Endosymbiont diversity across native and invasive brown widow spider populations.

The invasive brown widow spider, Latrodectus geometricus (Araneae: Theridiidae), has spread in multiple locations around the world and, along with it, brought associated organisms such as endosymbionts. We investigated endosymbiont diversity and prevalence across putative native and invasive populations of this spider, predicting lower endosymbiont diversity across the invasive range compared to the native range. First, we characterized the microbial community in the putative native (South Africa) and invasive (Israel and the United States) ranges via high throughput 16S sequencing of 103 adult females. All specimens were dominated by reads from only 1-3 amplicon sequence variants (ASV), and most individuals were infected with an apparently uniform strain of Rhabdochlamydia. We also found Rhabdochlamydia in spider eggs, indicating that it is a maternally-inherited endosymbiont. Relatively few other ASV were detected, but included two variant Rhabdochlamydia strains and several Wolbachia, Spiroplasma and Enterobacteriaceae strains. We then diagnostically screened 118 adult female spiders from native and invasive populations specifically for Rhabdochlamydia and Wolbachia. We found Rhabdochlamydia in 86% of individuals and represented in all populations, which suggests that it is a consistent and potentially important associate of L. geometricus. Wolbachia was found at lower overall prevalence (14%) and was represented in all countries, but not all populations. In addition, we found evidence for geographic variation in endosymbiont prevalence: spiders from Israel were more likely to carry Rhabdochlamydia than those from the US and South Africa, and Wolbachia was geographically clustered in both Israel and South Africa. Characterizing endosymbiont prevalence and diversity is a first step in understanding their function inside the host and may shed light on the process of spread and population variability in cosmopolitan invasive species.