MICROORGANISMS ASSOCIATED WITH HARD TICKS (IXODIDAE):FROM SYMBIONTS TO TICK-BORNE PATHOGENS

The research activity I developed during my doctorate can be divided into three parts. First, I investigated the genetic variability of Ixodes ricinus in Europe and North Africa also screening for the presence of bacteria belonging to Borrelia burgdorferi sensu latu complex. A second part of the work focused on Midichloria mitochondrii the principal endosymbiont of I. ricinus. This intramitochondrial bacterium is also present in other tick species but information on the exact intracellular location are not available for the majority of them. I decided to perform a molecular screening for detection of Midichloria-like organisms in vertebrates comprehending humans. Finally, the last part of this work provide information on presence of Ixodes ricinus and other tick species in areas (and from hosts) where record of the presence of these arthropods were absent or fragmentary; the ticks collected were also screened for the presence of different tick-borne pathogens. The genetic analysis on both two mitochondrial loci (COI and COII) and two nuclear loci (Defensin and TROSPA) of Ixodes ricinus confirmed the existence of two distinct groups of haplotypes showing a clear geographic pattern. The first group comprises individuals collected in the European continent, while the second group comprises individuals collected in Tunisia, North Africa. The existence of two groups of populations genetically differentiated in the two continents is also supported by the spatial analysis of molecular variance (SAMOVA), congruently for both mtDNA loci and the two nuclear loci. In previous studies on European and Northern African populations the lacking of data from intermediate areas did not allow determining the entity and the nature of the observed discontinuity. In our study, in addition to a considerable number of individuals from Central and Northern Europe, Southern populations of Ixodes ricinus, sampled in Italy (Sicily) were included. These individuals belong to European group, hence marking the existence of an abrupt and strong genetic discontinuity. Several hypotheses had been proposed to explain this pattern. It could be linked to geographic discontinuity due to the presence of the Mediterranean sea separating the two continents. Anyway, the absence of discontinuity in the entire European continent and the possibility for I. ricinus to cover large distance feeding on migratory birds, might make questionable this hypothesis. Besides, there could be implicated ecological factors linked to I. ricinus biology and interactions with its hosts. Another hypothesis, not necessary excluding the previous ones, considers the role of interaction between species. Indeed, I. ricinus is a parasites and a vector of other parasites/pathogens, interacting both with its hosts and with the transmitted pathogens, and this fact might be the cause of the genetic difference in the two populations. All these selective pressures might have contributed to the insurgence of the observed differences between European and Tunisian populations of I. ricinus. Nuclear and mitochondrial loci are concordant in evidencing a genetic discontinuity between the two continents, but the two markers show different patterns. For mitochondrial DNA, no Tunisian haplotypes were encountered in Europe and vice versa. Nuclear loci show a degree of sharing of haplotypes of the two different haplogroups, probably due to long-distance migration of avian hosts. Further interesting ecological and evolutionary scenarios, in a speculative way, might be suggested to explain such differences. For example, genetic drift is able to drive the loss of genetic variants in a totally random process. The flux of haplotypes might involve not only nuclear genes, but also mitoch