Conjugating effects of symbionts and environmental factors on gene expression in deep-sea hydrothermal vent mussels

Abstract

BackgroundThe deep-sea hydrothermal vent mussel Bathymodiolus azoricus harbors thiotrophic and methanotrophic symbiotic bacteria in its gills. While the symbiotic relationship between this hydrothermal mussel and these chemoautotrophic bacteria has been described, the molecular processes involved in the cross-talking between symbionts and host, in the maintenance of the symbiois, in the influence of environmental parameters on gene expression, and in transcriptome variation across individuals remain poorly understood. In an attempt to understand how, and to what extent, this double symbiosis affects host gene expression, we used a transcriptomic approach to identify genes potentially regulated by symbiont characteristics, environmental conditions or both. This study was done on mussels from two contrasting populations.ResultsSubtractive libraries allowed the identification of about 1000 genes putatively regulated by symbiosis and/or environmental factors. Microarray analysis showed that 120 genes (3.5% of all genes) were differentially expressed between the Menez Gwen (MG) and Rainbow (Rb) vent fields. The total number of regulated genes in mussels harboring a high versus a low symbiont content did not differ significantly. With regard to the impact of symbiont content, only 1% of all genes were regulated by thiotrophic (SOX) and methanotrophic (MOX) bacteria content in MG mussels whereas 5.6% were regulated in mussels collected at Rb. MOX symbionts also impacted a higher proportion of genes than SOX in both vent fields. When host transcriptome expression was analyzed with respect to symbiont gene expression, it was related to symbiont quantity in each field.ConclusionsOur study has produced a preliminary description of a transcriptomic response in a hydrothermal vent mussel host of both thiotrophic and methanotrophic symbiotic bacteria. This model can help to identify genes involved in the maintenance of symbiosis or regulated by environmental parameters. Our results provide evidence of symbiont effect on transcriptome regulation, with differences related to type of symbiont, even though the relative percentage of genes involved remains limited. Differences observed between the vent site indicate that environment strongly influences transcriptome regulation and impacts both activity and relative abundance of each symbiont. Among all these genes, those participating in recognition, the immune system, oxidative stress, and energy metabolism constitute new promising targets for extended studies on symbiosis and the effect of environmental parameters on the symbiotic relationships in B. azoricus.

Mercury accumulation in hydrothermal vent mollusks from the southern Tonga Arc, southwestern Pacific Ocean

“…In addition, metal accumulation in gills and digestive glands can be related to the nutrients uptake process from symbiotic bacteria. Provannid snails and mytilid mussels found in hydrothermal vent fields commonly harbor chemoautotrophic bacteria (methane-and sulfide-oxidizing bacteria) (Podowski et al, 2009(Podowski et al, , 2010Boutet et al, 2011;Duperron et al, 2011;Beinart et al, 2012;Rodriguez et al, 2013;Sen et al, 2013).…”

Identification and gene expression of multiple peptidoglycan recognition proteins (PGRPs) in the deep-sea mussel Bathymodiolus azoricus , involvement in symbiosis?

“…These bacteria receive all metabolites necessary for chemoautotrophy from the surrounding seawater -a mix between deep-sea water and hydrothermal fluid -through the bivalve gills, which maintain a flux usually serving nutrition by filtration. This symbiosis is known to be plastic and highly dependent on environmental conditions (Colaço et al, 2002;Fiala-Médioni et al, 2002;Salerno et al, 2005;Duperron et al, 2006;Boutet et al, 2011). Indeed the lack of hydrogen sulphur or methane can lead one or the other type of symbiont to take precedence on the other.…”

“…Halary and co-authors showed that a high concentration of hydrogen sulphur, in mussels kept in aquaria, induces a higher level of sulphur-oxidizing symbionts in B. azoricus gills (Halary et al, 2008). This flexibility is thought to be a host adaptation to cope with its heterogeneous and highly variable environment (Boutet et al, 2011). Despite the extensive research done on bathymodiolin symbiosis, the underlying molecular mechanisms that govern this symbiosis are largely unknown.…”

“…Specifically, mechanisms involved in the interaction between host and symbiont at the cellular and/or molecular level have not been explored. To date, only a few studies have examined related question through microscopic investigations or gene expression (Won et al, 2003;Kádár et al, 2005;Boutet et al, 2011;Bettencourt et al, 2014;Barros et al, 2015). As in other symbioses, B. azoricus may regulate its symbiont population through apoptosis mechanisms or immunological responses.…”

Transcriptomic response of the hydrothermal mussel Bathymodiolus azoricus in experimental exposure to heavy metals is modulated by the Pgm genotype and symbiont content

“…Genomic DNA of both mussel and bacteria was extracted together from gill tissue and muscle (used as a negative control in real-time PCR amplification) using a CTAB/PVP extraction procedure and the relative quantity of symbionts was estimated by real-time PCR amplification using 16S specific primers according to the protocol described in Boutet et al, 2011. All experiments were carried out using a Chroma4 thermal cycler (Bio-Rad Laboratories Inc, Hercules, CA) and 1X ABsolute TM QPCR SYBR® Green mix (ABgene, Epsom, UK), 70 nM of each primer, and diluted DNA (2.5 ng) in a final volume of 10 μl.…”