The nature and perception of environmental cues that orchestrate the pelago-benthic life cycle in the sponge Amphimedon queenslandica: molecular and behavioural perspectives

Abstract

The health and survival of benthic marine communities depends upon an ecological and developmental transition from pelagic larva to benthic adult in most invertebrate species, and this is orchestrated by the environment. The majority of 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 ontogenetically linked, but this is yet to be explored at the molecular level. My PhD research explores the nature and perception of environmental cues that regulate larval settlement by using parallel behavioural and transcriptomic assays on individual larvae of the sponge Amphimedon queenslandica, a representative of an early-branching animal lineage.Tracking the vertical position of A. queenslandica larvae over time revealed that larvae of this sponge 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. Larval settlement assays under three different light regimes (natural day-night, constant dark or constant light) were complimented with transcriptomic assays on individual larvae to identify candidate molecular pathways underlying the different settlement responses that were observed. Constant light prevents larval settlement in response to biochemical cues, likely via actively repressing chemostransduction; this is consistent with the sustained upregulation of a photosensory cryptochrome and two putative inactivators of G-protein signalling in the constant light only. These transcriptome data also suggest that the photo- and chemosensory systems may be hierarchically integrated into ontogeny to regulate larval settlement via nitric oxide (NO) and cyclic guanosine monophosphate (cGMP) signalling in this sponge.Larval responses to biochemical settlement cues also change with age, at both behavioural and transcriptional levels. The majority of larvae settled in response to biochemical cues when induced at 5 and 8 hours post emergence (hpe) from the adult sponge. Genome-wide gene expression spanning five time points through larval ontogeny identified candidate gene co-expression modules for underlying larval ability to respond to biochemical cues. Identification of transcription factors, with known functional roles in animal development, within these candidate co-expression modules suggests a potential mechanism for regulating the global developmental rates of larvae.To explore how larvae respond to different biochemical cues at the behavioural and molecular levels, larvae were presented with two different species of coralline algae (CA) at five time points through larval development (1.5, 3, 5 and 8 hpe). Despite significant differences in larval settlement responses to these two species of CA at 3 and 5 hpe, there was no difference in larval transcriptional profiles at 2 hours post induction (hpi). These data suggest that a small number of genes may have a large effect on settlement in this sponge and that components of the GPCR-NO signalling pathway may account for different behavioural responses to biochemical cues. We also identified carryover effects of inductive cue on the expression of a small suite of genes, including two components of the GPCR-NO pathway, in benthic post-larvae at 2 hpi. These data also provide another line of evidence to support a central role for the GPCR-NO pathway in mediating larval settlement in this sponge that belongs to one of the earliest branching of the extant animal lineages. The research presented in this dissertation are consistent with a conserved role for GPCR-NO signalling in regulating larval settlement and metamorphosis throughout the animal kingdom.