Population analyses of Amblyomma maculatum ticks and Rickettsia parkeri using single-strand conformation polymorphism

Gulf Coast ticks, Amblyomma maculatum, and the zoonotic agents they transmit, Rickettsia parkeri, are expanding into areas in the United States where they were not previously reported, and are emerging threats for public and veterinary health. The dynamics of this tick-pathogen system and implications for disease transmission are still unclear. To assess genetic variation of tick and rickettsial populations, we collected adult A. maculatum from 10 sites in Mississippi, 4 in the northern, one in the central, and 5 in the southern part of the state. PCR amplicons from tick mitochondrial 16S rRNA and rickettsial ompA genes as well as 5 intergenic spacer regions were evaluated for genetic variation using single-strand conformation polymorphism analysis. Frequencies of the 4 tick 16S haplotypes were not significantly different among regions of Mississippi, but within sites there were differences in distribution that can be explained by high migration rates. Phylogenetically, one lineage of tick haplotypes was a species-poor sister group to remaining haplotypes in the species-rich sister group. No genetic variation was identified in any of the 6 selected gene targets of R. parkeri examined in the infected ticks, suggesting high levels of intermixing.