2.b.ii. Southern-to-Southern Comparisons
Southern populations have the highest level of genetic diversity among populations in the sampling region and is notably not related to the geographic distance of populations. The DAPC results show many populations in the south form their own distinct units, except for a few populations that are geographically close to each other with few environmental barriers (e.g., no mountain ranges), like the Louisiana counties. The Georgia and Alabama populations are in the geographic and analytical center of our Southern populations. The central location of these populations is consistent with the hypothesis that I. scapularis originated in Mississippi and Georgia before expanding out to Florida and North Carolina 500,000 years ago (Van Zee et al., 2015).
The locus-level analysis between Southern populations further demonstrated the high levels of genetic diversity throughout the southern U.S. Aiken County has the lowest level of differentiation relative to other comparisons involving Osceola County (pairwise FST = 0.194), but there is a moderate number of loci that are still significantly different between the two populations. This is not the case for other southern population pairs that are more closely related, such as Aiken County and Rapides County (pairwise FST = 0.065).
We have further identified that the Southern clade, which was originally classified as one distinct unit occurring across several geographic regions, actually consists of multiple identifiable clades. This is seen in the LEA analysis, as the individuals in the south still assorted into three distinct groups out of our five major clusters, even without any associated metadata. The top six variant sites contributing the most to differentiating the five clusters are focused on differentiating Osceola County from the rest of the individuals. Osceola County samples have allele frequencies in opposite proportion to all other populations. The variant at position NW024609868.1:60000181 is within the annotated gene LOC8029585. The studied function of this gene is as a neuropeptide SIFamide receptor which is theorized to affect salivary gland regulation and/or reproductive behavior regulation (Simo et al., 2013). Changes in the functions of salivary gland regulation could affect how B. burgdorferi is transmitted, as the bacteria passes through the salivary glands to reach the host (Schwan & Piesman, 2002). However, we cannot be sure if the variant detected here affects function.
For this study, we assume that the Osceola County population is pureI. scapularis . The morphological and 16S rRNA data supported this assumption, but these individuals still form a distinct cluster with low levels of admixture. It is important to acknowledge that despite the current results, it is possible the Osceola County population is comprised of I. scapularis x I. affinis hybrids. Other studies have shown the ability of Ixodes species to hybridize (Kovalev et al., 2016; Patterson et al., 2017) but have not yet investigated I. scapularis and I. affinis . Further investigation of I. scapularis and I. affinis from Florida is warranted.