Disengaged response behavior when the response button is blocked: Evaluation of a micro-intervention.

In large-scale assessments, disengaged participants might rapidly guess on items or skip items, which can affect the score interpretation's validity. This study analyzes data from a linear computer-based assessment to evaluate a micro-intervention that blocked the possibility to respond for 2 s. The blocked response was implemented to prevent participants from accidental navigation and as a naive attempt to prevent rapid guesses and rapid omissions. The response process was analyzed by interpreting log event sequences within a finite-state machine approach. Responses were assigned to different response classes based on the event sequence. Additionally, post hoc methods for detecting rapid responses based on response time thresholds were applied to validate the classification. Rapid guesses and rapid omissions could be distinguished from accidental clicks by the log events following the micro-intervention. Results showed that the blocked response interfered with rapid responses but hardly led to behavioral changes. However, the blocked response could improve the post hoc detection of rapid responding by identifying responses that narrowly exceed time-bound thresholds. In an assessment context, it is desirable to prevent participants from accidentally skipping items, which in itself may lead to an increasing popularity of initially blocking responses. If, however, data from those assessments is analyzed for rapid responses, additional log data information should be considered.

Drosophila Kette coordinates myoblast junction dissolution and the ratio of Scar-to-WASp during myoblast fusion.

The fusion of founder cells and fusion-competent myoblasts (FCMs) is crucial for muscle formation in Drosophila Characteristic events of myoblast fusion include the recognition and adhesion of myoblasts, and the formation of branched F-actin by the Arp2/3 complex at the site of cell-cell contact. At the ultrastructural level, these events are reflected by the appearance of finger-like protrusions and electron-dense plaques that appear prior to fusion. Severe defects in myoblast fusion are caused by the loss of Kette (a homolog of Nap1 and Hem-2, also known as NCKAP1 and NCKAP1L, respectively), a member of the regulatory complex formed by Scar or WAVE proteins (represented by the single protein, Scar, in flies). kette mutants form finger-like protrusions, but the electron-dense plaques are extended. Here, we show that the electron-dense plaques in wild-type and kette mutant myoblasts resemble other electron-dense structures that are known to function as cellular junctions. Furthermore, analysis of double mutants and attempts to rescue the kette mutant phenotype with N-cadherin, wasp and genes of members of the regulatory Scar complex revealed that Kette has two functions during myoblast fusion. First, Kette controls the dissolution of electron-dense plaques. Second, Kette controls the ratio of the Arp2/3 activators Scar and WASp in FCMs.

Dock mediates Scar- and WASp-dependent actin polymerization through interaction with cell adhesion molecules in founder cells and fusion-competent myoblasts.

The formation of the larval body wall musculature of Drosophila depends on the asymmetric fusion of two myoblast types, founder cells (FCs) and fusion-competent myoblasts (FCMs). Recent studies have established an essential function of Arp2/3-based actin polymerization during myoblast fusion, formation of a dense actin focus at the site of fusion in FCMs, and a thin sheath of actin in FCs and/or growing muscles. The formation of these actin structures depends on recognition and adhesion of myoblasts that is mediated by cell surface receptors of the immunoglobulin superfamily. However, the connection of the cell surface receptors with Arp2/3-based actin polymerization is poorly understood. To date only the SH2-SH3 adaptor protein Crk has been suggested to link cell adhesion with Arp2/3-based actin polymerization in FCMs. Here, we propose that the SH2-SH3 adaptor protein Dock, like Crk, links cell adhesion with actin polymerization. We show that Dock is expressed in FCs and FCMs and colocalizes with the cell adhesion proteins Sns and Duf at cell-cell contact points. Biochemical data in this study indicate that different domains of Dock are involved in binding the cell adhesion molecules Duf, Rst, Sns and Hbs. We emphasize the importance of these interactions by quantifying the enhanced myoblast fusion defects in duf dock, sns dock and hbs dock double mutants. Additionally, we show that Dock interacts biochemically and genetically with Drosophila Scar, Vrp1 and WASp. Based on these data, we propose that Dock links cell adhesion in FCs and FCMs with either Scar- or Vrp1-WASp-dependent Arp2/3 activation.

The Arf-GEF Schizo/Loner regulates N-cadherin to induce fusion competence of Drosophila myoblasts.

Myoblast fusion is a key process in multinucleated muscle formation. Prior to fusion, myoblasts recognize and adhere to each other with the aid of cell-adhesion proteins integrated into the membrane. Their intracellular domains participate in signal transduction by binding to cytoplasmic proteins. Here we identified the calcium-dependent cell-adhesion protein N-cadherin as the binding partner of the guanine-nucleotide exchange factor Schizo/Loner in Drosophila melanogaster. N-cadherin was expressed in founder cells and fusion-competent myoblasts of Drosophila during the first fusion phase. Our genetic analyses demonstrated that the myoblast fusion defect of schizo/loner mutants is rescued in part by the loss-of-function mutation of N-cadherin, which suggests that Schizo/Loner is a negative regulator of N-cadherin. Based on our findings, we propose a model where N-cadherin must be removed from the myoblast membrane to induce a protein-free zone at the cell-cell contact point to permit fusion.

Three consecutive generations of nephridia occur during development of Platynereis dumerilii (Annelida, Polychaeta).

