Abstract
Invertebrates from various marine habitats form nutritional symbioses with chemosynthetic bacteria. In chemosynthetic symbioses, both the mode of symbiont transmission and the site of bacterial housing can affect the composition of the symbiont population. Vertically transmitted symbionts, as well as those hosted intracellularly, are more likely to form clonal populations within their host. Conversely, symbiont populations that are environmentally acquired and extracellular may be more likely to be heterogeneous/mixed within host individuals, as observed in some mytilid bivalves. The symbionts of thyasirid bivalves are also extracellular, but limited 16S rRNA sequencing data suggest that thyasirid individuals contain uniform symbiont populations. In a recent study, Thyasira cf. gouldi individuals from Bonne Bay, Newfoundland, Canada were found to host one of three 16S rRNA phylotypes of sulfur-oxidizing gammaproteobacteria, suggesting environmental acquisition of symbionts and some degree of site-specificity. Here, we use Sanger sequencing of both 16S RNA and the more variable ribulose-1,5-bisphosphate carboxylase (RuBisCO) PCR products to further examine Thyasira cf. gouldi symbiont diversity at the scale of host individuals, as well as to elucidate any temporal or spatial patterns in symbiont diversity within Bonne Bay, and relationships with host OTU or size. We obtained symbiont 16S rRNA and RuBisCO Form II sequences from 54 and 50 host individuals, respectively, during nine sampling trips to three locations over four years. Analyses uncovered the same three closely related 16S rRNA phylotypes obtained previously, as well as three divergent RuBisCO phylotypes; these were found in various pair combinations within host individuals, suggesting incidents of horizontal gene transfer during symbiont evolution. While we found no temporal patterns in phylotype distribution or relationships with host OTU or size, some spatial effects were noted, with some phylotypes only found within particular sampling sites. The sequencing also revealed symbiont populations within individual hosts that appeared to be a mixture of different phylotypes, based on multiple base callings at divergent sites. This work provides further evidence that Thyasira cf. gouldi acquires its symbionts from the environment, and supports the theory that hosts can harbour symbiont populations consisting of multiple, closely related bacterial phylotypes.
Highlights
Symbioses between animals and bacteria are ubiquitous and, in many cases, advantageous to the host (McFall-Ngai et al, 2013)
The degree of symbiont specificity varies among host species: some harbour clonal populations, while others can form symbioses with more than one bacterial strain, in either single or mixed populations (Duperron et al, 2008a; Petersen et al, 2012; Brissac et al, 2016)
From the bivalves examined in this study, we obtained 16S rRNA and RuBisCO Form II sequences from 54 and 50 host individuals, respectively (Table S1)
Summary
Symbioses between animals and bacteria are ubiquitous and, in many cases, advantageous to the host (McFall-Ngai et al, 2013). Hosts that transmit symbionts vertically (usually within the eggs) tend to form highly specific relationships with symbionts, which often form clonal populations (Goffredi et al, 2003; Wernegreen, 2005; Caro et al, 2007; Bright & Bulgheresi, 2010) Other host species, such as those that obtain symbionts environmentally from free-living bacterial populations (Nussbaumer, Fisher & Bright, 2006; Won, Jones & Vrijenhoek, 2008; Bright & Bulgheresi, 2010; Vrijenhoek, 2010), may be less specific and harbour mixed symbiont populations (Vrijenhoek, Duhaime & Jones, 2007; Moran, McCutcheon & Nakabachi, 2008; Ikuta et al, 2016). A different situation has been observed in thyasirid bivalves with extracellular symbionts: Thyasira cf. gouldi conspecifics from the same fjord (Bonne Bay, Newfoundland, Canada) associated with one of three
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