Abstract

The lateral zone of the gill filaments of the tropical shallow-water lucinid Codakia orbiculata is the zone of interest for chemoautotrophic symbiosis involving sulfur-oxidizing bacteria harboured in specialized cells called bacteriocytes. The understanding of bacterial infection process in gills of decolonized individuals will increase knowledge about bacterial symbiont transmission. This study was aimed at evidencing mechanisms underlying bacterial infection processes during bacterial recolonization processes of gill filaments. Collected individuals were starved in laboratory to bleed them. Then, they were treated with bromodeoxyuridine (BrdU) which is a cell division marker, or with cytosine arabinoside (Ara-C) which is an antimitotic agent, before being put back in their natural environment to allow the bacterial infection. We analysed gills with immunochemistry, histology and CARD-FISH techniques, in order to observe in the same, time the evolution of bacterial population, actin cytoskeleton organisation of bacteriocytes, and to evidence the freshly synthetized bacteria. First, we observed that the bacterial infection process induces an increase of bacterial number within bacteriocytes which were bleed in starvation for 3 and 5 months. On the other hand, no bacterial division was observed suggesting that this increase of bacterial endosymbiont is not due to the division of remaining bacteria but to the acquisition of a free-living symbiont form from the environment. The actin-like immunoreactivity of 3 months starved indidviduals confirms the hypothesis of an environmental transmission by endocytosis, since a strong labelling, totally superimposable on bacterial localization, has been detected to the apical area of the bacteriocytes.

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