Flashing light signaling circuit in sponges: endogenous light generation after tissue ablation in Suberites domuncula

The skeleton of siliceous sponges (phylum Porifera: classes Demospongiae and Hexactinellida), composed of tightly interacting spicules that assemble to a genetically fixed scaffold, is formed of bio-silica. This inorganic framework with the quality of quartz glass has been shown to operate as light waveguide in vitro and very likely has a similar function in vivo. Furthermore, the molecular toolkit for endogenous light generation (luciferase) and light/photon harvesting (cryptochrome) has been identified in the demosponge Suberites domuncula. These three components of a light signaling system, spicules-luciferase-cryptochrome, are concentrated in the surface layers (cortex) of the poriferan body. Specimens from which this cortex has been removed/ablated do not emit light. However, with regeneration and reconstitution of the cortex the animals re-gain the capacity to flash light. This newly discovered characteristic of sponges to generate light prompted us to investigate the genetic basis for the endogenous light signaling system. As a potential transcription factor involved in the expression of luciferase and cryptochrome, a SOX-related protein has been identified. In dark-adapted animals or in tissue from below the cortex region, the medulla, no gene or protein expression of SOX-related protein, luciferase, and cryptochrome could be detected. However, during the regeneration of the cortex, a stage-specific expression pattern was recorded: SOX-related protein > luciferase > cryptochrome. We conclude that a flashing light signaling circuit exists, which might control the retinoic acid-induced differentiation of stem cells into pulsating and contracting sponge cells, that is, pinacocytes and myocytes.