Genetic Diversity and Differentiation
Genetic variation among I. scapularis populations differed depending on geographic location of the populations being compared. Our overall FST value was 0.176, with paired population FST values ranging from 0 – 0.417. Northern-to-Northern comparisons had a mean FST of 0.049 (min = 0, max = 0.302, median = 0.029), and the Southern-to-Southern comparisons had a mean FST of 0.189 (min = 0.002, max= 0.367, median = 0.175) (See Supplemental Table 3 for all pairwise FSTvalues). These comparisons showed that Northern populations are more closely related to other Northern populations than Southern populations are to each other.
To further investigate population differentiation, focal populations from each quadrant were chosen based on their dispersed geographic locations and how well they represented their quadrant (Southeastern Atlantic quadrant represented by Aiken County, SC and Osceola County, FL; Southern Gulf quadrant by Rapides County, LA; Upper Midwest quadrant by Monroe County, WI; and Northeastern quadrant by Washington County, RI), and site-specific statistics were then calculated. Comparing populations across the Northern and Southern boundary produces a higher level of genetic differentiation across more loci, and Southern populations have more differentiation at more loci with other Southern populations, than when you compare Northern to Northern (Supplemental Figures 1A-C).
Basic diversity statistics (Table 1) confirm genetic variation within all populations. In general, Northern populations have a slightly higher level of allelic richness than those in the South, and populations close to the dividing boundaries of our quadrants have higher levels than populations farther to the north or south. The Southern populations also have a slightly higher degree of inbreeding (average=0.27) compared to northern populations (average=0.22). Additional results, including AMOVA and Hardy-Weinberg equilibrium analyses, are given in the supplemental text.