Studying selection in Anopheles stephensi under laboratory conditions
Abstract Background  An. stephensi exists in the peri-urban, urban, semi-urban, and arid ecosystems. It can survive in water polluted with nitrate, phosphate, and total organic carbon, indicating its ability to sustain and thrive in harsh urban environmental conditions. Compared to An. gambiae, An. stephensi has a wider range of thermal suitability for harbouring P. falciparum (15.3-37.2°C vs. 19.1-30.1°C) and P. vivax (15.7-32.5°C vs. 19.2-31.7°C); the high-temperature tolerance favours perennial malaria transmission by An. stephensi in more African regions. An. stephensi has spread to several countries in Africa—Djibouti, Sudan, Somalia, Ethiopia and Nigeria , in the last decade. Urbanization and immigration in African cities have provided a conducive ecological niche for An. stephensi; modeling suggests that ~126 million people in African cities are at risk. In 2017, An. stephensi was reported for the first time from Sri Lanka. The occurrence is worrisome as the country has been certified by WHO to have eliminated malaria in 2016.  The major motivation of the study was to study the genome plasticity and balancing selection in An. stephensi, for which we analysed the genome of laboratory-reared isofemale once every 25th generation—0 (Wild type), 25th, 50th, 100th, 125th, and 150th. Identifying loci that are maintained under balancing selection over 150 generations could give important insights into essential processes for its adaptability and survival. Results We sequenced whole genome for seven individuals from a clonal population, each belonging to a different generation and studied the genomic structure with Tajima'D and Beta scores for identifying signatures of selection. We observed five important genes which are undergoing balancing selection. The two genes, ASTEI04624 and ASTEI04623 are involved in the important processes such as sensory perception of taste and ASTEI10474 is involved in carbohydrate metabolism. Balancing selection was also observed in two genes ASTEI01081 and ASTEI00940 which are not yet functionally characterized. Conclusion Even though the clonal population is derived from an iso-female line and inbred under laboratory conditions, An. stephensi maintains a certain amount of heterozygosity in specific genes, essential for survival. We have identified candidate genes undergoing balancing selection at intermediate frequencies within the clonal population. With expansion of An. stephensi across urban Africa, understanding the function of genes involved in chemo sensation and other metabolic processes could be key for its control.