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

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 seven 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 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 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.