Rhodnius prolixus supergene families of enzymes potentially associated with insecticide resistance
Graphical abstract
Introduction
Chagas disease, or American trypanosomiasis, is a potentially life-threatening illness caused by the protozoan parasite, Trypanosoma cruzi. Recent estimates indicate that about seven million people around the world might be infected (Punta et al., 2014, World Health Organization, 2014). Once known as an endemic health problem of poor rural populations in Latin American countries, where the primary insect vectors live, it has now spread worldwide with as many as 300 thousand infected individuals in the USA alone (Bern and Montgomery, 2009, CDC, 2013, Coura and Viñas, 2010). Because of growing population movements, Chagas disease is now a worldwide concern that can have severe consequences in the long term. The economic costs associated with disability and mortality of this disease are enormous and will continue to grow because of the high proportion of infected people that will develop the chronic form (Rassi et al., 2010, World Health Organization, 2008).
The parasite can be transmitted through blood-sucking triatomine bugs, contaminated food or transfusion of infected blood (World Health Organization, 2014). The disease is endemic in most Latin American countries where it is mainly vector-borne transmitted by contact with feces of some species of triatomine bugs (kissing bugs), of which Rhodnius prolixus (Hemiptera, Reduviidae, Triatominae) is one of the vectors and a model organism. R. prolixus occurs mainly in Central and South American countries where Chagas disease is endemic and a very serious health problem.
Classified as a tropical neglected disease, this illness does not receive attention from pharmaceutical and biotechnology industries and, consequently, there is little expectation for new drugs, diagnostic kits or even a vaccine. In this scenario, disease prevention focuses on vector control programs that, in general, rely intensely on chemical control (Moncayo and Silveira, 2009, World Health Organization, 2014). Some progress in containing disease transmission has been attained (Dias et al., 2002, Moncayo and Silveira, 2009) and R. prolixus has been recently considered eradicated from Central America (Hashimoto and Schofield, 2012). However, it is possible that other triatomine bugs may take its place. One of the reasons why species of Reduviidae have been important vectors is that most of them are very effective in adopting a domestic lifestyle, with Rhodnius being the most efficient one (Schofield et al., 1999). However, a sylvatic life cycle of T. cruzi has involved more than 150 species of triatomines and another 100 wild mammals for millions of years (Coura and Dias, 2009, Coura and Viñas, 2010).
R. prolixus is also an important model species and is responsible for many insights into the hematophagous Reduviidae life cycle, basic biology and development. The R. prolixus genome has recently been sequenced and is available for comparative studies with other insect species. These studies might help us better understand triatomine life cycle and evolution and identify new targets for insecticide development thus aiding the control of pathogen transmission.
Although it is expected that new vector control strategies should be developed for all insect species (McGraw and O'Neill, 2013), chemical control still plays an important part on integrated pest management programs. The massive use of chemical products can lead to insecticide resistance, as has happened with many insect vectors (Bass and Field, 2011, Li et al., 2007). In Triatoma infestans, the major Chagas disease vector in southern South America, for example, resistance to deltamethrin and other insecticides has been reported in different areas of the Gran Chaco region of Argentina and Bolivia (Germano et al., 2010, Picollo et al., 2005). Although only one study has documented insecticide resistance in R. prolixus (Vassena et al., 2000), parallel evolution seems to be especially common in this phenotypic trait (ffrench-Constant, 2013). Therefore, monitoring of insecticide resistance and the mechanisms involved is of critical importance.
Two major types of insecticide resistance mechanisms have been observed in natural populations: point mutations in insecticide targeted genes and over-expression or mutations in genes encoding detoxification enzymes. The latter, also known as metabolic resistance, implies a greater capacity to detoxify insecticides due to an increase in the expression or in the activity of genes related to detoxification metabolism. Four enzyme families – esterases, glutathione s-transferases (GST), cytochrome P450s (CYP) and, more recently, ABC transporters have been implicated in insecticide resistance and are therefore highly relevant to public health issues (Bass and Field, 2011, Dermauw and Van Leeuwen, 2014, Li et al., 2007).
In this study we investigate the gene content of three major detoxification enzyme families (esterases, GSTs and CYPs) in the R. prolixus genome. We compare both the identity and the number of genes found with other insect species whose genome sequences are available. Phylogenetic analysis was used to infer gene classification and evolution. R. prolixus exhibits a diverse set of genes encoding detoxification enzymes. In each family, gene numbers in some classes (or 'clans', according to the usual nomenclature adopted for CYPs) are similar to those of other hemipteran species while others resemble the honeybee Apis mellifera that has a genome depleted of detoxification genes. Comparative genomics of enzyme families conserved across organisms helps not only in gene annotation but also in unraveling their evolutionary structure and function. The identification of all these genes is of utmost importance as basic knowledge and also to direct detoxification studies on triatomines that can help insecticide management strategies in control programs.
Section snippets
Identification of R. prolixus detoxification enzymes
Sequences encoding GSTs and esterases were identified from the predicted protein set of R. prolixus (version Rhodnius-prolixus-CDC_PEPTIDES_RproC1.2), Drosophila melanogaster (version dmel-all-translation-r5), Acyrthosiphum pisum (version aphidbase_v2_pep), Ap. mellifera (version Apismellifera4) and Anopheles gambiae (version Anopheles-gambiae-PEST_PEPTIDES_AgamP3) using the FAT program (Seabra-Junior et al., 2011) with their respective protein family domains. The carboxylesterase (CCE)
Esterases
Most esterases belong to the carboxylesterase (CCE or COEsterase, Pfam PF00135 domain; Punta et al., 2014) gene family within the alpha/beta hydrolase fold superfamily (Hotelier et al., 2010, Hotelier et al., 2004). Although this has historically been the most studied domain, other types of esterases can be involved in different detoxification routes and may play important roles in insecticide resistance as well (Montella et al., 2012). The alpha/beta hydrolase fold domain is one of the most
Conclusions
The age of genomics and transcriptomics has brought a new baseline for insecticide resistance studies (ffrench-Constant, 2014). Instead of trying to find the unique gene responsible for insecticide resistance in a specific population, the omics approach makes it possible now to understand the intricate resistance response that seems to be the norm in insecticide resistance studies in vector and pest species. There have always been more differences than similarities when genes were investigated
Financial support
Several Brazilian governmental agencies contributed to this research: CNPq, FAPERJ, CAPES, SVS, INCT-EM. Sponsors did not have any role in the study design, sampling, analysis of the data or manuscript writing and submission.
Acknowledgments
The authors would like to thank RM Albano for a critical reading of an earlier version of the manuscript. NP and MPJ are members of the CONICET Researcher's Career, Argentina.
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2021, Pesticide Biochemistry and PhysiologyCitation Excerpt :However, another two members of CYP3 clan (CYP3085B1 and CYP3092A6) were not constitutively overexpressed in other pyrethroid-resistant T. infestans populations (Grosso et al., 2016). All these families were first described in R. prolixus genome, clustering separated from other insect species (Schama et al., 2016). Thus, the potential function of these new families should be deeply investigated in future studies.