Lithium‐Induced respecification of pattern in Paramecium

The long‐known teratogenic effects (dorsalisation) of lithium on amphibian embryos has recently raised renewed interest. As it is known that lithium blocks the polyphosphoinositide (PI) cycle, causing a depressed level of myo ‐inositol, and as injections of myo ‐inostiol have been shown to rescue the effects of Li + , it was postulated that Li + causes a flattening of gradients of PI cycle activity underlying the developmental polarities. We have studied the effect of Li + on the morphogenesis of the unicellular organism, Paramecium. We show (1) that exposure to 25 mM Li + during division yields precise distorsions of the cortical pattern that can be explained by a uniformisation of surface growth i.e. partial suppression of the right/left and antero/posterior asymmetries and (2) that Li + effects are rescued by injection of myo ‐ inositol. These results suggest that spatially graded activity of the PI cycle (ensuring in turn a spatially graded distribution of secondary messengers directly involved in the morphogenetic processes) appeared early in evolution.