Loss of the starvation-and-light fruitbody formation trigger in the myxomycete Physarum roseum.

Abstract

Myxomycetes are unicellular amoebozoans that form fruiting bodies to reproduce, a process known as sporulation. In the model species Physarum polycephalum, plasmodia form fruiting bodies only after several days of starvation followed by light exposure. It has long been assumed that the same starvation-plus-light trigger applies to the genus 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 rare for other species of Physarum, their phenotypes and molecular mechanisms remain unclear. Consequently, we investigated Physarum rigidum and Physarum roseum under starvation and light conditions. Four of the six P. rigidum plasmodia sporulated by day 6, whereas P. roseum did not sporulate within 7 days. RNA-seq of P. roseum across nutrient-rich/starved and dark/light conditions revealed differential expression was driven chiefly by nutrition; light caused only minor changes and did not elicit the transcriptional programme 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 suggest that P. roseum responds only minimally to light stimulation and that the starvation-plus-light trigger is not universally retained within the genus Physarum.