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Résumé |
The 'social' amoeba D. discoideum is facultatively multicellular. Starvation
triggers a life cycle where single cells come together to form multicellular
fruiting bodies, essential for efficient dispersal and long-term survival. In this
process, part of the cells die while promoting the survival of the spores. The
evolution of self-sacrificial behaviour is more easily understood when all cells
in the body share the same genome. It is therefore puzzling to observe that in
natural conditions multicellular aggregates tend to be genetic chimeras, so that
genetic conflicts are unavailable. Theory predicts that the spread of genotypes
that reap more than their fair share of benefits from the group -- the so-called
cheaters -- should prevent cooperative behaviour to be evolutionary stable. We
compared the social performance in chimeras composed of isogenic cells harvested
at different phases of population growth, and found that social behavior is
modulated by phenotipic plasticity as well as genetic background. By tracing the
origin of spore biases to the process of aggregation from single cells, we
explored the single-cell determinants of differences in social behaviour. Finally,
we show that biases due to non-genetic sources of phenotypic variation are
comparable to genetic effects, and can dominate over genetic differences,
overturning classical definitions of social behaviour. Our observations suggest
that inevitable heterogeneity in cell-level physical properties may act - by
breaking heritability of social behaviour - as a hindrance to the evolutionary
success of cheaters, and this even when social interactions within the
multicellular body are neglected. |
