Wolbachia infection and host shifts in scale insects

Abstract

Wolbachia is one of the most successful endosymbiotic bacteria of arthropods. Known as the “master of manipulation”. These bacteria can induce a wide range of phenotypes in their hosts, resulting in far-reaching ecological and evolutionary consequences. These consequences can subsequently be exploited for disease and pest control. Over the last decades, research into these bacteria has thrived at an ever-increasing pace, covering many different aspects of Wolbachia. However, there is a prominent feature of Wolbachia that has received less scientific attention: its surprisingly well-developed ability to switch from one host species to another. In this thesis, I first provide a thorough review of our current knowledge of Wolbachia host shifts that have accumulated over the last three decades (Chapter 2). To this end, I synthesise insights from diverse disciplines, including cell biology, microbiology, immunology, developmental biology, ecology and evolutionary biology.While Wolbachia can be observed in many arthropods, our knowledge of Wolbachia’s infection rate is unevenly distributed across arthropod groups. In Chapter 3, I addressed this knowledge gap by conducting the first survey for Wolbachia infection in scale insects. I fitted a distribution of within-species prevalence of Wolbachia to my data and compared it to distributions fitted of an up-to-date dataset compiled from surveys across all arthropods. Prevalence of Wolbachia in scale insects follows a distribution similar to exponential decline (most species are predicted to have low to intermediate prevalence), in contrast to the U-shaped distribution estimated for other taxa (most host species are predicted to either have a very low or very high Wolbachia prevalence). I observed no significant associations between Wolbachia infection and five scale insect traits (mode of reproduction, distribution, number of known parasitoid species, host breadth, and ability to induce galls).In Chapter 4, I studied the genetic diversity of Wolbachia strains in scale insects, determined their phylogenetic relationships and detected host-shift events. Host phylogeny and geographic distributions are the two main factors determining host shifts, however, their effect in Wolbachia host-shifting is mainly unknown. I used scale insects as a study model to evaluate the contribution of these two factors in host shifting. Using Illumina pooled amplicon sequencing together with a novel bioinformatic pipeline, I first revealed the strain diversity and composition (from single to multiple infections) of Wolbachia in Australian scale insects. 32% of all samples were co-infected with two or more strains of Wolbachia. wSph1 was identified as the dominant strain group among not only scale insects but other associate species, suggesting this strain group can be considered a “super-spreader”. I fitted a modified version of the GAMM model to host phylogeny and home range distribution in order to statistically evaluate factors influencing host shifts in scale insects. The results showed that host-shifts in scale insects can be mainly explained by the phylogenetic distance effect, whereas geographic distribution has no significant contribution.Ecological interactions, such as prey-predator, host-parasitoid, and sharing common food resources are among the common pathways of horizontal transfer of Wolbachia, also referred as routes of transfer. The direct interactions of scale insects with many ecological associates such as ants and parasitoids can give rise to potential routes of transfer. To ascertain whether this is the case, in Chapter 5, I screened for Wolbachia in associates (such as ants, parasitoid wasps, beetles) that were directly collected with the scale insects. I detected a positive correlation between Wolbachia infection in scale insects and their ant associates, pointing to a possible route of Wolbachia horizontal transmission. Based on the sequencing data, most of the associates I tested in the current study could be involved in host-shifting. Overall, this thesis contributes to our knowledge of Wolbachia distributions and host-shifting, especially on the infection rate, distribution and routes of horizontal transfer of this symbiont in scale insects.