evolution of prior selfing may promote coexistence, whereas reproductive interference in the absence of such evolution results in competitive exclusion. We found that lower pollinator availability is likely to favour rapid evolutionary shifts to higher prior selfing rates, thereby neutralising the negative effects of reproductive interference in both species. When the strength of inbreeding depression decreased with an increase in the population-level selfing rate, moderate pollinator availability resulted in long-term coexistence in which relative abundance-dependent selection on the prior selfing rate served to intermittently maintain the population density of the inferior competitor. 4. Synthesis. We demonstrate that the evolution of prior selfing may increase population growth rates of inferior competitors and may consequently promote long-term coexistence via an evolutionary rescue. This constitutes a novel mechanism explaining the co-evolutionary coexistence of closely related plant species without niche partitioning, and is consistent with recent studies reporting that closely related species with mixed mating systems can co-occur sympatrically, even under conditions of mutual reproductive interference

Immunolocalization of antioxidant enzymes in high-pressure frozen root and stem nodules of Sesbania rostrata

34 Pags.- 7 Figs.- 1 Tabl. The definitive version is available at: http://onlinelibrary.wiley.com/journal/10.1111/(ISSN)1469-8137The activities and localizations of superoxide dismutases (SODs) were compared in root and stem nodules of the semi-aquatic legume Sesbania rostrata using gel-activity assays and immunogold labelling, respectively. Nodules were fixed by high-pressure freezing and dehydrated by freeze substitution. Stem nodules showed more total and specific SOD activities than root nodules because of the presence of chloroplastic CuZnSOD. Most of the total SOD activity of stem and root nodules resulted from 'cytosolic' CuZnSOD, localized in the cytoplasm and chromatin, and from MnSOD in the bacteroids and in the mitochondria of vascular tissue. FeSOD was present in nodule plastids and in leaf chloroplasts, and was found to be associated with chromatin. Superoxide production was detected histochemically in the vascular bundles and in the infected tissue of stem and root nodules, whereas peroxide accumulation was observed in the cortical cell walls and intercellular spaces, as well as within the infection threads of both nodule types. These data suggest a role of CuZnSOD and FeSOD in protecting nuclear DNA from reactive oxygen species and/or in modulating gene activity. The enhanced levels of CuZnSOD, MnSOD and superoxide production in vascular bundle cells are consistent with a role of CuZnSOD and superoxide in the lignification of xylem vessels, but also suggest additional functions in coping with superoxide production by the high respiratory activity of parenchyma cells.This work was supported by the Royal Society (UK), Ministerio de Educación y Ciencia-Fondos Europeos de Desarrollo Regional (AGL2005-01404 and AGL2008-01298) and Gobierno de Aragón (group A53). E.K.J. thanks the Royal Society (UK) and Gobierno de Aragón-Caja Inmaculada (Spain) for funding a sabbatical leave (‘Programa Europa’). M.C.R. was the recipient of a postdoctoral contract (Program I3P) of Consejo Superior de Investigaciones Científicas-Fondo Social Europeo.Peer Reviewe

APC/C-Cdh1-dependent anaphase and telophase progression during mitotic slippage

<p>Abstract</p> <p>Background</p> <p>The spindle assembly checkpoint (SAC) inhibits anaphase progression in the presence of insufficient kinetochore-microtubule attachments, but cells can eventually override mitotic arrest by a process known as mitotic slippage or adaptation. This is a problem for cancer chemotherapy using microtubule poisons.</p> <p>Results</p> <p>Here we describe mitotic slippage in yeast <it>bub2Δ </it>mutant cells that are defective in the repression of precocious telophase onset (mitotic exit). Precocious activation of anaphase promoting complex/cyclosome (APC/C)-Cdh1 caused mitotic slippage in the presence of nocodazole, while the SAC was still active. APC/C-Cdh1, but not APC/C-Cdc20, triggered anaphase progression (securin degradation, separase-mediated cohesin cleavage, sister-chromatid separation and chromosome missegregation), in addition to telophase onset (mitotic exit), during mitotic slippage. This demonstrates that an inhibitory system not only of APC/C-Cdc20 but also of APC/C-Cdh1 is critical for accurate chromosome segregation in the presence of insufficient kinetochore-microtubule attachments.</p> <p>Conclusions</p> <p>The sequential activation of APC/C-Cdc20 to APC/C-Cdh1 during mitosis is central to accurate mitosis. Precocious activation of APC/C-Cdh1 in metaphase (pre-anaphase) causes mitotic slippage in SAC-activated cells. For the prevention of mitotic slippage, concomitant inhibition of APC/C-Cdh1 may be effective for tumor therapy with mitotic spindle poisons in humans.</p