hide
Free keywords:
-
Abstract:
Symbioses between eukaryotic hosts and chemosynthetic, sulfur-oxidizing bacteria are both
phylogenetically and geographically widespread. Most of the known host species are animals
(Metazoa), but ciliates (Ciliophora) are also known to be host organisms. The karyorelictean
ciliate Kentrophoros has ectosymbionts that oxidize sulfide and fix carbon. They are found
interstitially in shallow-water marine sediments, at the interface between sulfidic and oxic
layers. Kentrophoros appears to phagocytose and digest its endosymbionts for nutrition.
Although it was first described almost a century ago, the phylogenetic affiliation of the
symbionts remains unknown. The aim of this study was to identify the ectosymbionts of
Kentrophoros.
Kentrophoros was collected from the islands of Elba (Mediterranean Sea) and Sylt (North
Sea). 16S rRNA sequences show a diversity of bacterial types associated with Kentrophoros,
including Proteobacteria from various classes, and Bacteroidetes. One cluster of sequences
that falls within the basal Gammaproteobacteria was considered to be a likely candidate for
the symbiont, and specific probes for fluorescence in-situ hybridization (FISH) were designed
for it. FISH with the specific probes shows hybridization with the ectosymbionts of some, but
not all, Kentrophoros individuals examined. The ectosymbionts are thus either belonging to
more than one clade, or there is greater than expected sequence variation at the probe target
site. We also reconstructed the phylogeny of Kentrophoros and related ciliates using 18S
rRNA sequences. However, we could not recover a well-supported monophyletic clade
containing all Kentrophoros sequences, suggesting that the genus as currently defined may be
polyphyletic. The molecular and morphological diversity of Kentrophoros is consistent with
the apparent diversity of its ectosymbionts. These unexpected results indicate that broader
taxon and gene sampling are needed to understand the diversity of both host and symbiont in
this system
