<i>Wolbachia</i> Infect Ovaries in the Course of Their Maturation: Last Minute Passengers and Priority Travellers?

<div><p><i>Wolbachia</i> are widespread endosymbiotic bacteria of arthropods and nematodes. Studies on such models suggest that <i>Wolbachia</i>'s remarkable aptitude to infect offspring may rely on a re-infection of ovaries from somatic tissues instead of direct cellular segregation between oogonia and oocytes. In the terrestrial isopod <i>Armadillidium vulgare</i>, <i>Wolbachia</i> are vertically transmitted to the host offspring, even though ovary cells are cyclically renewed. Using Fluorescence <i>in situ</i> hybridization (FISH), we showed that the proportion of infected oocytes increased in the course of ovary and oocyte maturation, starting with 31.5% of infected oocytes only. At the end of ovary maturation, this proportion reached 87.6% for the most mature oocytes, which is close to the known transmission rate to offspring. This enrichment can be explained by a secondary acquisition of the bacteria by oocytes (<i>Wolbachia</i> can be seen as last minute passengers) and/or by a preferential selection of oocytes infected with <i>Wolbachia</i> (as priority travellers).</p>