Variability is found in all kinds of biological systems across all scales, from genes to cells to social groups to ecosystems. So far, most studies have been conducted in multicellular organisms but the complexity of these systems impedes the identification and characterisation of the underlying sources of variations. In this context, we propose to examine the sources of variation in a unicellular organism Physarum polycephalum (myxomycete), which is a huge, multi-nuclear single cell. We take advantage of the fact that P. polycephalum forms clones allowing us to measure the variability between clones of the same strain (genetically identical individuals) and between clones of different strains (genetically different individuals). Our main goal is to identify the mechanisms underlying the variability in P. polycephalum and to understand how variability translates into the expression of the differential plasticity observed between strains.