Population genetics of Aedes vexans and its role in vectoring dog heartworm

The floodwater mosquito Aedes vexans is a potential vector of West Nile Virus and it has a worldwide distribution. Limited information is available on the genetic variability and population structure in relation to a small and large geographical area, and its capacity as a vector of dog heartworm needs to be understood. The first two objectives of this research involved sequencing a 423bp region of the NADH 5 subunit from the mitochondrial DNA genome to: (1) determine the temporal genetic variability and population structure of Ae. vexans in a limited geographical area (New Orleans, before and after hurricane Katrina; (2) determine the global genetic variability of Ae. vexans, using mainly specimens from North American localities. The third objective employed a PCR-base approach to screen Aedes vexans for Dirofilaria immitis, the causal agent of dog heartworm. In New Orleans, we compared mosquito specimens collected during 2005 (n = 99) before the hurricane, with specimens from 2006 (n = 103), after the hurricane. Thirty-eight and 62 haplotypes were observed for 2005 and 2006, respectively. Average haplotype diversity (Hd) was high (Hd > 0.88) in 2005 and 2006 for both New Orleans and Jefferson parishes. Neither genetic differentiation nor isolation by distance was observed across parishes, and gene flow and/or population growth was considerable. It does not appear that Hurricane Katrina had any impact on genetic diversity and, despite the intense efforts to control mosquitoes in New Orleans, Ae. vexans has not undergone a population bottleneck. The lack of a bottleneck effect maybe due to Ae. vexans breeding outside of the city where no mosquito control is conducted, and adults migrating into the city. For the second objective, we examined the genetic variation within 242 Ae. vexans specimens, the majority belonging to North American localities. In addition, specimens from Germany, Switzerland, and American Samoa were obtained. All specimens were collected during late spring and summer of 2006 and 2007. Ninety-seven haplotypes were found from the total population sampled. The average haplotype diversity of 0.944+/-0.009 indicated a high level of genetic diversity. Genetic variability was seen across locations and partitioned as 92.6% for within populations and 7.4% of the variation among populations. There was some level of correlation (0.33) between genetic and geographic distance as evidenced by the Mantel Test (p=0.08). Haplotypes 1 (16.7%) and 4 (18.6%) were the most abundant, and present in most of the locations sampled. Since there is considerable gene flow and this mosquito is able to pursue long flights, it is very likely that this mosquito can be an important vector of several viral diseases. The third objective focused on using a PCR based approach to detect Dirofilaria immitis in Ae. vexans. Traditional methods like morphological identification of larval stages from mosquito body parts are time consuming and require high technical expertise while a PCR-base approach can be faster and reliable. A 378-bp DNA fragment from a single repeated element was amplified through PCR using specific primers from 436 Aedes vexans mosquitoes. Heartworm was not detected in any of the specimens. A greater sample size and different mosquito trapping strategy could potentially result in positive detection of D. immitis in Ae. vexans populations.