Silencing OsHI-LOX makes rice more susceptible to chewing herbivores, but enhances resistance to a phloem feeder

The jasmonic acid (JA) pathway plays a central role in plant defense responses against insects. Some phloem-feeding insects also induce the salicylic acid (SA) pathway, thereby suppressing the plant’s JA response. These phenomena have been well studied in dicotyledonous plants, but little is known about them in monocotyledons. We cloned a chloroplast-localized type 2 13-lipoxygenase gene of rice, OsHI-LOX, whose transcripts were up-regulated in response to feeding by the rice striped stem borer (SSB) Chilo suppressalis and the rice brown planthopper (BPH) Niaparvata lugens, as well as by mechanical wounding and treatment with JA. Antisense expression of OsHI-LOX (as-lox) reduced SSB- or BPH-induced JA and trypsin protease inhibitor (TrypPI) levels, improved the larval performance of SBB as well as that of the rice leaf folder (LF) Cnaphalocrocis medinalis, and increased the damage caused by SSB and LF larvae. In contrast, BPH, a phloem-feeding herbivore, showed a preference for settling and ovipositing on WT plants, on which they consumed more and survived better than on as-lox plants. The enhanced resistance of as-lox plants to BPH infestation correlated with higher levels of BPH-induced H2O2 and SA, as well as with increased hypersensitive response-like cell death. These results imply that OsHI-LOX is involved in herbivore-induced JA biosynthesis, and plays contrasting roles in controlling rice resistance to chewing and phloem-feeding herbivores. The observation that suppression of JA activity results in increased resistance to an insect indicates that revision of the generalized plant defense models in monocotyledons is required, and may help develop novel strategies to protect rice against insect pests.

KRAS and HRAS mutations confer resistance to MET targeting in preclinical models of MET-expressing tumor cells.

The MET receptor tyrosine kinase is often deregulated in human cancers and several MET inhibitors are evaluated in clinical trials. Similarly to EGFR, MET signals through the RAS-RAF-ERK/MAPK pathway which plays key roles in cell proliferation and survival. Mutations of genes encoding for RAS proteins, particularly in KRAS, are commonly found in various tumors and are associated with constitutive activation of the MAPK pathway. It was shown for EGFR, that KRAS mutations render upstream EGFR inhibition ineffective in EGFR-positive colorectal cancers. Currently, there are no clinical studies evaluating MET inhibition impairment due to RAS mutations. To test the impact of RAS mutations on MET targeting, we generated tumor cells responsive to the MET inhibitor EMD1214063 that express KRAS G12V, G12D, G13D and HRAS G12V variants. We demonstrate that these MAPK-activating RAS mutations differentially interfere with MET-mediated biological effects of MET inhibition. We report increased residual ERK1/2 phosphorylation indicating that the downstream pathway remains active in presence of MET inhibition. Consequently, RAS variants counteracted MET inhibition-induced morphological changes as well as anti-proliferative and anchorage-independent growth effects. The effect of RAS mutants was reversed when MET inhibition was combined with MEK inhibitors AZD6244 and UO126. In an in vivo mouse xenograft model, MET-driven tumors harboring mutated RAS displayed resistance to MET inhibition. Taken together, our results demonstrate for the first time in details the role of KRAS and HRAS mutations in resistance to MET inhibition and suggest targeting both MET and MEK as an effective strategy when both oncogenic drivers are expressed.

Subunit composition of VRAC channels determines substrate specificity and cellular resistance to Pt-based anti-cancer drugs

Although platinum-based drugs are widely used chemotherapeutics for cancer treatment, the determinants of tumor cell responsiveness remain poorly understood. We show that the loss of subunits LRRC8A and LRRC8D of the heteromeric LRRC8 volume-regulated anion channels (VRACs) increased resistance to clinically relevant cisplatin/carboplatin concentrations. Under isotonic conditions, about 50% of cisplatin uptake depended on LRRC8A and LRRC8D, but neither on LRRC8C nor on LRRC8E. Cell swelling strongly enhanced LRRC8-dependent cisplatin uptake, bolstering the notion that cisplatin enters cells through VRAC. LRRC8A disruption also suppressed drug-induced apoptosis independently from drug uptake, possibly by impairing VRAC-dependent apoptotic cell volume decrease. Hence, by mediating cisplatin uptake and facilitating apoptosis, VRAC plays a dual role in the cellular drug response. Incorporation of the LRRC8D subunit into VRAC substantially increased its permeability for cisplatin and the cellular osmolyte taurine, indicating that LRRC8 proteins form the channel pore. Our work suggests that LRRC8D-containing VRACs are crucial for cell volume regulation by an important organic osmolyte and may influence cisplatin/carboplatin responsiveness of tumors.

