Nitric oxide is not a negative regulator of metamorphic induction in the abalone Haliotis asinina

Abstract

Nitric oxide (NO) is a second messenger molecule synthesized by the enzyme nitric oxide synthase (NOS) that requires the molecular chaperone heat shock protein 90 (HSP90) for normal enzymatic activity. Past studies have revealed that both NO and HSP90 act as negative regulators (repressors) of metamorphosis in a diverse range of marine invertebrates, including several molluscan species. Here, we test the role of NO in the metamorphic induction of a Vetigastropod mollusc, the tropical abalone Haliotis asinina. Specifically, we (1) test the effects of NO-manipulating pharmacological agents, (2) measure the temporal expression of NOS and HSP90 genes through metamorphosis, and (3) assess the spatial expression of NOS and HSP90 in larvae. We find that inhibition of NOS reduces rates of metamorphosis, indicating that NO facilitates, rather than represses, induction of metamorphosis in H. asinina. The marked increase in NOS expression in putative sensory cells localized to the anterior foot of competent larvae is consistent with NO as an inductive molecule for metamorphosis. In contrast to NOS, HSP90 transcript abundance decreases at competence and there is no evidence of NOS and HSP90 transcript co-localization. This study provides the first evidence of NO as an inductive facilitator of molluscan metamorphosis. Our experimental data suggest that NO modulates signals derived from live inductive substrates via the larval foot to regulate metamorphosis. Inter-specific comparisons of spatial NOS expression in molluscs suggest that the localized pattern of NOS or its protein product is related to the regulatory action of NO in metamorphosis.

An ancient role for nitric oxide in regulating the animal pelagobenthic life cycle: evidence from a marine sponge

“…Together, these results indicate that NO signalling is necessary – and perhaps also sufficient – for the induction of settlement and initiation of metamorphosis of A. queenslandica larvae. The activating role of NO in A. queenslandica settlement and metamorphosis contrasts with observations in a range of bilaterian larvae where NO appears to play a repressive or inhibitory role202122232425262728, but is similar to other species where it appears to play an inductive or stimulatory role3031323334.…”

“…We find that NO appears to induce, rather than represses, settlement and metamorphosis as has been observed in some ascidians and molluscs313233. As has been documented in the ascidian Ciona 334464, NO signalling appears to work via – or in concert with – the ERK signalling pathway in A. queenslandica .…”

“…7). NO typically represses the initiation of metamorphosis in multiple bilaterian marine invertebrates20212223242526272829, although there are other reported cases where NO acts as an activator of metamorphosis3031323334. The results presented here for the demosponge A. queenslandica phylogenetically extend the role of NO in animal metamorphosis beyond the bilaterians, thus raising the possibility that its action harks back to the last common animal ancestor, and that it has been able to switch between being an activator and repressor of life cycle transitions over the course of animal evolution.…”

“…The phylogenetic diversity of taxa in which this has been experimentally observed has led to the proposition that NO maintains larvae in a prepared state for metamorphosis25 and that this role for NO may be a highly conserved feature of marine invertebrate settlement and metamorphosis2930. However, there are documented cases in ascidians and a gastropod of NO playing a positive role in regulating the initiation and rate of metamorphosis31323334, raising the possibility that differing roles of NO may be associated with different animal life histories and ecologies30.…”

Bivalves are NO Different: Nitric Oxide as Negative Regulator of Metamorphosis in the Pacific Oyster, Crassostrea Gigas.

“…Inhibition of the NO pathway by exogenous NO scavengers or inhibitors to NOS has successfully induced metamorphosis in these species. However, interestingly for some gastropod, ascidian and sponge species, NO had a positive regulatory effect on metamorphosis instead (27)(28)(29)(30)(31), suggesting some species-speci c adaptation in response to NO (32). Nevertheless, NO biosynthesis by NOS is a conserved pathway, found in all types of living organisms from prokaryotes, plants to metazoans with a remarkable conservation of animal NOS despite several duplication events in invertebrates and vertebrates (33)(34)(35).…”