Stable and dynamic microtubules coordinately shape the myosin activation zone during cytokinetic furrow formation

The cytokinetic furrow arises from spatial and temporal regulation of cortical contractility. To test the role microtubules play in furrow specification, we studied myosin II activation in echinoderm zygotes by assessing serine19-phosphorylated regulatory light chain (pRLC) localization after precisely timed drug treatments. Cortical pRLC was globally depressed before cytokinesis, then elevated only at the equator. We implicated cell cycle biochemistry (not microtubules) in pRLC depression, and differential microtubule stability in localizing the subsequent myosin activation. With no microtubules, pRLC accumulation occurred globally instead of equatorially, and loss of just dynamic microtubules increased equatorial pRLC recruitment. Nocodazole treatment revealed a population of stable astral microtubules that formed during anaphase; among these, those aimed toward the equator grew longer, and their tips coincided with cortical pRLC accumulation. Shrinking the mitotic apparatus with colchicine revealed pRLC suppression near dynamic microtubule arrays. We conclude that opposite effects of stable versus dynamic microtubules focuses myosin activation to the cell equator during cytokinesis

MCAK facilitates chromosome movement by promoting kinetochore microtubule turnover

Mitotic centromere-associated kinesin (MCAK)/Kif2C is the most potent microtubule (MT)-destabilizing enzyme identified thus far. However, MCAK's function at the centromere has remained mechanistically elusive because of interference from cytoplasmic MCAK's global regulation of MT dynamics. In this study, we present MCAK chimeras and mutants designed to target centromere-associated MCAK for mechanistic analysis. Live imaging reveals that depletion of centromere-associated MCAK considerably decreases the directional coordination between sister kinetochores. Sister centromere directional antagonism results in decreased movement speed and increased tension. Sister centromeres appear unable to detach from kinetochore MTs efficiently in response to directional switching cues during oscillatory movement. These effects are reversed by anchoring ectopic MCAK to the centromere. We propose that MCAK increases the turnover of kinetochore MTs at all centromeres to coordinate directional switching between sister centromeres and facilitate smooth translocation. This may contribute to error correction during chromosome segregation either directly via slow MT turnover or indirectly by mechanical release of MTs during facilitated movement

Geomorphic Evidence for Differential Rock Uplift Across the Southern Cascadia Forearc

The nature of upper plate deformation along the Cascadia subduction zone (CSZ) is poorly understood. Systematic covariation among topographic relief, geodetically determined uplift rates, decadal to millennial erosion rates, and the frequency of episodic tremor and slip (ETS) along the Cascadia forearc suggest a genetic association between forearc topography and plate boundary deformation. Although spatial variations in uplift are commonly attributed to the inboard position of the locked plate interface, the association of high topographic relief and rapid erosion across the boundary between the central Oregon Coast Ranges and the Klamath mountains suggests that ongoing rock uplift may contribute to this signal. Here, I test this hypothesis using a combination of stream profile analysis, observations of fluvial terraces, and existing determinations of watershed-average erosion rate. In the Rogue River watershed, these analyses reveal systematic spatial patterns in channel steepness (a measure of channel gradient normalized for contributing basin area) that delineate a western block characterized by high channel steepness and high local relief (up to 1.5 km) from an eastern portion characterized by lower relief and gentler channels. The boundary between these two domains is a sharp, linear mountain front that trends NNE and is approximately coincident with a bedrock fault zone within the Western Klamath Terrane. East of this structure, herein referred to as the Eight Dollar Fault, fluvial networks are characterized by low-gradient, alluviated valleys, whereas western channels are decorated with flights of strath terraces and perched gravels attesting to recent incision. Longitudinal profiles of the trunk streams of the Rogue and Illinois Rivers exhibit a broad steepening across this transition that is manifest as a convex longitudinal profile. Knickpoints are observed throughout the watershed, but are not systematically associated with the elevation or drainage area of the convexities along trunk channels. Comparison of channel steepness values to existing data on erosion rate suggest that these differences in landscape morphology, channel profile steepness, and geomorphic character of channels are associated with sustained high rates of rock uplift and erosion in the western Klamath mountains. These results suggest that the western Klamath mountains are actively uplifting relative to the eastern forearc. Permanent strain associated with active deformation within the southern Cascadia forearc may be a non-negligible component geodetically measured uplift rates in the region

