Morphometric and genome-wide variation in Ixodes ticks at an expanding range edge

Background/Question/Methods

Ixodid ticks (Acari: Ixodidae) are important vectors of zoonotic disease in humans, second only to mosquitos. Three genera comprise most of the medically important species in North America: Ixodes, Dermacentor, and Amblyomma. Ixodes scapularis (blacklegged tick) is of particular concern because it’s rapidly expanding its range along with Borrelia burgdorferi, the spirochete bacteria that causes Lyme disease. Hybridization within and between species can facilitate rapid spread with implications for pathogen risk, but this is rarely assessed in expanding populations. We developed a multiplex PCR for simple and cost-efficient screening of genome-wide variation via high-throughput sequencing. After filtering for polymorphic loci and off-target amplification, we surveyed genetic variation in ixodid ticks (N = 518) sampled from twenty locations across a Lyme disease hotspot at the leading edge of range expansion in Eastern, Ontario between 2017 and 2021. Sampled ticks were imaged and analyzed using 10 dorsal and 16 ventral landmarks known to vary among and within species. We assessed morphological and genetic diversity to address three questions: (1) How many genetically distinct species are present in the sample? (2) Is there evidence for interspecific hybridization? (3) How much morphological and genetic variation exists within vs among species and sample sites?

Results/Conclusions

After filtering genetic markers, 166 loci were sequenced in 518 individuals. Preliminary analysis of molecular data on 37 I. scapularis samples shows 29.5% polymorphic loci. Our preliminary principal component analysis using the polymorphic loci showed no clustering difference between male (n=18) and female (n=19) ticks nor between collection sites. However, our results showed that most of the variation in the ticks collected from the Queen’s University Biological Station were found along PC1(9.28%). Meanwhile, most of the variation in ticks collected form the “Odessa” site were found along PC2 (8.32%). Lastly, despite no evident clustering between sex or collection site, we identified four individuals that diverged from the main cluster which could be an indication of hybridization between species. For morphometrics, 10 dorsal and 16 ventral common landmarks were able to distinguish between species and sex with a high degree of accuracy. Dorsally, PC1 accounted for 82.69% of the morphologic variation while PC2 accounted for 6.87% of the variation. Male I. scapularis ticks were found to resemble ticks from the Dermacentor genus more than female I. scapularis ticks dorsally; however, the same trend was not found for the ventral aspect.