Human risk of tick-borne illness has typically been estimated based on characterizations of host species composition and diversity. However it is becoming increasingly clear that the species composition within the tick itself, the tick microbiome, plays a crucial role in pathogen transmission dynamics. Interactions between vertically-transmitted endosymbionts and commensal bacteria found within the tick have been shown to affect pathogen acquisition, virulence, and transmission. Prior studies have found the tick microbiome to vary by species, sex, and geography, yet it remains unclear what natural factors dictate the composition of the tick microbiome. We investigated the role of environmental exposure and clutch identity on the microbiome of Ixodes pacificus ticks, the primary Lyme disease vector in the western U.S. We placed larval I. pacificus ticks in enclosed mesh bags in oak woodland habitat in Northern California, and performed microbiome analysis on a subset of these larvae at two-week intervals. Replicate larvae from three known maternal lines were included in order to tease apart the role of egg clutch identity and to compare abiotic versus biotic effects. Microbiome characterization of ticks from these treatment groups involved 16s gene sequencing on an Illumina MiSeq, and computational sequence analysis using Quantitative Insights in Microbial Ecology.
Microbiome characterizations were conducted on a total of 93 I. pacificus (90 larvae and the corresponding 3 maternal adults) yielding a list of microbial species present within each tick as well as their relative abundances. This sequencing data revealed that the length of environmental exposure as well as clutch identity influenced microbiome composition. Specifically, ticks within a single clutch displayed decreased microbiome species richness and diversity with increased environmental exposure. This finding, while contrary to expectation, is potentially explained by the increase in proportion of Rickettsia, a known bacterial endosymbiont of I. pacificus. We found the proportional abundance of Rickettsia increased as the tick matured in the environment while overall species richness and diversity declined suggesting a potential competitive interaction between Rickettsia and other microbiome species. Microbiome species composition was also analyzed for individuals within a single exposure treatment using Principal Coordinates Analysis which indicated clear clustering patterns based on clutch identity. Overall we conclude that both environmental exposure time and clutch identity cause natural variations in larval tick microbiomes which, when combined with prior transmission studies, suggest potential differences in human disease risk based on the age and maternal identity of the encountered tick.