“MITOCHONDRION, SWEET MITOCHONDRION”: THE ENDOSYMBIONT “CANDIDATUS BANDIELLA WOODRUFFII” AND ITS POSSIBLE LOCALIZATION INSIDE EUPLOTES WOODRUFFI RESPIRATORY ORGANELLE

Euplotes species (Ciliophora, Spirotrichea) display a certain attitude to harbor microbial consortia, especially if they present the obligate symbiosis with Polynucleobacter necessarius (Heckmann et al., 1983; Boscaro et al., 2012, 2013; Schrallhammer et al., 2013; Vannini et al, 2010, 2012, 2014). Herein we describe the case of Euplotes woodruffi strain NDG2, sampled in an inner channel of Visakhapatnam harbor (Andhra Pradesh, India), in which three endosymbiotic bacteria have been detected by means of FISH analysis: Polynucleobacter necessarius (Burkholderiales, Burkholderiaceae), “Candidatus (Ca.) Megaira polyxenophyla” (Rickettsiales, Rickettsiaceae), and “Ca. Bandiella woodruffii” (Rickettsiales, Midichloriaceae). Among those three endosymbionts, P. necessarius, is known to be an obligate endosymbiont of several Euplotes species (Vannini et al., 2012) and “Ca. Megaira polyxenophyla” has been repeatedly found in several ciliate hosts, belonging to different genera and even classes (Schrallhammer et al., 2013) and in metazoans as well (Penn et al., 2006; Fraune & Bosch, 2007; Sunagawa et al., 2009; Murakami et al., 2017). “Ca. Bandiella woodruffii”, on the contrary, has been described from an E. woodruffi strain from Brazil (Senra et al., 2015) and, so far, detected exclusively in this ciliate species. P. necessarius and “Ca. Megaira polyxenophyla” are cytoplasmic endosymbionts, and, similarly, “Ca. Bandiella woodruffii” has been supposed to be harbored in the host cytoplasm according to FISH analysis in the original description of the species (Senra et al., 2015); however, to date, no data concerning its morphology and ultrastructure are available. Preliminary TEM analysis carried out in the present work on E. woodruffi strain NDG2 consortium, suggest that “Ca. Bandiella woodruffii” could occupy the mitochondrion of the ciliate. Interestingly, this feature has been previously described in other Midichloriaceae, such as “Ca. Midichloria mitochondrii”. Indeed, this close relative of “Ca. Bandiella woodruffii”, is specifically adapted for colonizing these cellular compartments in the ovarian cells of the tick Ixodes ricinus (Sassera et al., 2006). Further analyses with imaging techniques are currently undergoing with the aim to validate these ultrastructural observations.