Caratterizzazione genomica ed evolutiva dei simbionti batterici nelle zecche

Obligate symbionts allow many arthropod lineages to thrive on a restricted diet. An interesting model for the study of these symbiosis are ticks, as they are dependent on - mostly - maternally inherited bacteria to complement their blood diet. For my PhD I studied the bacteria association in three different ticks: Coxiella endosymbiont with Amblyomma nuttalli, Midichloria and Francisella endosymbiont with Hyalomma marginatum, Spiroplasma and Midichloria with Ixodes frontalis. In the first project, I characterised the bacterial symbiont present in the tick, and then did a comparative genomic and phylogenomic analysis of the Coxiella endosymbionts, comparing them to the pathogen Coxiella burnetii to infer an evolutionary scenario, including the Coxiella common ancestor traits, to clarify if the symbionts are deriving from a pathogen, or pathogenicity is a specific novel trait of the C. burnetii lineage. For the second project, I studied the presence of a multi-partite symbiosis that involves the tick H. marginatum with two different bacteria for a complete provision of nutrients. Our analysis shows that the Francisella endosymbiont in H. marginatum has lost part of the pathway for biotin, and the co-presence with Midichloria restores the genomic capability for biotin synthesis. At same time the biosynthethic capabilities of Midichloria are not sufficient to replace Francisella, making the presence of both bacteria necessary for the host's survival. For the third, I characterise the symbiont population in I. frontalis, considering also if there is relation of the two different sublinages of the tick with a specific symbiont. We observed the presence of Midichloria and Spiroplasma in both sublineages, but with a limited presence for Midichloria in one lineage. Then, for the last project I studied the deep phylogeny of the order Rickettsiales which includes some of the tick pathogens and mutualists, but also many other bacteria with a wide range of hosts. According to the newly inferred scenario, Rickettsiales evolved intracellularity parallelly in different lineages, developing a dependance on their host through horizontal transfers of transporter genes. The results presented here show how similar selective pressures, due the symbiotic relationship with ticks, in phylogenetically close systems can results in different outcomes: Coxiella is present as a single symbiont derived after convergent reduction from a pathogen, in H. marginatum we have a dual bacterial symbiosis with interlocking genomic capabilities, and in I. frontalis we have two very different symbionts alternating their presence to provide the same metabolic capabilities. This diversity of outcomes becomes even starker when we enlarge our analysis to the whole Rickettsiales order, where we have different cellular localisations, metabolic capabilities, host species preference and relation with the host.