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 species Physarum polycephalum, this morphogenesis is triggered when starvation is followed by starvation-plus-light cue has been considered broadly conserved throughout Physarum. 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 other Physarum species, their phenotypes and molecular mechanisms remains unclear. Consequently, we investigated Physarum rigidum and Physarum roseum under starvation and light. Four of six P. rigidum plasmodia sporulated by day 6, whereas P. roseum did not sporulate within seven days. RNA-seq of P. roseum across 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 of P. polycephalum sporulation. The photoreceptor genes that drive sporulation in P. polycephalum were not detected in P. roseum, and 92 candidate photoreceptor genes showed no significant regulation. These findings indicate that P. roseum responds only minimally to light stimulation, and that the starvation-plus-light trigger is not universally retained within Physarum.