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

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.

Marine Invertebrate Larvae Associated with Symbiodinium: A Mutualism from the Start?

“…Interestingly, homologous strains have been shown to distribute differently in larval tissues than heterologous strains (Rodriguez-Lanetty et al, 2006), and it has also been reported that homologous symbionts establish a more benefitting symbiosis Schwarz et al, 1999;Dunn and Weis, 2009). However, Mies et al (2017c) found no differences in survival in coral, nudibranch, and giant clam larvae associated with both homologous and heterologous Symbiodinium clades. Further, a recent study by Biquand et al (2017) suggests that at least part of the specificity in the host-symbiont relationship may be determined by symbiont cell size, where Symbiodinium strains with a larger cell size are less likely to establish a symbiotic relationship than taxa with a smaller cell size using the laboratory model Aiptasia (Baumgarten et al, 2015) and corals.…”

“…However, that same study found that these compounds contribute rather insignificantly to the larval nutrition. Other reports show that Symbiodinium types within clades A and C associated with coral and giant clam larvae produce a higher amount of the fatty acid DHA (docosahexaenoic acid, 22:6ω3- Mies et al, 2017c) and that Symbiodinium associated with giant clam larvae express a glycerol-synthesizing gene (Mies et al, 2017b), but in neither of those cases translocation was effectively confirmed.…”

“…Its lecithotrophic veliger larvae undergo metamorphosis in the absence of Symbiodinium (Carroll and Kempf, 1990;Kempf, 1991), but recent studies suggest that this species does not engage in a mutualistic relationship (Mies et al, 2017c). While nudibranchs usually acquire symbionts horizontally (Loh et al, 2006;Burghardt and Wägele, 2014;Ahmadian et al, 2016), a case of vertical transmission has been recently reported (Wecker et al, 2015).…”

“…For acroporid corals Cumbo et al (2012) report less than 10 symbiont cells acquired by larvae, while Harii et al (2009) registered an average of 50-60 symbiont cells, but with high variability. Mies et al (2017c) report about 30 cells for both giant clam and nudibranch larvae and more than 100 for coral larvae. Animal hosts are able to select Symbiodinium strains through post-phagocytic recognition mechanisms involving lectin/glycan interactions (Koike et al, 2004;Wood-Charlson et al, 2006;Fransolet et al, 2012).…”

“…At this time, the expression of heat-shock proteins and oxidative-stress genes becomes detectable (DeSalvo et al, 2008(DeSalvo et al, , 2010RodriguezLanetty et al, 2009;Meyer et al, 2011). Data by Mies et al (2017c) show that coral and giant clam larvae from Vietnam bleach at 29 • C and hardly survive past 32 • C. However, the highlytolerant larvae of Pocillopora damicornis (from Okinawa, Japan) have been shown to withstand a temperature of 32 • C, despite losing the majority of its symbiont cells (Haryanti et al, 2015). Nevertheless, the bleaching threshold may be greatly influenced by host and symbiont physiology and genetics, and also by the oceanographic conditions in situ (Roik et al, 2016).…”