Selective resistance to the PARP inhibitor olaparib in a mouse model for BRCA1-deficient metaplastic breast cancer

Metaplastic breast carcinoma (MBC) is a rare histological breast cancer subtype characterized by mesenchymal elements and poor clinical outcome. A large fraction of MBCs harbor defects in breast cancer 1 (BRCA1). As BRCA1 deficiency sensitizes tumors to DNA cross-linking agents and poly(ADP-ribose) polymerase (PARP) inhibitors, we sought to investigate the response of BRCA1-deficient MBCs to the PARP inhibitor olaparib. To this end, we established a genetically engineered mouse model (GEMM) for BRCA1-deficient MBC by introducing the MET proto-oncogene into a BRCA1-associated breast cancer model, using our novel female GEMM ES cell (ESC) pipeline. In contrast to carcinomas, BRCA1-deficient mouse carcinosarcomas resembling MBC show intrinsic resistance to olaparib caused by increased P-glycoprotein (Pgp) drug efflux transporter expression. Indeed, resistance could be circumvented by using another PARP inhibitor, AZD2461, which is a poor Pgp substrate. These preclinical findings suggest that patients with BRCA1-associated MBC may show poor response to olaparib and illustrate the value of GEMM-ESC models of human cancer for evaluation of novel therapeutics.

Blocking the epithelial-to-mesenchymal transition pathway abrogates resistance to anti-folate chemotherapy in lung cancer.

Anticancer therapies currently used in the clinic often can neither eradicate the tumor nor prevent disease recurrence due to tumor resistance. In this study, we showed that chemoresistance to pemetrexed, a multi-target anti-folate (MTA) chemotherapeutic agent for non-small cell lung cancer (NSCLC), is associated with a stem cell-like phenotype characterized by an enriched stem cell gene signature, augmented aldehyde dehydrogenase activity and greater clonogenic potential. Mechanistically, chemoresistance to MTA requires activation of epithelial-to-mesenchymal transition (EMT) pathway in that an experimentally induced EMT per se promotes chemoresistance in NSCLC and inhibition of EMT signaling by kaempferol renders the otherwise chemoresistant cancer cells susceptible to MTA. Relevant to the clinical setting, human primary NSCLC cells with an elevated EMT signaling feature a significantly enhanced potential to resist MTA, whereas concomitant administration of kaempferol abrogates MTA chemoresistance, regardless of whether it is due to an intrinsic or induced activation of the EMT pathway. Collectively, our findings reveal that a bona fide activation of EMT pathway is required and sufficient for chemoresistance to MTA and that kaempferol potently regresses this chemotherapy refractory phenotype, highlighting the potential of EMT pathway inhibition to enhance chemotherapeutic response of lung cancer.

Mechanisms and evolution of plant resistance to aphids

Aphids are important herbivores of both wild and cultivated plants. Plants rely on unique mechanisms of recognition, signalling and defence to cope with the specialized mode of phloem feeding by aphids. Aspects of the molecular mechanisms underlying aphid-plant interactions are beginning to be understood. Recent advances include the identification of aphid salivary proteins involved in host plant manipulation, and plant receptors involved in aphid recognition. However, a complete picture of aphid-plant interactions requires consideration of the ecological outcome of these mechanisms in nature, and the evolutionary processes that shaped them. Here we identify general patterns of resistance, with a special focus on recognition, phytohormonal signalling, secondary metabolites and induction of plant resistance. We discuss how host specialization can enable aphids to co-opt both the phytohormonal responses and defensive compounds of plants for their own benefit at a local scale. In response, systemically induced resistance in plants is common and often involves targeted responses to specific aphid species or even genotypes. As co-evolutionary adaptation between plants and aphids is ongoing, the stealthy nature of aphid feeding makes both the mechanisms and outcomes of these interactions highly distinct from those of other herbivore-plant interactions. © 2016 Macmillan Publishers Limited.

