Data from: Closely coupled evolutionary history of ecto- and endosymbionts from two distantly related animal phyla

The level of integration between associated partners can range from
ectosymbioses to extracellular and intracellular endosymbioses, and this
range has been assumed to reflect a continuum from less intimate to
evolutionarily highly stable associations. In this study, we examined the
specificity and evolutionary history of marine symbioses in a group of
closely related sulphur-oxidizing bacteria, called Candidatus Thiosymbion,
that have established ecto- and endosymbioses with two distantly related
animal phyla, Nematoda and Annelida. Intriguingly, in the ectosymbiotic
associations of stilbonematine nematodes, we observed a high degree of
congruence between symbiont and host phylogenies, based on their ribosomal
RNA (rRNA) genes. In contrast, for the endosymbioses of gutless
phallodriline annelids (oligochaetes), we found only a weak congruence
between symbiont and host phylogenies, based on analyses of symbiont 16S
rRNA genes and six host genetic markers. The much higher degree of
congruence between nematodes and their ectosymbionts compared to those of
annelids and their endosymbionts was confirmed by cophylogenetic analyses.
These revealed 15 significant codivergence events between stilbonematine
nematodes and their ectosymbionts, but only one event between gutless
phallodrilines and their endosymbionts. Phylogenetic analyses of 16S rRNA
gene sequences from 50 Cand. Thiosymbion species revealed seven
well-supported clades that contained both stilbonematine ectosymbionts and
phallodriline endosymbionts. This closely coupled evolutionary history of
marine ecto- and endosymbionts suggests that switches between symbiotic
lifestyles and between the two host phyla occurred multiple times during
the evolution of the Cand. Thiosymbion clade, and highlights the
remarkable flexibility of these symbiotic bacteria.