Carboxylesterase E3 gene evolution: Selection effects and geographic distribution of mutations associated to insecticide resistance in Cochliomyia hominivorax (Diptera: Calliphoridae)

Livestock production is an important economic activity in Brazil, but has been suffering significant losses due to the impact of parasites. The New World screwworm fly (NWS), Cochliomyia hominivorax, is an important ectoparasite and myiasis causing fly endemic in the Americas which stand out in this scenario. The current distribution of NWS includes part of Caribbean and all South America and has been successfully eradicated from North and part of Central America by sterile insect technique (SIT). In South America, NWS is controlled by chemical insecticides, which indiscriminate use has caused the selection of resistant individuals and hence reducing their effectiveness. The Gly137Asp and Trp251Leu substitutions in the active site of carboxylesterase E3 have been associated to resistance of diethyl and dimethyl organophosphates, respectively. To infer selective constraints on synonymous substitutions we aligned cDNA sequences of this esterase gene from sixteen Muscomorpha species available in Genebank. Codon substitution models were used, in which both likelihood ratio tests between the mutation-selection models implemented in CODEML program from PAML 4.6 (FMutSel0-M0 X FMutSel-M0 and FMutSel0-M3 X FMutSel-M3) were statistically significant (p=0.000, d.f.=41), indicating that only the mutational bias is not sufficient to explain the synonymous substitutions, but selection might be an important factor on their evolution. A second approach, based on the nucleotide composition at the third codon position and codon usage bias with RSCU (Relative Synonymous Codon Usage), showed the existence of a trend between the most abundant codons and a cytosine in that position of part of the four and six-fold degenerate amino acids. These results are in agreement with the idea that C-ending codons stabilize the mRNA and the efficiency of translation, and then would be favoured by selection. Future analysis will investigate positive selection on non-synonymous substitutions of C. hominivorax E3 gene.