Sphingosine kinase 2 deficient mice exhibit reduced experimental autoimmune encephalomyelitis: Resistance to FTY720 but not ST-968 treatments

The immunomodulatory drug FTY720 is presently approved for the treatment of relapsing-remitting multiple sclerosis. It is a prodrug that requires activation by sphingosine kinase 2 (SK-2) to induce T cell homing to secondary lymphoid tissue. In this study, we have investigated the role of SK-2 in experimental autoimmune encephalomyelitis (EAE) in C57BL/6 mice. We show that SK-2 deficiency reduced clinical symptoms of EAE. Furthermore, in SK-2-deficient mice, the protective effect of FTY720 on EAE was abolished, while the non-prodrug FTY720-derivative ST-968 was still fully active. Protection was paralleled by reduced numbers of T-lymphocytes in blood and a reduced blood-brain-barrier leakage. This correlated with reduced mRNA expression of ICAM-1, VCAM-1, but enhanced expression of PECAM-1. A similar regulation of permeability and of PECAM-1 was seen in primary cultures of isolated mouse brain vascular endothelial cells and in a human immortalized cell line upon SK-2 knockdown. In summary, these data demonstrated that deletion of SK-2 exerts a protective effect on the pathogenesis of EAE in C57BL/6 mice and that SK-2 is essential for the protective effect of FTY720 but not of ST-968. Thus, ST-968 is a promising novel immunomodulatory compound that may be a valuable alternative to FTY720 under conditions where SK-2 activity is limited.

The transcriptional repressor Delta EF1 controls resistance to the EGFR inhibitor erlotinib in human HNSCC lines

Epidermal Growth Factor Receptor (EGFR) overexpression occurs in about 90% of Head and Neck Squamous Cell Carcinoma (HNSCC) cases. Aberrant EGFR signaling has been implicated in the malignant features of HNSCC. Thus, EGFR appears to be a logical therapeutic target with increased tumor specificity for the treatment of HNSCC. Erlotinib, a small molecule tyrosine kinase inhibitor, specifically inhibits aberrant EGFR signaling in HNSCC. Only a minority of HNSCC patients were able to derive a substantial clinical benefit from erlotinib. ^ This dissertation identifies Epithelial to Mesenchymal Transition (EMT) as the biological marker that distinguishes EGFR-dependent (erlotinib-sensitive) tumors from the EGFR-independent (erlotinib-resistant) tumors. This will allow us to prospectively identify the patients who are most likely to benefit from EGFR-directed therapy. More importantly, our data identifies the transcriptional repressor DeltaEF1 as the mesenchymal marker that controls EMT phenotype and resistance to erlotinib in human HNSCC lines. si-RNA mediated knockdown of DeltaEF1 in the erlotinib-resistant lines resulted in reversal of the mesenchymal phenotype to an epithelial phenotype and significant increase in sensitivity to erlotinib. ^ DeltaEF1 represses the expression of the epithelial markers by recruiting HDACs to chromatin. This observation allows us to translate our findings into clinical application. To test whether the transcriptional repression by DeltaEF1 underlines the mechanism responsible for erlotinib resistance, erlotinib-resistant lines were treated with an HDAC inhibitor (SAHA) followed by erlotinib. This resulted in a synergistic effect and substantial increase in sensitivity to erlotinib in the resistant cell lines. Thus, combining an HDAC inhibitor with erlotinib represents a novel promising pharmacologic strategy for reversing resistance to erlotinib in HNSCC patients. ^

Impact of elevated pCO2 on acid-base regulation of the sea urchin Echinometra mathaei and its relation to resistance to ocean acidification: A study in mesocosms

Due to their low metabolism and apparent poor ion regulation ability, sea urchins could be particularly sensitive to ocean acidification resulting from increased dissolution of atmospheric carbon dioxide. Therefore, we evaluated the acid-base regulation ability of the coral reef sea urchin Echinometra mathaei and the impact of decreased pH on its growth and respiration activity. The study was conducted in two identical artificial reef mesocosms during seven weeks. Experimental tanks were maintained respectively at mean pHT 7.7 and 8.05 (with field-like night and day variations). The major physico-chemical parameters were identical, only pCO2 and pHT differed. Results indicate that E. mathaei can regulate the pH of its coelomic fluid in the considered range of pH, allowing a sustainable growth and ensuring an unaffected respiratory metabolism, at least at short term.

