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Applied and Environmental Microbiology, July 2000, p. 3024-3030, Vol. 66, No. 7
Department of Biology, University of
California, Santa Cruz, Santa Cruz, California 95064
Received 3 December 1998/Accepted 18 August 1999
The phylogenetic relationships of bacterial symbionts from three
gall-bearing species in the marine red algal genus
Prionitis (Rhodophyta) were inferred from 16S rDNA sequence
analysis and compared to host phylogeny also inferred from sequence
comparisons (nuclear ribosomal internal-transcribed-spacer region).
Gall formation has been described previously on two species of
Prionitis, P. lanceolata (from central
California) and P. decipiens (from Peru). This
investigation reports gall formation on a third related host, Prionitis filiformis. Phylogenetic analyses based on
sequence comparisons place the bacteria as a single lineage within the Roseobacter grouping of the
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Copyright © 2000, American Society for Microbiology. All rights reserved.
Molecular and Ecological Evidence for Species
Specificity and Coevolution in a Group of Marine Algal-Bacterial
Symbioses
subclass of the division
Proteobacteria (99.4 to 98.25% sequence identity among
phylotypes). Comparison of symbiont and host molecular phylogenies
confirms the presence of three gall-bearing algal lineages and is
consistent with the hypothesis that these red seaweeds and their
bacterial symbionts are coevolving. The species specificity of these
associations was investigated in nature by whole-cell hybridization of
gall bacteria and in the laboratory by using cross-inoculation trials. Whole-cell in situ hybridization confirmed that a single bacterial symbiont phylotype is present in galls on each host. In laboratory trials, bacterial symbionts were incapable of inducing galls on alternate hosts (including two non-gall-bearing species). Symbiont-host specificity in Prionitis gall formation indicates an
effective ecological separation between these closely related symbiont
phylotypes and provides an example of a biological context in which to
consider the organismic significance of 16S rDNA sequence variation.
*
Corresponding author. Present address: NASA Ames
Research Center, Mailstop 239-11, Moffett Field, CA 94035-1000. Phone:
(650) 604-3279. Fax: (650) 604-3279. E-mail:
JASHEN{at}MAIL.ARC.NASA.GOV.
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