A host as an ecosystem: <scp> <i>W</i> </scp> <i>olbachia</i> coping with environmental constraints

<jats:title>Summary</jats:title> <jats:p> The <jats:styled-content style="fixed-case"> <jats:italic>W</jats:italic> </jats:styled-content> <jats:italic>olbachia</jats:italic> are intracellular endosymbionts widely distributed among invertebrates. These primarily vertically transmitted <jats:italic>α‐proteobacteria</jats:italic> have been intensively studied during the last decades because of their intriguing interactions with hosts, ranging from reproductive manipulations to mutualism. To optimize their vertical transmission from mother to offspring, the <jats:styled-content style="fixed-case"> <jats:italic>W</jats:italic> </jats:styled-content> <jats:italic>olbachia</jats:italic> have developed fine‐tuned strategies. However, the <jats:styled-content style="fixed-case"> <jats:italic>W</jats:italic> </jats:styled-content> <jats:italic>olbachia</jats:italic> are not restricted to the female gonads and frequently exhibit wide intra‐host distributions. This extensive colonization of somatic organs might be necessary for <jats:styled-content style="fixed-case"> <jats:italic>W</jats:italic> </jats:styled-content> <jats:italic>olbachia</jats:italic> to develop their diverse extended phenotypes. From an endosymbiont's perspective, the within‐host environment potentially presents different environmental constraints. Hence, the <jats:styled-content style="fixed-case"> <jats:italic>W</jats:italic> </jats:styled-content> <jats:italic>olbachia</jats:italic> have to face different intracellular habitats, their host's immune system as well as other microorganisms co‐occurring in the same host individual and sometimes even in the same cell. A means for the <jats:styled-content style="fixed-case"> <jats:italic>W</jats:italic> </jats:styled-content> <jats:italic>olbachia</jats:italic> to protect themselves from these environmental constraints may be to live ‘hidden’ in vacuoles within host cells. In this review, we summarize the current knowledge regarding the extent of the <jats:styled-content style="fixed-case"> <jats:italic>W</jats:italic> </jats:styled-content> <jats:italic>olbachia</jats:italic> pandemic and discuss the various environmental constraints these bacteria may have to face within their ‘host ecosystem’. Finally, we identify new avenues for future research to better understand the complexity of <jats:styled-content style="fixed-case"> <jats:italic>W</jats:italic> </jats:styled-content> <jats:italic>olbachia</jats:italic> 's interactions with their intracellular environment. </jats:p>