Biomineralization changes with food supply confer juvenile scallops (Argopecten purpuratus) resistance to ocean acidification

Future ocean acidification (OA) will affect physiological traits of marine species, with calcifying species being particularly vulnerable. As OA entails high energy demands, particularly during the rapid juvenile growth phase, food supply may play a key role in the response of marine organisms to OA. We experimentally evaluated the role of food supply in modulating physiological responses and biomineralization processes in juveniles of the Chilean scallop, Argopecten purpuratus, that were exposed to control (pH 8.0) and low pH (pH 7.6) conditions using three food supply treatments (high, intermediate, and low). We found that pH and food levels had additive effects on the physiological response of the juvenile scallops. Metabolic rates, shell growth, net calcification, and ingestion rates increased significantly at low pH conditions, independent of food. These physiological responses increased significantly in organisms exposed to intermediate and high levels of food supply. Hence, food supply seems to play a major role modulating organismal response by providing the energetic means to bolster the physiological response of OA stress. On the contrary, the relative expression of chitin synthase, a functional molecule for biomineralization, increased significantly in scallops exposed to low food supply and low pH, which resulted in a thicker periostracum enriched with chitin polysaccharides. Under reduced food and low pH conditions, the adaptive organismal response was to trade-off growth for the expression of biomineralization molecules and altering of the organic composition of shell periostracum, suggesting that the future performance of these calcifiers will depend on the trajectories of both OA and food supply. Thus, incorporating a suite of traits and multiple stressors in future studies of the adaptive organismal response may provide key insights on OA impacts on marine calcifiers.

Tough as a rock-boring urchin: adult Echinometra sp. EE from the Red Sea show high resistance to ocean acidification over long-term exposures

Ocean acidification, a process caused by the continuous rise of atmospheric CO2 levels, is expected to have a profound impact on marine invertebrates. Findings of the numerous studies conducted in this field indicate high variability in species responses to future ocean conditions. This study aimed at understanding the effects of long-term exposure to elevated pCO2 conditions on the performance of adult Echinometra sp. EE from the Gulf of Aqaba (Red Sea). During an 11-month incubation under high pCO2 (1,433 µatm, pHNBS 7.7) and control (435 µatm, pHNBS 8.1) conditions, we examined the urchins' somatic and gonadal growth, gametogenesis and skeletal microstructure. Somatic and gonadal growths were exhibited with no significant differences between the treatments. In addition, all urchins in the experiment completed a full reproductive cycle, typical of natural populations, with no detectable impact of increased pCO2 on the timing, duration or progression of the cycle. Furthermore, scanning electron microscopy imaging of urchin tests and spines revealed no signs of the usual observed effects of acidosis, such as skeletal dissolution, widened stereom pores or non-smoothed structures. Our results, which yielded no significant impact of the high pCO2 treatment on any of the examined processes in the urchins studied, suggest high resistance of adult Echinometra sp. EE to near future ocean acidification conditions. With respect to other findings in this area, the outcome of this study provides an example of the complicated and diverse responses of echinoids to the predicted environmental changes.

Arabidopsis heterotrimeric G-protein regulates cell wall defense and resistance to necrotrophic fungi

The Arabidopsis heterotrimeric G-protein controls defense responses to necrotrophic and vascular fungi. The agb1 mutant impaired in the Gβ subunit displays enhanced susceptibility to these pathogens. Gβ/AGB1 forms an obligate dimer with either one of the Arabidopsis Gγ subunits (γ1/AGG1 and γ2/AGG2). Accordingly, we now demonstrate that the agg1 agg2 double mutant is as susceptible as agb1 plants to the necrotrophic fungus Plectosphaerella cucumerina. To elucidate the molecular basis of heterotrimeric G-protein-mediated resistance, we performed a comparative transcriptomic analysis of agb1-1 mutant and wild-type plants upon inoculation with P. cucumerina. This analysis, together with metabolomic studies, demonstrated that G-protein-mediated resistance was independent of defensive pathways required for resistance to necrotrophic fungi, such as the salicylic acid, jasmonic acid, ethylene, abscisic acid, and tryptophan-derived metabolites signaling, as these pathways were not impaired in agb1 and agg1 agg2 mutants. Notably, many mis-regulated genes in agb1 plants were related with cell wall functions, which was also the case in agg1 agg2 mutant. Biochemical analyses and Fourier Transform InfraRed (FTIR) spectroscopy of cell walls from G-protein mutants revealed that the xylose content was lower in agb1 and agg1 agg2 mutants than in wild-type plants, and that mutant walls had similar FTIR spectratypes, which differed from that of wild-type plants. The data presented here suggest a canonical functionality of the Gβ and Gγ1/γ2 subunits in the control of Arabidopsis immune responses and the regulation of cell wall composition.

