ISSN:
1550-7408
Source:
Blackwell Publishing Journal Backfiles 1879-2005
Topics:
Biology
Notes:
Bacteria belonging to the genus Polynucleobacter were previously described as endosymbionts inhabiting the cytoplasm of freshwater ciliates of the genus Euplotes having a 9 type I cirrus pattern. They were all found to be necessary for the division process of their hosts. Here, we report the finding of a Polynucleobacter bacterium as symbiont of E. harpa, a brackish species with ten frontoventral cirri. Identity of the symbionts was unambiguously determined by morphological description and by 16S rRNA gene sequence characterization. Furthermore, antibiotic treatments confirmed that these bacteria play the same functional role of the other Polynucleobacter symbionts; aposymbiotic E. harpa ceases to divide and eventually die. Phylogenetic analyses based on 18S rRNA gene show that Euplotes species harboring Polynucleobacter symbionts, included E. harpa, steadily cluster together. In order to clear whether this symbiosis was established as a single evolutionary event by a common ancestor of the ciliate hosts, or by multiple acquisition of different bacteria by the different Euplotes species, comparative phylogenetic analyses were performed within the so-called Polynucleobacter-cluster, comprising bacterial symbionts as well as strictly related free-living bacteria. Due to a high degree of conservation of 16S rRNA gene sequences within this group, phylogenetic trees based on this gene did not resolve the matter. Surprisingly, even the characterization and the analysis of the 16-23S ITS sequences, usually exhibiting faster evolutionary rates, revealed a high similarity (〉98%), thus hindering a stable and clear phylogenetic reconstruction. Data obtained cannot at present definitively confirm or reject any hypothesis on the origin of this symbiosis, revealing, at the same time, that we are dealing with a relatively young symbiotic system. As shown by the close phylogenetic relationship between free-living and symbiotic Polynucleobacter, this is an ideal model for studying development and adaptation to symbiotic lifestyle.
Type of Medium:
Electronic Resource
URL:
http://dx.doi.org/10.1111/j.1550-7408.2005.05202003_6_16.x
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