Temporal variation and lack of host specificity among bacterial endosymbionts of Osedax bone worms (Polychaeta: Siboglinidae)

Rahel M Salathé,Robert C Vrijenhoek

doi:10.1186/1471-2148-12-189

Abstract

BackgroundOsedax worms use a proliferative root system to extract nutrients from the bones of sunken vertebrate carcasses. The roots contain bacterial endosymbionts that contribute to the nutrition of these mouthless and gutless worms. The worms acquire these essential endosymbionts locally from the environment in which their larvae settle. Here we report on the temporal dynamics of endosymbiont diversity hosted by nine Osedax species sampled during a three-year investigation of an experimental whale fall at 1820-m depth in the Monterey Bay, California. The host species were identified by their unique mitochondrial COI haplotypes. The endosymbionts were identified by ribotyping with PCR primers specifically designed to target Oceanospirillales.ResultsThirty-two endosymbiont ribotypes associated with these worms clustered into two distinct bacterial ribospecies that together comprise a monophyletic group, mostly restricted to deep waters (>1000 m). Statistical analyses confirmed significant changes in the relative abundances of host species and the two dominant endosymbiont ribospecies during the three-year sampling period. Bone type (whale vs. cow) also had a significant effect on host species, but not on the two dominant symbiont ribospecies. No statistically significant association existed between the host species and endosymbiont ribospecies.ConclusionsStandard PCR and direct sequencing proved to be an efficient method for ribotyping the numerically dominant endosymbiont strains infecting a large sample of host individuals; however, this method did not adequately represent the frequency of mixed infections, which appears to be the rule rather than an exception for Osedax individuals. Through cloning and the use of experimental dilution series, we determined that minority ribotypes constituting less than 30% of a mixture would not likely be detected, leading to underestimates of the frequency of multiple infections in host individuals.

Highlights

Osedax worms use a proliferative root system to extract nutrients from the bones of sunken vertebrate carcasses
Host diversity Based on previously published c-oxidase subunit-1 (COI) sequences [15], two host taxa, MB16 and MB17, were new to this study (Figure 3)
Osedax MB16 is related to white-collar, whereas Osedax MB17’s is most closely related to O. rubiplumus (D = 20.3%)

Summary

Introduction

Osedax worms use a proliferative root system to extract nutrients from the bones of sunken vertebrate carcasses. The roots contain bacterial endosymbionts that contribute to the nutrition of these mouthless and gutless worms. The worms acquire these essential endosymbionts locally from the environment in which their larvae settle. We report on the temporal dynamics of endosymbiont diversity hosted by nine Osedax species sampled during a three-year investigation of an experimental whale fall at 1820-m depth in the Monterey Bay, California. The siboglinid tubeworms commonly found at vents, seeps and debris-falls acquire their endosymbionts environmentally [4]. Vestimentiferans, monoliferans and frenulates host chemoautotrophic endosymbionts [9,10], whereas the bone-eating Osedax worms (Figure 1) host heterotrophic bacteria [11]

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Results

Discussion

Conclusion