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.