Disruption of abscisic acid signaling constitutively activates Arabidopsis resistance to the necrotrophic fungus Plectosphaerella cucumerina.

Plant resistance to necrotrophic fungi is regulated by a complex set of signaling pathways that includes those mediated by the hormones salicylic acid (SA), ethylene (ET), jasmonic acid (JA), and abscisic acid (ABA). The role of ABA in plant resistance remains controversial, as positive and negative regulatory functions have been described depending on the plant-pathogen interaction analyzed. Here, we show that ABA signaling negatively regulates Arabidopsis (Arabidopsis thaliana) resistance to the necrotrophic fungus Plectosphaerella cucumerina. Arabidopsis plants impaired in ABA biosynthesis, such as the aba1-6 mutant, or in ABA signaling, like the quadruple pyr/pyl mutant (pyr1pyl1pyl2pyl4), were more resistant to P. cucumerina than wild-type plants. In contrast, the hab1-1abi1-2abi2-2 mutant impaired in three phosphatases that negatively regulate ABA signaling displayed an enhanced susceptibility phenotype to this fungus. Comparative transcriptomic analyses of aba1-6 and wild-type plants revealed that the ABA pathway negatively regulates defense genes, many of which are controlled by the SA, JA, or ET pathway. In line with these data, we found that aba1-6 resistance to P. cucumerina was partially compromised when the SA, JA, or ET pathway was disrupted in this mutant. Additionally, in the aba1-6 plants, some genes encoding cell wall-related proteins were misregulated. Fourier transform infrared spectroscopy and biochemical analyses of cell walls from aba1-6 and wild-type plants revealed significant differences in their Fourier transform infrared spectratypes and uronic acid and cellulose contents. All these data suggest that ABA signaling has a complex function in Arabidopsis basal resistance, negatively regulating SA/JA/ET-mediated resistance to necrotrophic fungi.

Resistance to malathion and lambda-cyhalothrin in Ceratitis capitata (Wiedemann) (Diptera: Tephritidae)

