Acquisition of a Novel Sulfur-Oxidizing Symbiont in the Gutless Marine Worm Inanidrilus exumae

Abstract

Gutless phallodrilines are marine annelid worms without a mouth or gut, which live in an obligate association with multiple bacterial endosymbionts that supply them with nutrition. In this study, we discovered an unusual symbiont community in the gutless phallodriline Inanidrilus exumae that differs markedly from the microbiomes of all 22 of the other host species examined. Comparative 16S rRNA gene sequence analysis and fluorescence in situ hybridization revealed that I. exumae harbors cooccurring gamma-, alpha-, and deltaproteobacterial symbionts, while all other known host species harbor gamma- and either alpha- or deltaproteobacterial symbionts. Surprisingly, the primary chemoautotrophic sulfur oxidizer “Candidatus Thiosymbion” that occurs in all other gutless phallodriline hosts does not appear to be present in I. exumae. Instead, I. exumae harbors a bacterial endosymbiont that resembles “Ca. Thiosymbion” morphologically and metabolically but originates from a novel lineage within the class Gammaproteobacteria. This endosymbiont, named Gamma 4 symbiont here, had a 16S rRNA gene sequence that differed by at least 7% from those of other free-living and symbiotic bacteria and by 10% from that of “Ca. Thiosymbion.” Sulfur globules in the Gamma 4 symbiont cells, as well as the presence of genes characteristic for autotrophy (cbbL) and sulfur oxidation (aprA), indicate that this symbiont is a chemoautotrophic sulfur oxidizer. Our results suggest that a novel lineage of free-living bacteria was able to establish a stable and specific association with I. exumae and appears to have displaced the “Ca. Thiosymbion” symbionts originally associated with these hosts.IMPORTANCE All 22 gutless marine phallodriline species examined to date live in a highly specific association with endosymbiotic, chemoautotrophic sulfur oxidizers called “Ca. Thiosymbion.” These symbionts evolved from a single common ancestor and represent the ancestral trait for this host group. They are transmitted vertically and assumed to be in transition to becoming obligate endosymbionts. It is therefore surprising that despite this ancient, evolutionary relationship between phallodriline hosts and “Ca. Thiosymbion,” these symbionts are apparently no longer present in Inanidrilus exumae. They appear to have been displaced by a novel lineage of sulfur-oxidizing bacteria only very distantly related to “Ca. Thiosymbion.” Thus, this study highlights the remarkable plasticity of both animals and bacteria in establishing beneficial associations: the phallodriline hosts were able to acquire and maintain symbionts from two very different lineages of bacteria, while sulfur-oxidizing bacteria from two very distantly related lineages were able to independently establish symbiotic relationships with phallodriline hosts.

Fidelity varies in the symbiosis between a gutless marine worm and its microbial consortium

“…Thiosymbion), to a mixed pattern explained by both host matrilines and locations (Gamma3), one explained by locations (Delta4), those that could not be explained by matrilines or locations (Delta1a and Delta1b), and a highly intermixed pattern among host groups (spirochete). Instead of the expected general flexibility and low fidelity of symbiotic associations inferred in previous studies of gutless annelid symbioses5,[47][48][49] , these results together indicate various levels of partner fidelity among the symbionts at the microevolutionary scale. The high degree of congruence between host mitochondria and Ca.…”

“…Increasing sequencing efforts have uncovered variability of symbiont composition in multi-member symbioses among closely related host species 6,8,43,47,48,62 and between conspecific populations 63,64 .…”

“…The monophyletic group of marine annelids in the subfamily Phallodrilinae (Clitellata, Naididae, Genera Olavius and Inanidrilus; sensu Erséus et al 44 ) completely lack a digestive tract and excretion organs, and instead live in an obligate symbiosis with several bacterial partners 35,45,46 (Figure 1a and 1b). Bacterial partners of these gutless marine annelids belong to a limited set of taxa in the Alpha-, Delta-and Gamma-proteobacteria, and Spirochaeta, and appear to form host species-specific bacterial consortia 45,[47][48][49][50][51] . Among six species of gutless marine annelids whose symbionts have been studied, all species associate with the gammaproteobacterium Candidatus (Ca.)…”

“…Thiosymbion and host annelids do not show a co-speciation pattern 5 , and Ca. Thiosymbion has been replaced by the Gamma4 symbiont in Inanidrilus exumae 48 . In the best researched species Olavius algarvensis, seven endosymbiont genera have been identified thus far -two sulphide-oxidizing Gammaproteobacteria; Ca.…”

Microbiome Shifts Associated With the Introduction of Wild Atlantic Horseshoe Crabs (Limulus polyphemus) Into a Touch-Tank Exhibit

“…Similarly, the high abundance and diversity of sulfide-oxidizing bacteria (SOB) in the genus Thiothrix in wild animals is consistent with the presence of SRB-derived sulfide. Symbioses between sulfur-oxidizing microorganisms and aquatic invertebrates are widespread in sulfide-rich marine environments and thought to have evolved independently in many organisms ( Dubilier et al, 2008 ; Dattagupta et al, 2009 ; Distel et al, 2017 ; Bergin et al, 2018 ). Thiothrix is a common ectosymbiont of marine invertebrates ( Temara et al, 1993 ; Brigmon and De Ridder, 1998 ; Bauermeister et al, 2012 ).…”