Loss of starvation‑and‑light sporulation trigger in Myxomycetes Physarum roseum

Myxomycetes are unicellular amoebozoans that form fruiting bodies to reproduce (sporulation). In the model speciesPhysarum polycephalum, this morphogenesis is triggered when starvation is followed by starvation-plus-light cue has been considered broadly conserved throughoutPhysarum. Recent observations of congeners that fail to sporulate under the same conditions have raised doubts about this assumption and prompted tentative taxonomic reconsideration. Because comparable starvation and light tests are scarce for otherPhysarumspecies, their phenotypes and molecular mechanisms remains unclear. Consequently, we investigatedPhysarum rigidumandPhysarum roseumunder starvation and light. Four of sixP. rigidumplasmodia sporulated by day 6, whereasP. roseumdid not sporulate within seven days. RNA-seq ofP. roseumacross nutrient-rich/starved and dark/light conditions showed that differential expression was driven chiefly by nutrition; light caused only minor changes and did not elicit the transcriptional program characteristic ofP. polycephalumsporulation. The photoreceptor genes that drive sporulation inP. polycephalumwere not detected inP. roseum, and 92 candidate photoreceptor genes showed no significant regulation. These findings indicate thatP. roseumresponds only minimally to light stimulation, and that the starvation-plus-light trigger is not universally retained withinPhysarum.