La mosca mediterránea de la fruta, Ceratitis capitata (Wiedemann, 1824) (Diptera: Tephritidae), es una de las plagas de mayor incidencia económica en cítricos y otros frutales a nivel mundial. En España las medidas de control de esta plaga en cítricos, desde mediados de los 90 hasta 2009, se basaron principalmente en el monitoreo de las poblaciones y en la aplicación de tratamientos aéreos y terrestres con malatión cebo. Sin embargo, desde la retirada en la Unión Europea en 2009 de los productos fitosanitarios que contienen malatión, los insecticidas más utilizados para el control de esta plaga han sido lambda-cihalotrina y spinosad. En 2004-2005 se detectaron poblaciones españolas de C. capitata resistentes a malatión. Esta resistencia se ha asociado a una mutación (G328A) en la acetilcolinesterasa (AChE), a una duplicación del gen de la AChE (Ccace2) (una de las copias lleva la mutación G328A) y a resistencia metabólica mediada por esterasas (posiblemente aliesterasas). Sin embargo, cuando se secuenció la aliesterasa CcE7 en individuos de una línea resistente a malatión, no se encontró ninguna de las mutaciones (G137D y/o W251L/S/G) asociadas a resistencia en otras especies, si bien se encontraron otras mutaciones al compararlos con individuos de una línea susceptible. Asimismo, mediante la selección en laboratorio de una línea resistente a malatión (W-4Km) con lambda-cihalotrina, se ha podido obtener una línea resistente a lambda-cihalotrina (W-1K). Finalmente, se ha demostrado la capacidad de esta especie para desarrollar resistencia a spinosad mediante selección en laboratorio. Los múltiples mecanismos de resistencia identificados evidencian el potencial de esta especie para desarrollar resistencia a insecticidas con diferentes modos de acción. Los objetivos de esta tesis doctoral son: 1) evaluar la susceptibilidad de poblaciones españolas de campo de C. capitata a lambda-cihalotrina y dilucidar los mecanismos de resistencia en la línea W-1Kλ; 2) comparar la herencia, el coste biológico y la estabilidad de la resistencia a malatión mediada por la mutación G328A y la duplicación del gen Ccace2 (una de las copias lleva la mutación G328A); y 3) investigar el papel de las mutaciones identificadas en la aliesterasa CcαE7 en la resistencia a malatión. Estos estudios son de utilidad para el desarrollo de estrategias de manejo de la resistencia que puedan prevenir o retrasar la aparición de resistencia y aumentar la sostenibilidad de los insecticidas disponibles para el control de esta plaga. Nuestros resultados indican que las poblaciones españolas de C. capitata analizadas han desarrollado resistencia a lambda-cihalotrina. Los valores de CL50 estimados para las poblaciones recogidas en la Comunidad Valenciana, Cataluña y Andalucía oscilaron entre 129 ppm y 287 ppm, igualando o sobrepasando la concentración recomendada para los tratamientos de campo (125 ppm). Estos resultados contrastan con los obtenidos con tres poblaciones de campo recogidas en Túnez, cuya susceptibilidad fue similar a la de la línea control (C). La línea resistente a lambda-cihalotrina W-1K se continuó seleccionando en el laboratorio alcanzándose unos niveles de resistencia de 205 veces con respecto a la línea C, siendo su CL50 (4224 ppm) más de 30 veces superior a la concentración recomendada para los tratamientos de campo. Esta línea resistente mostró altos niveles de resistencia cruzada a deltametrina (150 veces) y a etofenprox (240 veces), lo que sugiere que el desarrollo de resistencia a lambda-cihalotrina podría comprometer la eficacia de otros piretroides para el control de esta plaga. Hemos demostrado que la resistencia de la línea W-1K a lambda-cihalotrina fue casi completamente suprimida por el sinergista PBO, lo que indica que las enzimas P450 desempeñan un papel muy importante en la resistencia a este insecticida. Sin embargo, tanto las moscas de la línea susceptible C como las de la línea resistente W-1K perdieron inmediatamente la capacidad de caminar (efecto “knock-down”) al ser tratadas tópicamente con lambda-cihalotrina, lo que sugiere que la resistencia no está mediada por alteraciones en la molécula diana (resistencia tipo “kdr”). La resistencia metabólica mediada por P450 fue analizada comparando la expresión de 53 genes CYP (codifican enzimas P450) de las familias CYP4, CYP6, CYP9 y CYP12 en adultos de la línea resistente W-1K y de la línea susceptible C. Nuestros resultados muestran que el gen CYP6A51 (número de acceso GenBank XM_004534804) fue sobreexpresado (13-18 veces) en la línea W-1K. Por otra parte, la expresión del gen CYP6A51 fue inducida tanto en adultos de la línea W-1K como de la línea C al ser tratados con lambda-cihalotrina. Sin embargo, no se obtuvieron diferencias significativas entre la línea susceptible C y la línea resistente W-1K al