Phenotypic variations in the gills of the symbiont-containing bivalve Lucinoma aequizonata |
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Authors: | U Hentschel D S Millikan C Arndt S C Cary H Felbeck |
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Institution: | Institut für Molekulare Infektionsbiologie, Universit?t Würzburg, D-97070 Würzburg, Germany e-mail: ute.hentschel@mail.uni-wuerzburg.de Tel.: +49-931-312588; Fax: +49-931-312578, DE Pacific Biomedical Research Center, Department of Microbiology, University of Hawaii, Honolulu, Hawaii 96813, USA, US Marine Biology Research Division-0202, Scripps Institution of Oceanography, La Jolla, California 92093, USA, US College of Marine Studies, University of Delaware, Lewes, Delaware 19958, USA, US
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Abstract: | The marine bivalve Lucinoma aequizonata (Lucinidae) maintains a population of sulfide-oxidizing chemoautotrophic bacteria in its gill tissue. These are housed in
large numbers intracellularly in specialized host cells, termed bacteriocytes. In a natural population of L. aequizonata, striking variations of the gill colors occur, ranging from yellow to grey, brown and black. The aim of the present study
was to investigate how this phenomenon relates to the physiology and numbers of the symbiont population. Our results show
that in aquarium-maintained animals, black gills contained fewer numbers of bacteria as well as lower concentrations of sulfur
and total protein. Nitrate respiration was stimulated by sulfide (but not by thiosulfate) 33-fold in homogenates of black
gills and threefold in yellow gill homogenates. The total rates of sulfide-stimulated nitrate respiration were the same. Oxygen
respiration could be measured in animals with yellow gills but not in animals with black gills. The cumulative data suggest
that black-gilled clams maintained in the aquarium represent a starvation state. When collected from their natural habitat
black gills contain the same number of bacteria as yellow gills. Also, no significant difference in glycogen concentrations
of the host tissues was observed. Therefore, starvation is unlikely the cause of black gill color in a natural population.
Alternative sources of nutrition to sulfur-based metabolism are discussed. Denaturing gradient gel electrophoresis (DGGE)
performed on the different gill tissues, as well as on isolated symbionts, resulted in a single gill symbiont amplification
product, the sequence of which is identical to published data. These findings provide molecular evidence that one dominant
phylotype is present in the morphologically different gill tissues. Nevertheless, the presence of other phylotypes cannot
formally be excluded. The implications of this study are that the gill of L. aequizonata is a highly dynamic organ which lends itself to more detailed studies regarding the molecular and cellular processes underlying
nutrient transfer, regulation of bacterial numbers and host–symbiont communication.
Received: 1 September 1999 / Accepted: 1 February 2000 |
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