The Phylogeographic History of the New World Screwworm Fly, Inferred by Approximate Bayesian Computation Analysis

<div><p>Insect pest phylogeography might be shaped both by biogeographic events and by human influence. Here, we conducted an approximate Bayesian computation (ABC) analysis to investigate the phylogeography of the New World screwworm fly, <i>Cochliomyia hominivorax</i>, with the aim of understanding its population history and its order and time of divergence. Our ABC analysis supports that populations spread from North to South in the Americas, in at least two different moments. The first split occurred between the North/Central American and South American populations in the end of the Last Glacial Maximum (15,300-19,000 YBP). The second split occurred between the North and South Amazonian populations in the transition between the Pleistocene and the Holocene eras (9,100-11,000 YBP). The species also experienced population expansion. Phylogenetic analysis likewise suggests this north to south colonization and Maxent models suggest an increase in the number of suitable areas in South America from the past to present. We found that the phylogeographic patterns observed in <i>C. hominivorax</i> cannot be explained only by climatic oscillations and can be connected to host population histories. Interestingly we found these patterns are very coincident with general patterns of ancient human movements in the Americas, suggesting that humans might have played a crucial role in shaping the distribution and population structure of this insect pest. This work presents the first hypothesis test regarding the processes that shaped the current phylogeographic structure of <i>C. hominivorax</i> and represents an alternate perspective on investigating the problem of insect pests.</p> </div

Carboxylesterase E3 <i>ChαE7</i> schematic view.

<p>Comparison of exon and intron lengths and position between (A) <i>L</i>. <i>cuprina</i> and (B) <i>C</i>. <i>hominivorax</i>. Exons and introns were named according to <i>L</i>. <i>cuprina</i> (e = exon; I = intron) [<a href="http://www.plosone.org/article/info:doi/10.1371/journal.pone.0128441#pone.0128441.ref027" target="_blank">27</a>]. The position of primers used for gene region characterization (7F0a, RN2, 7FIn1 and 7R1aN) and for posterior populational analyses (7FIn2, RN2 and 7R3a) are indicated by blue and red arrows, respectively. RN2 is indicated by a green arrow (used for both characterization and population analyses).</p

Incongruent Nuclear and Mitochondrial Genetic Structure of New World Screwworm Fly Populations Due to Positive Selection of Mutations Associated with Dimethyl- and Diethyl-Organophosphates Resistance

<div><p>Livestock production is an important economic activity in Brazil, which has been suffering significant losses due to the impact of parasites. The New World screwworm (NWS) fly, <i>Cochliomyia hominivorax</i>, is an ectoparasite and one of the most important myiasis-causing flies endemic to the Americas. The geographic distribution of NWS has been reduced after the implementation of the Sterile Insect Technique (SIT), being eradicated in North America and part of Central America. In South America, <i>C</i>. <i>hominivorax</i> is controlled by chemical insecticides, although indiscriminate use can cause selection of resistant individuals. Previous studies have associated the Gly137Asp and Trp251Leu mutations in the active site of carboxylesterase E3 to resistance of diethyl and dimethyl-organophosphates insecticides, respectively. Here, we have sequenced a fragment of the carboxylesterase E3 gene (<i>ChαE7</i>), comprising part of intron iII, exon eIII, intron iIII and part of exon eIV, and three mitochondrial gene sequences (CR, COI and COII), of NWS flies from 21 locations in South America. These markers were used for population structure analyses and the <i>ChαE7</i> gene was also investigated to gain insight into the selective pressures that have shaped its evolution. Analysis of molecular variance (AMOVA) and pairwise FST analysis indicated an increased genetic structure between locations in the <i>ChαE7</i> compared to the concatenated mitochondrial genes. Discriminant analysis of principal components (DAPC) and spatial analysis of molecular variance (SAMOVA) indicated different degrees of genetic structure for all markers, in agreement with the AMOVA results, but with low correlation to geographic data. The NWS fly is considered a panmitic species based on mitochondrial data, while it is structured into three groups considering the <i>ChαE7</i> gene. A negative association between the two mutations related to organophosphate resistance and Fay & Wu’s H significant negative values for the exons, suggest that these mutations evolved under positive selection.</p></div

Frequencies of polymorphic nucleotides related to the determination of susceptibility and resistance in the codons 137 and 251 of the <i>ChαE7</i> sequence.

<p>The structure in groups I, II and III (K = 3) was analyzed. Codons were named for the encoded amino acid, <i>ChαE7</i> sequence position (amino acid number) and the nucleotide substitution of that codon (Gly137-G, Asp137-A, Trp251-G, Leu251-T and Ser251-C).</p

Structure analyses results for esterase data (K = 3).

<p>(A) SAMOVA F indices. F_CT: differentiation between groups; F_SC: differentiation between sampling locations within groups; F_ST: differentiation between sites between groups. (B) Pie charts of the posterior probability to be from a group estimated by DAPC (i.e. Red, Yellow and Green) and SAMOVA grouping (i.e. I, II and III). SAMOVA groups (I, II and III), considered for posterior analyses, are defined by the grey, purple and blue contours.</p

Hardy-Weinberg equilibrium tests for the two polymorphic codon positions related to the determination of susceptibility and resistance.

<p>Statistically significant values are in bold. Ho = observed heterozygosity; He = expected heterozygosity; s.d. = standard deviation.</p><p>Hardy-Weinberg equilibrium tests for the two polymorphic codon positions related to the determination of susceptibility and resistance.</p

Mismatch distribution analyses results.

<p>(A) Mitochondrial sequences considering the complete sample in a group. (B) <i>ChαE7</i> sequences considering Group I. (C) <i>ChαE7</i> sequences considering Group II. (D) <i>ChαE7</i> sequences considering Group III from SAMOVA, respectively.</p