Investigating Cascade Magmatism Through Dating and Chemical Analysis of the Hatchet Mountain and Pe Ell Formations, SW WA

Modern Cascade arc magmatism began ~45 Ma, shortly after accretion of the Siletzia terrane culminated at ~50 Ma. The earliest expressions of this magmatism are several petrologically-diverse volcanic units in SW Washington including: (1) the Goble Volcanics (GV) / Hatchet Mountain Formation (HM), (2) the Pe Ell Formation (PE), and (3) scattered exposures of unnamed basalts (UB). These rocks, all dominantly subaerial lavas, occur west of the modern arc where they are interbedded with marine and deltaic sedimentary units, suggesting eruption in a forearc or volcanic front setting. Goals of this study are: (1) to characterize the elemental and Sr-Nd isotopic compositions of these early Cascades lavas, (2) to determine whether there are chemical differences between the HM and GV, and (3) to investigate the long-term chemical evolution of the Cascade arc by comparing the compositions of these early lavas with those of younger Cascade arc rocks. Although mapped separately, the HM and GV are chemically indistinguishable. GV ranges from basalt to dacite (48.1%- 66.7%) encompassing the range of silica for HM (50.4%- 60.1%). Both units range from tholeiitic to calc-alkaline (up to 1.75 wt. % K2O) and have HFSE depletions and LILE enrichments characteristic of an arc setting. PE basalts are alkaline (shoshonite series) with OIB-like spidergram trends and La/Yb \u3e 25. The UB are Mg rich tholeiites (45.6-46.3 wt.% SiO2, 7.6-10.9 wt.% MgO) with moderate REE fractionation (La/Yb = 5.5-18.4) and OIB affinities (Ba/Nb = 4 – 7). These primitive basalts are chemically similar to the Basalt of Wolf Point (Evarts, 2001) and the Quaternary low K tholeiites of Leeman et al (2005). The compositional diversity of these lavas implies significantly different mantle sources / melting conditions. The PE and UB both require an OIB-like source that contained garnet. The higher La/Yb ratio for PE suggests a source with more garnet, perhaps the result of a lower degree of melting. Application of the Lee et al (2010) thermobarometer to UB magmas yields T = 1572-1590°C and P = 3.5-4.2 Gpa (105-126 km depth). These conditions suggest an asthenospheric source, possibly associated with a slab tear. The arc traits of the MF and GV imply a mantle wedge source that had been modified by slab-derived components

Genome size differentiates co-occurring populations of the planktonic diatom Ditylum brightwellii (Bacillariophyta)

