Abstract
Insects are the most abundant and diverse species on Earth. Numerous microorganisms have a broad spectrum of associations with insects, impacting their biology and evolution. In fact, insect hosts provide a valuable ecological niche for microorganisms, living in body cavities, gut, cuticles and host cells. The relationship between insects and microorganisms comprises a continuum, ranging from obligate symbionts which are crucial for host survival to facultative symbionts that exert different effects on the insect hosts. In agriculture, insect pests cause great losses in food production. Globally, invasive species are increasing their distribution range, contributing to the spread of diseases in new regions and supporting local dynamics of disease transmission. Phytoplasmas are vector borne pathogens transmitted to various plant species, including many agriculturally important crops. The vectors responsible for their transmission are hemipterans belonging to several families. In Europe, the American grapevine leafhopper Scaphoideus titanus is the main vector of ‘Candidatus Phytoplasma vitis’ the causative agent of Flavescence dorée in grapevine plants. S. titanus was introduced accidentally in Europe in 1958 and despite decades of mandatory use of insecticides, the spreading of S. titanus is increasing, reaching new countries inside and outside the European Union. Although many aspects of the biology of S. titanus are well understood, there is still incomplete knowledge about the ecology and taxonomy of this vector. This thesis focused on the investigation of the bacterial communities of S. titanus, as well as its genetic diversity through high-throughput sequencing approaches. First, I focused on studying the ecology of bacterial communities in S. titanus across Europe, comparing bacterial composition between different populations and life stages using 16S rRNA gene metabarcoding. In this context, I applied methods in microbial ecology and bioinformatics to characterize the bacterial communities in S. titanus (Chapter I and II). Consequently, using high-throughput sequencing approaches, I assembled the complete mitochondrial genome of five Scaphoideus species to reconstruct their phylogenetic relationships with emphasis on S. titanus, to assess the structure of the genetic populations in S. titanus across Europe and North America thus revealing potential routes of invasion (Chapter III). For the last chapter I investigated the seasonal variations of the primary symbionts of Cacopsylla pyricola, the vector of the causative agent of pear decline, ‘Candidatus Phytoplasma pyri’ (Chapter IV). These studies provide new insights about the introduction of the American grapevine leafhopper in Europe and the composition of its bacterial communities.