PHYLOGENETIC ANALYSIS OF HORIZONTAL TRANSFER OF WOLBACHIA AMONG INSECT HOSTS

Abstract

Wolbachia are highly prevalent endosymbiotic bacteria infecting more than two-thirds of arthropods. While Wolbachia are mainly transmitted vertically through the female germline, experiments and phylogenetic studies have suggested that horizontal transmission between hosts is not uncommon. However, there are no broad-scale surveys of Wolbachia horizontal transfer that describe patterns at a large scale, such as host order or continent. In this thesis, we aim to get a better understanding of horizontal transfer events and how they might occur in insects. Here, we report a comprehensive analysis of horizontal transfers in different host insect orders and continents using a large database containing nucleotide sequences of the genes wsp and ftsZ. We found heterogenous and unidirectional Wolbachia transition rates between insect orders and continents yet without any specific pattern of emergence and spread, based on both analysis of clades of high gene sequence similarity and best fitting models to the phylogenetic trees. Although each gene had a distinctive model of transfer and a distinctive phylogenetic signal, they exhibited alike indications within clades of similar sequences, suggesting that discrepancy in their phylogenetic histories of ancestry is more visible on a larger timescale. We were able to detect a signal of the host phylogeny and geography in the Wolbachia phylogeny of clades comprising highly similar sequences by Procrustean approach to co-phylogeny, however, host shifts between phylogenetically and geographically distant species was evident and frequent. Together, these results contribute to understanding the dynamics and patterns of Wolbachia host shifts. We anticipate this thesis to be a starting point for more sophisticated experiments, for example, Wolbachia trans-infection experiments between insects of the orders that showed zero rates of transfer and analysis of the possible physiological and/or ecological factors that might prevent horizontal transfer.