<p>Abstract</p> <p>Background</p> <p>Diatoms are one of the most species-rich groups of eukaryotic microbes known. Diatoms are also the only group of eukaryotic micro-algae with a diplontic life history, suggesting that the ancestral diatom switched to a life history dominated by a duplicated genome. A key mechanism of speciation among diatoms could be a propensity for additional stable genome duplications. Across eukaryotic taxa, genome size is directly correlated to cell size and inversely correlated to physiological rates. Differences in relative genome size, cell size, and acclimated growth rates were analyzed in isolates of the diatom <it>Ditylum brightwellii</it>. <it>Ditylum brightwellii </it>consists of two main populations with identical 18s rDNA sequences; one population is distributed globally at temperate latitudes and the second appears to be localized to the Pacific Northwest coast of the USA. These two populations co-occur within the Puget Sound estuary of WA, USA, although their peak abundances differ depending on local conditions.</p> <p>Results</p> <p>All isolates from the more regionally-localized population (population 2) possessed 1.94 ± 0.74 times the amount of DNA, grew more slowly, and were generally larger than isolates from the more globally distributed population (population 1). The ITS1 sequences, cell sizes, and genome sizes of isolates from New Zealand were the same as population 1 isolates from Puget Sound, but their growth rates were within the range of the slower-growing population 2 isolates. Importantly, the observed genome size difference between isolates from the two populations was stable regardless of time in culture or the changes in cell size that accompany the diatom life history.</p> <p>Conclusions</p> <p>The observed two-fold difference in genome size between the <it>D. brightwellii </it>populations suggests that whole genome duplication occurred within cells of population 1 ultimately giving rise to population 2 cells. The apparent regional localization of population 2 is consistent with a recent divergence between the populations, which are likely cryptic species. Genome size variation is known to occur in other diatom genera; we hypothesize that genome duplication may be an active and important mechanism of genetic and physiological diversification and speciation in diatoms.</p

Patterning of the cell cortex by Rho GTPase Dynamics

The Rho GTPases — RHOA, RAC1 and CDC42 — are small GTP binding proteins that regulate basic biological processes such as cell locomotion, cell division and morphogenesis by promoting cytoskeleton-based changes in the cell cortex. This regulation results from active (GTP-bound) Rho GTPases stimulating target proteins that, in turn, promote actin assembly and myosin 2-based contraction to organize the cortex. This basic regulatory scheme, well supported by in vitro studies, led to the natural assumption that Rho GTPases function in vivo in an essentially linear matter, with a given process being initiated by GTPase activation and terminated by GTPase inactivation. However, a growing body of evidence based on live cell imaging, modelling and experimental manipulation indicates that Rho GTPase activation and inactivation are often tightly coupled in space and time via signalling circuits and networks based on positive and negative feedback. In this Review, we present and discuss this evidence, and we address one of the fundamental consequences of coupled activation and inactivation: the ability of the Rho GTPases to self-organize, that is, direct their own transition from states of low order to states of high order. We discuss how Rho GTPase self-organization results in the formation of diverse spatiotemporal cortical patterns such as static clusters, oscillatory pulses, travelling wave trains and ring-like waves. Finally, we discuss the advantages of Rho GTPase self-organization and pattern formation for cell function

Effective Theories for Circuits and Automata

Abstracting an effective theory from a complicated process is central to the study of complexity. Even when the underlying mechanisms are understood, or at least measurable, the presence of dissipation and irreversibility in biological, computational and social systems makes the problem harder. Here we demonstrate the construction of effective theories in the presence of both irreversibility and noise, in a dynamical model with underlying feedback. We use the Krohn-Rhodes theorem to show how the composition of underlying mechanisms can lead to innovations in the emergent effective theory. We show how dissipation and irreversibility fundamentally limit the lifetimes of these emergent structures, even though, on short timescales, the group properties may be enriched compared to their noiseless counterparts.Comment: 11 pages, 9 figure

A Rho GTPase Signal Treadmill Backs a Contractile Array

SummaryContractile arrays of actin filaments (F-actin) and myosin-2 power diverse biological processes. Contractile array formation is stimulated by the Rho GTPases Rho and Cdc42; after assembly, array movement is thought to result from contraction itself. Contractile array movement and GTPase activity were analyzed during cellular wound repair, in which arrays close in association with zones of Rho and Cdc42 activity. Remarkably, contraction suppression prevents translocation of F-actin and myosin-2 without preventing array or zone closure. Closure is driven by an underlying “signal treadmill” in which the GTPases are preferentially activated at the leading edges and preferentially lost from the trailing edges of their zones. Treadmill organization requires myosin-2-powered contraction and F-actin turnover. Thus, directional gradients in Rho GTPase turnover impart directional information to contractile arrays, and proper functioning of these gradients is dependent on both contraction and F-actin turnover.Video Abstrac