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SeMoVi - October 13th, 2010

14h00 - 15h00	Raymond Goldstein, University of Cambridge Stirring Tails of Evolution More information on the Raymond Goldstein website
15h00 - 15h30	Pause café
15h30 - 16h30	Vincent Langlois, LST - ENSL Escape and attack jumps of planktonic copepods: kinematics and energetics Jocelyn Etienne, LSP-UJF Trying to take cells by surprise
16h30 - 17h00	Discussion générale

Raymond Goldstein, University of Cambridge
Stirring Tails of Evolution

One of the most fundamental issues in biology is the nature of evolutionary transitions from single cell organisms to multicellular ones. Not surprisingly for microscopic life in a fluid environment, many of the processes involved are related to transport and locomotion, for efficient exchange of chemical species with the environment is one of the most basic features of life. This is particularly so in the case of flagellated eukaryotes such as green algae, whose members serve as model organisms for the study of transitions to multicellularity. In this talk I will summarize recent theoretical and experimental work addressing a number of interrelated aspects of evolutionary transitions in the Volvocine green algae, which range from unicellular Chlamydomonas to Volvox , with thousands of cells. Phenomena to be discussed include allometry of nutrient uptake, phenotypic plasticity, flagellar synchronization, hydrodynamic bound states, and the dynamics of adaptive phototaxis.

Vincent Langlois, LST - ENSL
Escape and attack jumps of planktonic copepods: kinematics and energetics

Copepods constitute the dominating zooplankton group in the oceans, and may be the most abundant metazoans on Earth. Their success lies in particular in their ability to detect and escape their predators at high speed, as well as the capacity of some species of feeding by ambushing their prey. We have studied the kinematics of these two kinds of displacement through high-speed video observations of free-swimming copepods of various species. Simple numerical simulations allow us to estimate the power requirements and muscle forces in play in escape and attack jumps. We analyze the scaling of kinematics and muscular performan with organism size and compare them with other animals.

Jocelyn Etienne, LSP-UJF
Trying to take cells by surprise

Living cells are puzzling mechanical objects. Their internal structure is now relatively well described: they possess a dense cytoskeleton of semi-rigid polymers on which molecular motors are constantly acting, and which they re-model by polymerisation and depolymerisation. In this context, one of the difficulties for modelling is to distinguish between passive mechanical response to a sollicitation and "active" response, where biochemical reactions are involved and lead to a change of the cell material itself, e.g. polymerisation or molecular bindings occuring selectively in a specific area.

I will present two experimental setups in which we believe however that cell response can be described on the basis of a purely mechanical model. In the first experiment, cells, which are initially suspended in a fluid, are spreading on a solid substrate. Using numerical simulations of simple mechanical models, we show that the retarding force for this spreading originates from the dissipation incurred by the cell cortex when rearranging during spreading. The second experiment confronts the cell with a spring-like device whose stiffness can be tuned arbitrarily. Up to a threshold stiffness, the cell is able to adjust its own contractility so as to attain a fixed shape, independent of the stiffness. A simple visco-elastic model, with the addition of a source term for molecular motor-generated stress and of a model for polymerisation, allows to reproduce faithfully these features.

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