Molecular and behavioural evidence that interdependent photo - and chemosensory systems regulate larval settlement in a marine sponge.

Marine pelagic larvae use a hierarchy of environmental cues to identify a suitable benthic habitat on which to settle and metamorphose into the adult phase of the life cycle. Most larvae are induced to settle by biochemical cues and many species have long been known to preferentially settle in the dark. Combined, these data suggest that larval responses to light and biochemical cues may be linked, but this has yet to be explored at the molecular level. Here, we track the vertical position of larvae of the sponge Amphimedon queenslandica to show that they descend to the benthos at twilight, by which time they are competent to respond to biochemical cues, consistent with them naturally settling in the dark. We use larval settlement assays under three different light regimes, combined with transcriptomics on individual larvae, to identify candidate molecular pathways underlying larval settlement. We find that larvae do not settle in response to biochemical cues if maintained in constant light. Our transcriptome data suggest that constant light actively represses settlement via the sustained up-regulation of two putative inactivators of chemotransduction in constant light only. Our data suggest that photo- and chemosensory systems interact to regulate larval settlement via nitric oxide and cyclic guanosine monophosphate signalling in this sponge, which belongs to one of the earliest-branching animal phyla.

Pluripotency and the origin of animal multicellularity.

A widely held-but rarely tested-hypothesis for the origin of animals is that they evolved from a unicellular ancestor, with an apical cilium surrounded by a microvillar collar, that structurally resembled modern sponge choanocytes and choanoflagellates1-4. Here we test this view of animal origins by comparing the transcriptomes, fates and behaviours of the three primary sponge cell types-choanocytes, pluripotent mesenchymal archaeocytes and epithelial pinacocytes-with choanoflagellates and other unicellular holozoans. Unexpectedly, we find that the transcriptome of sponge choanocytes is the least similar to the transcriptomes of choanoflagellates and is significantly enriched in genes unique to either animals or sponges alone. By contrast, pluripotent archaeocytes upregulate genes that control cell proliferation and gene expression, as in other metazoan stem cells and in the proliferating stages of two unicellular holozoans, including a colonial choanoflagellate. Choanocytes in the sponge Amphimedon queenslandica exist in a transient metastable state and readily transdifferentiate into archaeocytes, which can differentiate into a range of other cell types. These sponge cell-type conversions are similar to the temporal cell-state changes that occur in unicellular holozoans5. Together, these analyses argue against homology of sponge choanocytes and choanoflagellates, and the view that the first multicellular animals were simple balls of cells with limited capacity to differentiate. Instead, our results are consistent with the first animal cell being able to transition between multiple states in a manner similar to modern transdifferentiating and stem cells.