comparar la cantidad de P450 y la actividad NADPH-citocromo c reductasa presente en fracciones microsomales obtenidas a partir de abdómenes. Asimismo, no hemos podido correlacionar el metabolismo de deltametrina, estimado in vitro mediante la incubación de este insecticida con fracciones microsomales, con el nivel de resistencia a este piretroide observado en los bioensayos con la línea W-1K. Por otro lado, no se encontró ninguna alteración en la región promotora 5'UTR del gen CYP6A51 (-500 pb desde el inicio de la traducción) que pudiera explicar su sobreexpresión en la línea W-1K. Los datos obtenidos sugieren que la resistencia a lambda-cihalotrina en la línea W-1K está mediada por P450 y que la sobreexpresión de CYP6A51 puede desempeñar un papel importante, aunque se necesitan más evidencias para establecer una asociación directa de la resistencia con este gen. Hemos estudiado la herencia, el coste biológico y la estabilidad de la resistencia a malatión mediada por la mutación G328A y la duplicación del gen Ccace2 (una de las copias lleva la mutación G328A). La línea susceptible C, donde no se encuentra la mutación G328A (genotipo S/S), se cruzó con dos isolíneas establecidas para representar genotipos únicos correspondientes a los dos mecanismos de resistencia asociados a la molécula diana: 1) la isolínea 267Y (genotipo R/R) establecida a partir de una pareja que portaba la mutación G328A en homocigosis; 2) la isolínea 306TY (genotipo RS/RS) establecida a partir de una pareja que portaba en homocigosis la duplicación del gen Ccace2. No se realizaron cruces recíprocos, ya que mediante experimentos de hibridación in situ en cromosomas politénicos se pudo comprobar que el locus de la AChE y la duplicación (probablemente en tándem) se localizan en el cromosoma autosómico 2L. La susceptibilidad al malatión de los parentales resistentes (R/R o RS/RS) y susceptibles (S/S), los cruces F1 (S/R, S/RS y R/RS) y los retrocruzamientos indican que la resistencia a malatión es semi-dominante en ambos casos. Sin embargo, nuestros resultados no fueron concluyentes con respecto a la naturaleza monogénica de la resistencia a malatión en estas isolíneas. Por lo tanto, no podemos descartar que otros genes que contribuyan a la resistencia, además de la mutación G328A (isolínea 267Y) y de la duplicación del gen Ccace2 (isolínea 306TY), puedan haber sido seleccionados durante el proceso de selección de 267Y y 306TY. Varios parámetros biológicos fueron evaluados para determinar si estos dos mecanismos de resistencia a malatión suponen un coste biológico para los genotipos resistentes. Individuos con genotipo R/R mostraron un retraso en el tiempo de desarrollo de huevo a pupa, un peso de pupa reducido y una menor longevidad de los adultos, en comparación con los individuos con genotipo S/S. Sin embargo, el peso de pupa de los individuos con genotipo RS/RS fue similar al de los individuos S/S, y su desarrollo de huevo a pupa intermedio entre S/S y R/R. Estas diferencias en el coste biológico pueden estar relacionadas con la reducción de la eficiencia catalítica de la AChE mutada en los individuos R/R, y al efecto compensatorio que la copia no mutada del gen tiene en los individuos RS/RS que portan la duplicación. La estabilidad de la resistencia a malatión mediada por la mutación G328A y la duplicación se analizó mediante el seguimiento de los caracteres de resistencia en la progenie de retrocruzamientos S/R x R/R y S/RS x RS/RS a lo largo de varias generaciones en ausencia de presión de selección con insecticidas. Nuestros resultados muestran que la frecuencia del alelo que porta la mutación G328A disminuyó desde 67,5% en la primera generación del retrocruzamiento S/R x R/R (75% esperado, asumiendo segregación mendeliana y que sólo hay dos alelos: uno mutado y otro no mutado) a 12% después de 10 generaciones. Por el contrario, la frecuencia de la duplicación sólo disminuyó desde 75% en en la primera generación del retrocruzamiento S/RS x RS/RS (75% esperado, asumiendo segregación Mendeliana y que la duplicación segrega como un único alelo) a 50% en el mismo período, lo que indica que la duplicación es más estable que la mutación. Asimismo, se analizó la presencia de la mutación y de la duplicación en poblaciones de campo recogidas en seis localidades en 2004-2007, cuando todavía se usaba el malatión, y se comparó con poblaciones recogidas en los mismos campos en 2010, un año después de la prohibición del malatión en la Unión Europea. La frecuencia media del genotipo susceptible (S/S) aumentó del 55,9% en el período 2004-2007 a 70,8% en 2010, mientras que la frecuencia de los genotipos portadores de la mutación en homocigosis o heterocigosis (R/R y S/R) disminuyó del 30,4 al 9,2%, los que llevan la duplicación en homocigosis o heterocigosis (RS/RS y S/RS) aumentaron