Abstract
Infections by maternally inherited bacterial endosymbionts, especially
Wolbachia, are common in insects and other invertebrates but
infection dynamics across species ranges are largely under studied.
Specifically, we lack a broad understanding of the origin of
Wolbachia infections in novel hosts and the factors governing
their spread. We used Genotype-by-Sequencing (GBS) data from previous
population genomics studies for range-wide surveys of Wolbachia
presence and genetic diversity in over 2,700 North American butterflies
of the genus Lycaeides. As few as one sequence read identified by
assembly to a Wolbachia pan-reference genome provided high
accuracy in detecting infections as determined by confirmatory PCR
tests. Using a conservative threshold of five reads, we detected
Wolbachia in all but two of the 107 sampling localities spanning
the continent, and with most localities having high infection
frequencies (mean = 91\% infection rate). Three major
lineages of Wolbachia were identified as separate strains that
appear to represent three separate invasions of Lycaeides
butterflies. Overall, we found extensive evidence for acquisition of
Wolbachia through interspecific transfer between host lineages.
Strain wLycC was confined to a single butterfly taxon, hybrid
lineages derived from it, and closely adjacent populations in other
taxa. While the other two strains were detected throughout the rest of
the continent, strain wLycB almost always co-occurred with
wLycA. Our demographic modeling suggests wLycB is a recent
invasion. These results demonstrate the utility of using resequencing
data from hosts to quantify Wolbachia genetic variation and
provide evidence of multiple colonizations of novel hosts through
hybridization between butterfly lineages and complex dynamics between
Wolbachia strains.