High degree of specificity in the association between symbiotic betaproteobacteria and the host Euplotes (Ciliophora,Euplotia)

Vannini, Claudia; Sigona, Cristiana; Hahn, Martin; Petroni, Giulio; Fujishima, Masahiro

doi:10.1016/j.ejop.2017.04.003

TheBetaproteobacteria-Euplotesassociationisanobligatorysymbioticsysteminvolvingamonophyleticgroupofciliatespeciesandtwobetaproteobacteriaspecieswhichcanbealternativelypresent.Recentdatashowedthatthisrelationshiphasbeenestablishedmorethanonceandthatseveralsymbiont-substitutioneventstookplace,revealingacomplexandintriguingevolutionarypath.Duetothedifferentevolutionarypathwaysfollowedbythedifferentsymbionts,eachbacterialstraincouldhavedifferentiallyevolvedand/orlostfunctionaltraits.Therefore,weperformedre-infectionexperiments,bothbyphagocytosisandbymicroinjection,totestthepossiblefunctionalroleofthedifferentbacteriatowardstheciliates.Ourresultsconfirmthatthegrowthcapacityofthehostisindissolublylinkedtothepresenceofitsoriginalsymbionts.Resultsoftheattemptsofre-infectionbyphagocytosisshowedthatnoneofthebacteriaisabletosuccessfullycolonizethehostcytoplasminthisway,evenifregularlyingested.Re-infectionbymicroinjectionsucceedonlyinonecase.SuchresultspointtoahighdegreeofspecificityintheinteractionsbetweenbacteriaandEuplotesevenaftertheinvasionstep.Duetoaco-evolutivepathwayofreciprocaladaptation,differentdegreeofre-colonizationabilitycouldhavebeenconservedbythedifferentspeciesandstrainsofthesymbionts.