Molecular data for nephridial development in polychaetes are not available yet. The scope of our work was to establish a reference system for future investigations using two markers for nephridial development: beta-tubulin as marker for cilia and alkaline phosphatase (AP) activity for secretory epithelia. The markers identified, unexpectedly, three consecutively forming generations of nephridia: (1) a transitory unciliated, but AP-positive head kidney, (2) a transitory larval nephridium, which undergoes a morphological transition from a protonephridium to a funnelled nephridium concomitant with the development of the coelomic cavity and finally, (3) the serially arranged metanephridia. The spatial arrangement of larval and definitive nephridia, revealed an up to now unknown developmental boundary between the synchronously forming larval and the serially proliferating definitive segments. Development of three consecutive sets of nephridia with different morphology and biochemical properties was unexpected and reveals an interesting multistep process in the development of excretory structures in Platynereis.

WASP and SCAR have distinct roles in activating the Arp2/3 complex during myoblast fusion.

Myoblast fusion takes place in two steps in mammals and in Drosophila. First, founder cells (FCs) and fusion-competent myoblasts (FCMs) fuse to form a trinucleated precursor, which then recruits further FCMs. This process depends on the formation of the fusion-restricted myogenic-adhesive structure (FuRMAS), which contains filamentous actin (F-actin) plugs at the sites of cell contact. Fusion relies on the HEM2 (NAP1) homolog Kette, as well as Blow and WASP, a member of the Wiskott-Aldrich-syndrome protein family. Here, we show the identification and characterization of schwächling--a new Arp3-null allele. Ultrastructural analyses demonstrate that Arp3 schwächling mutants can form a fusion pore, but fail to integrate the fusing FCM. Double-mutant experiments revealed that fusion is blocked completely in Arp3 and wasp double mutants, suggesting the involvement of a further F-actin regulator. Indeed, double-mutant analyses with scar/WAVE and with the WASP-interacting partner vrp1 (sltr, wip)/WIP show that the F-actin regulator scar also controls F-actin formation during myoblast fusion. Furthermore, the synergistic phenotype observed in Arp3 wasp and in scar vrp1 double mutants suggests that WASP and SCAR have distinct roles in controlling F-actin formation. From these findings we derived a new model for actin regulation during myoblast fusion.

Blown fuse regulates stretching and outgrowth but not myoblast fusion of the circular visceral muscles in Drosophila.

Circular visceral muscles of Drosophila are binuclear syncytia arising from fusion of two different kinds of myoblasts: a circular visceral founder cell and one visceral fusion-competent myoblast. In contrast to fusion leading to the somatic body-wall musculature, myoblast fusion for the circular visceral muscles does not result in massive syncytia but instead in syncytia interconnected with multiple cytoplasmic bridges, which differentiate into large web-shaped muscles. Here, we show that these syncytial circular visceral muscles build a gut-enclosing network with the interwoven longitudinal visceral muscles. At the ultrastructural level, during circular visceral myoblast fusion and the first step of somatic myoblast fusion prefusion complexes and electron-dense plaques were not detectable which was surprising as these structures are characteristic for the second step of somatic myoblast fusion. Moreover, we demonstrate that Blown fuse (Blow), a cytoplasmic protein essential for the second step of somatic myoblast fusion, plays a different role in circular visceral myogenesis. Blow is known to be essential for progression beyond the prefusion complex in the somatic mesoderm; however, analysis of blow mutants established that it has a restricted role in stretching and outgrowth of the syncytia in the circular visceral muscles. Furthermore, we also found that in the visceral mesoderm, Blow is expressed in both the fusion-competent myoblasts and circular visceral founders, while expression in the somatic mesoderm is initially restricted to fusion-competent myoblasts. We also demonstrate that different enhancer elements in the first intron of blow are responsible for this distinct expression pattern. Thus, we propose a model for Blow in which this protein is involved in at least two clearly differing processes during Drosophila muscle formation, namely somatic myoblast fusion on the one hand and stretching and outgrowth of circular visceral muscles on the other.

kette and blown fuse interact genetically during the second fusion step of myogenesis in Drosophila.

Drosophila myoblast fusion proceeds in two steps. The first one gives rise to small syncytia, the muscle precursor cells, which then recruit further fusion competent myoblasts to reach the final muscle size. We have identified Kette as an essential component for myoblast fusion. In kette mutants, founder cells and fusion-competent myoblasts are determined correctly and overcome the very first fusion. But then, at the precursor cell stage, fusion is interrupted. At the ultrastructural level, fusion is characterised by cell-cell recognition, alignment, formation of prefusion complexes, electron dense plaques and membrane breakdown. In kette mutants, electron dense plaques of aberrant length accumulate and fusion is interrupted owing to a complete failure of membrane breakdown. Furthermore, we show that kette interacts genetically with blown fuse (blow) which is known to be required to proceed from prefusion complexes to the formation of the electron dense plaques. Interestingly, a surplus of Kette can replace Blow function during myogenesis. We propose a model in which Dumbfounded/Sticks and stones-dependent cell adhesion is mediated over Rolling Pebbles, Myoblast city, Crk, Blown fuse and Kette, and thus induces membrane fusion.

Embryonic development of the oligochaete Enchytraeus coronatus: an SEM and histological study of embryogenesis from one-cell stage to hatching.

We describe the embryonic development of the soil-living oligochaete Enchytraeus coronatus (Enchytraeidae, Oligochaeta, Annelida). Enchytraeus coronatus is a direct developer. It follows the typical spiral cleavage mode of development that is highly conserved among annelids and a large number of other lophotrochozoan taxa that are collectively named "Spiralia." Scanning electron microscopy (SEM) was combined with light microscopic analysis of wholemounted and sectioned embryos, differentially processed through histological stainings, to reconstruct and document cellular movements and organogenesis from early cleavage stages until hatching. With the help of these data we have established a scheme of morphologically defined stages in order to facilitate future studies on the molecular and histological level that will allow a detailed cross-species comparison among annelids and other phyla.