Meeting Abstract
Nonvisual photoreceptors have been studied for their ability to allow organisms to interact with their external environment, but their role in host-symbiont communication is not well known. We studied the involvement of bacterial partners in regulating host transcription of putative circadian photoreceptors in the light-organ symbiosis between the squid Euprymna scolopes and its luminous symbiont Vibrio fischeri. This binary model for bacterial colonization of a host organ is characterized by daily transcriptional rhythms in both partners, as well as by daily rhythms in symbiont luminescence and host-cell morphology. Two transcripts encoding cryptochromes (escry1 and escry2), blue-light receptors that entrain circadian rhythms, were identified in the host. Whereas transcription of both cycle in the head with a pattern suggesting entrainment to environmental light, escry1 cycles in the symbiont-colonized light organ with an 8-fold upregulation coincident not with environmental light but with the rhythms of bacterial luminescence. Manipulating the colonization process revealed that escry1 transcription patterns in the light organ were dependent upon the presence of symbionts. Mutants of V. fischeri defective in luminescence failed to induce cry1 expression to wild-type levels, providing evidence that bacterial luminescence entrains host cryptochrome expression. This study demonstrates that bacterial symbionts have the potential to be active participants in the setting of host biological rhythms. The conservation of both bacterial-epithelial interactions and circadian gene regulation across the metazoa suggests that symbiont-induced circadian rhythms may be widespread.
