Evolution of Adaptive Variation in the Mosquito Culex pipiens: Multiple Independent Origins of Insecticide Resistance Mutations

SIMPLE SUMMARY: The application of insecticides to control insect pests can result in the evolution of resistance. Within a population, the individuals carrying the resistant mutation survive after insecticide application while the others die, leading to the spread of resistance. The finding that the same mutations confer insecticide resistance in different species or populations raises the question how often these mutations arise in natural populations. Resistant mutations can originate once within a population and then spread. Alternatively, multiple origins can occur within the same population or in different geographic areas. Here, we used a phylogenetic approach to investigate the origin of three mutations conferring resistance to diflubenzuron insecticide in Italian and Turkish populations of the mosquito Culex pipiens. Our results support a scenario of multiple origins of the mutations. Resistance is a major threat to mosquito control, and these findings help inform resistance management. At the same time, insecticide resistance is an informative model for studying the origin of adaptive variation. In the words of Rachel Carson, “If Darwin were alive today, he would be astounded and delighted by the impressive verification that his theories of the survival of the fittest are receiving from the insect world” (Silent Spring, 1962) ABSTRACT: Insecticide resistance is an informative model for studying the appearance of adaptive traits. Simultaneously, understanding how many times resistance mutations originate is essential to design effective resistance management. In the mosquito Culex pipiens, target–site resistance to the insecticide diflubenzuron (DFB) has been recently found in Italian and Turkish populations. Three point mutations confer it at the codon 1043 of the chitin synthase 1 gene (chs-1): I1043L, I1043M, and I1043F. Whether the resistant mutations originated independently from different susceptible alleles or sequentially from resistant alleles and whether resistant alleles from Italy and Turkey have originated once or multiple times remain unresolved. Here, we sequenced a fragment of the chs-1 gene carrying the resistant mutations and inferred the phylogenetic relationships among susceptible and resistant alleles. Confirming previous findings, we found the three mutations in Italy and the I1043M in Turkey. Notably, the I1043F was also found for the first time in Turkish samples, highlighting the need for extensive monitoring activities. Phylogenetic analyses are consistent with an independent origin of the I1043F, I1043M, and I1043L mutations from different susceptible alleles and with multiple independent origins of the Italian and Turkish I1043M and I1043F alleles.