Aedes aegypti vector competence and gene flow in Mexico. Association mapping software for testing candidates genes associated with a phenotype

Abstract

I have discovered an abrupt barrier to gene flow in Ae. aegypti associated with the Neovolcanic axis (NVA) along the Gulf Coast of Mexico. The mountains associated with the NV A limit the distribution of Ae. aegypti to ~ 15 km between the mountains and the Atlantic Ocean. Most importantly the change in gene flow appears to have also caused an abrupt increase in the frequency of mosquitoes with a midgut infection barrier (MIB) to the south of the NVA and a gradual increase in mosquitoes with a midgut escape barrier (MEB) south of the axis. The results of this study demonstrate how a barrier in gene flow can also cause a shift in the frequencies of alleles at MIB and MEB loci and thereby cause changes in susceptibility. Early trypsin (ET) is considered a candidate gene controlling MIB and MEB. It has been shown that Ae. aegypti females fed an infectious blood meal, containing a soybean trypsin inhibitor, presented a decrease in midgut DEN2 RNA copies. DEN2 proteolytic processing, mediated by midgut trypsins, probably influenced the infection (Molina-Cruz et al. 2004). Six criteria were applied to minimize the detection of false positive QTNs when testing the association between segregating sites in ET and DEN2 susceptibility in Ae. aegypti. First, a priori hypothesis that ET is a candidate gene was established. Second, large sample sizes were used to maximize the analysis power. Third, the independence of segregation of alternate nucleotides at segregating sites was tested. Fourth, I used mtDNA (neutral marker) to identify false QTNs arising from analysis of genetically distinct populations. Fifth, populations were subdivided (panmictic units) to independently validated QTNs. Sixth, patterns of association between alleles and genotypes were examined when presumptive QTNs arise. No consistent associations between segregating sites in ET and susceptibility were found. However, little systematic disequilibrium was detected in the ET gene allowing the possibility that cis elements in close proximity to the ET gene were undetected. Additionally, mutations in enhancers or promoters residing outside the analyzed region, which could be associated with the phenotype, remain a de facto qualification of QTN studies.