Expression of a symbiosis-specific gene in Symbiodinium type A1 associated with coral, nudibranch and giant clam larvae.

M Mies,E N Calderon,C R Voolstra,P Y G Sumida,C B Castro,D O Pires

doi:10.1098/rsos.170253

Abstract

Symbiodinium are responsible for the majority of primary production in coral reefs and found in a mutualistic symbiosis with multiple animal phyla. However, little is known about the molecular signals involved in the establishment of this symbiosis and whether it initiates during host larval development. To address this question, we monitored the expression of a putative symbiosis-specific gene (H+-ATPase) in Symbiodinium A1 ex hospite and in association with larvae of a scleractinian coral (Mussismilia hispida), a nudibranch (Berghia stephanieae) and a giant clam (Tridacna crocea). We acquired broodstock for each host, induced spawning and cultured the larvae. Symbiodinium cells were offered and larval samples taken for each host during the first 72 h after symbiont addition. In addition, control samples including free-living Symbiodinium and broodstock tissue containing symbionts for each host were collected. RNA extraction and RT-PCR were performed and amplified products cloned and sequenced. Our results show that H+-ATPase was expressed in Symbiodinium associated with coral and giant clam larvae, but not with nudibranch larvae, which digested the symbionts. Broodstock tissue for coral and giant clam also expressed H+-ATPase, but not the nudibranch tissue sample. Our results of the expression of H+-ATPase as a marker gene suggest that symbiosis between Symbiodinium and M. hispida and T. crocea is established during host larval development. Conversely, in the case of B. stephanieae larvae, evidence does not support a mutualistic relationship. Our study supports the utilization of H+-ATPase expression as a marker for assessing Symbiodinium–invertebrate relationships with applications for the differentiation of symbiotic and non-symbiotic associations. At the same time, insights from a single marker gene approach are limited and future studies should direct the identification of additional symbiosis-specific genes, ideally from both symbiont and host.

Highlights

Coral reefs are marine coastal environments found in tropical areas, noted for their remarkable biodiversity [1]
This diversity is supported by the complexity of habitats created by the CaCO3 structure produced by reef-building corals [2] and by the high primary production performed by symbiotic dinoflagellates, called zooxanthellae [3,4,5]
In order to better understand the symbiotic relationship between Symbiodinium and coral reef larvae and to assess the general suitability of H+-ATPase as a symbiosis marker, we investigated H+-ATPase expression by Symbiodinium when associated with larvae of three different hosts: (i) Mussismilia hispida, a scleractinian coral; (ii) Berghia stephanieae, a nudibranch and (iii) Tridacna crocea, a giant clam

Summary

Introduction

Coral reefs are marine coastal environments found in tropical areas, noted for their remarkable biodiversity [1] This diversity is supported by the complexity of habitats created by the CaCO3 structure produced by reef-building corals [2] and by the high primary production performed by symbiotic dinoflagellates, called zooxanthellae [3,4,5]. These dinoflagellates (genus Symbiodinium) are found in an endosymbiotic association with multiple metazoan and protist phyla [6], being harboured inside the host tissues at high densities, typically 1010 cells per m2 of coral reef [7]. Each clade tends to associate with a particular selection of hosts [7,17,18], and in cases of acquisition of heterologous clades, the host typically displays reduced fitness and growth [19,20]

Methods

Results

Conclusion