Knoll, A. | Search bioRxiv

bioRxiv2019

Biological data questions the support of the self inhibition required for pattern generation in the half center model.

Locomotion control in mammals has been hypothesized to be governed by a central pattern generator (CPG) located in the circuitry of the spinal cord. The most common model of the CPG is the half center model, where two pools of neurons generate alternating, oscillatory activity. In this model, the pools reciprocally inhibit each other ensuring alternating activity. There is experimental support for reciprocal inhibition. However another crucial part of the half center model is a self inhibitory mechanism which prevents the neurons of each individual pool from infinite firing. Self-inhibition is hence necessary to obtain alternating activity. But critical parts of the experimental bases for the proposed mechanisms for self-inhibition were obtained in vitro, in preparations of juvenile animals. The commonly used adaptation of spike firing does not appear to be present in adult animals in vivo. We therefore modeled several possible self inhibitory mechanisms for locomotor control. Based on currently published data, previously proposed hypotheses of the self inhibitory mechanism, necessary to support the CPG hypothesis, seems to be put into question by functional evaluation tests or by in vivo data. This opens for alternative explanations of how locomotion activity patterns in the adult mammal could be generated. Author summaryLocomotion control in animals is hypothesized to be controlled through an intrinsic central pattern generator in the spinal cord. This was proposed over a hundred years ago and has subsequently been formed into a consistent theory, through experimentation and computer modeling. However, critical data that support the neuronal circuitry mechanisms underpinning this theory has been obtained in experiments that greatly differ from intact animals. We propose, after trying to fill in this critical part, that new ideas are required to explain locomotion of intact animals.

neuroscience↗

bioRxiv2019

Neoproterozoic origin and multiple transitions to macroscopic growth in green seaweeds

The Neoproterozoic Era records the transition from a largely bacterial to a predominantly eukaryotic phototrophic world, creating the foundation for the complex benthic ecosystems that have sustained Metazoa from the Ediacaran Period onward. This study focusses on the evolutionary origins of green seaweeds, which play an important ecological role in the benthos of modern sunlit oceans and likely played a crucial part in the evolution of early animals by structuring benthic habitats and providing novel niches. By applying a phylogenomic approach, we resolve deep relationships of the core Chlorophyta (Ulvophyceae or green seaweeds, and freshwater or terrestrial Chlorophyceae and Trebouxiophyceae) and unveil a rapid radiation of Chlorophyceae and the principal lineages of the Ulvophyceae late in the Neoproterozoic Era. Our time-calibrated tree points to an origin and early diversification of green seaweeds in the late Tonian and Cryogenian periods, an interval marked by two global glaciations, with strong consequent changes in the amount of available marine benthic habitat. We hypothesize that the unicellular and simple multicellular ancestors of green seaweeds survived these extreme climate events in isolated refugia, and diversified following recolonization of benthic environments that became increasingly available as sea ice retreated. An increased supply of nutrients and biotic interactions such as grazing pressure has likely triggered the independent evolution of macroscopic growth via different strategies, including both true multicellularity, and multiple types of giant celled forms.\n\nSignificance StatementGreen seaweeds are important primary producers along coastlines worldwide, and likely played a key role in the evolution of animals. To understand their origin and diversification, we resolve key relationships among extant green algae using a phylotranscriptomic approach. We calibrate our tree using available fossil data, to reconstruct important evolutionary events such as transitions to benthic environments, and evolution of macroscopic growth. We estimate green seaweeds to have originated in the late Tonian/Cryogenian Period, followed by a marked Ordovician diversification of macroscopic forms. This ancient proliferation of green seaweeds likely modified shallow marine ecosystems, which set off an evolutionary arms race between ever larger seaweeds and grazers.

evolutionary biology↗