levemente desde 12,8 hasta 13,3%, y los que llevan a la vez la mutación y la duplicación (R/RS) también aumentaron del 1 al 6,7%. Estos resultados son consistentes con que la duplicación del gen Ccace2 (con una copia con la mutación G328A y la otra copia no mutada) es más ventajosa que la mutación G328A por si sola, ya que la duplicación mantiene los niveles de resistencia a la vez que limita el coste biológico. Para investigar la asociación entre la resistencia a malatión y las mutaciones encontradas previamente en CcE7, hemos generado isolíneas con mutaciones específicas seleccionadas por su ubicación próxima a la entrada al centro activo de la enzima. La isolínea Sm2 (procedente de una hembra heterocigota para la mutación V96L y un macho homocigoto para el alelo no mutado) mantuvo altos niveles de resistencia a malatión, incluso después de 30 generaciones sin presión de selección. Por el contrario, la isolínea 267Y (compuesta por individuos homocigotos para la mutación L267Y) y la línea 306TY (compuesta por individuos homocigotos para la doble mutación R306T-N307Y) mostraron una reducción significativa en los niveles de resistencia. También hemos encontrado que la resistencia a malatión de la línea Sm2 fue parcialmente revertida por DEF y TPP, y que Sm2 mostró una reducción significativa en la actividad MTB, como se ha descrito en otras especies que muestran resistencia específica a malatión mediada por aliesterases. Además, fue posible asociar la presencia de la mutación V96L en individuos de la línea Sm2 con supervivencia a una concentración discriminante de malatión (5,000 ppm) y con una baja actividad MTB. Estos resultados sugieren una posible relación entre la mutación V96L en la aliesterasa CcE7 y la resistencia a malatión, aunque todavía no se puede concluir que la resistencia es causada por esta mutación, siendo necesarios más estudios para comprobar su contribución a la resistencia. En conclusión, se ha encontrado por primera vez resistencia a lambda-cihalotrina en poblaciones de campo de C. capitata, y nuestros resultados indican que las P450 son el principal mecanismo de resistencia en la línea W-1K. Esta situación se suma al caso previamente descrito de resistencia en campo a malatión asociada a la mutación G328A, a la duplicación del gen Ccace2 (una de las copias lleva la mutación G328A) y a resistencia metabólica mediada por esterasas. Nuestros resultados también indican que la alteración de la molécula diana AChE parece ser responsable de un cierto nivel de resistencia a malatión en C. capitata, que puede ser estimada como aproximadamente 25-40 veces para la mutación G328A y 40-60 veces para la duplicación; mientras que la resistencia mediada por esterasas y que ha sido asociada en este estudio con la mutación V96L en CcE7 puede conferir un efecto multiplicativo (por un factor de 5 a 10) aumentando la resistencia a malatión a 200-400 veces. Por otra parte, hemos demostrado que los insectos resistentes que llevan la duplicación tienen un coste biológico menor y muestran una estabilidad mayor que aquellos con la mutación G328A en ausencia de presión de selección con insecticidas. Esto representa un escenario en el que los genotipos con la duplicación permanecerán en el campo en frecuencias bajas a moderadas, pero podrían ser seleccionados rápidamente si se utilizan malatión u otros insecticidas que muestren resistencia cruzada. Estos resultados tienen importantes implicaciones para los programas de manejo de la resistencia, ya que el repertorio de insecticidas eficaces para el control de C. capitata es cada vez más limitado. Además, la coexistencia de múltiples mecanismos de resistencia en poblaciones de campo ofrece el potencial para desarrollar resistencia frente a otros insecticidas disponibles para el control de esta plaga. Estrategias para de manejo de la resistencia basadas en la alternancia de insecticidas con diferentes modos de acción, y su combinación con otros métodos de control, deben ser implementadas para evitar el desarrollo de resistencia en campo. ABSTRACT The Mediterranean fruit fly (Medfly), Ceratitis capitata (Wiedemann, 1824) (Diptera: Tephritidae), is one of the most economically damaging pests of citrus and other fruit crops worldwide. Control measures in citrus crops in Spain from the mid 90's to 2009 were mainly based on field monitoring of population levels and aerial and ground treatments with malathion bait sprays. However, since the withdrawal of phytosanitary products containing malathion in the European Union in 2009, lambda-cyhalothrin and spinosad have become the most widely used insecticides for the control of this pest. Resistance to malathion was found in Spanish field populations of C. capitata in 2004-2005. This resistance has been associated with a mutation G328A in the acetylcholinesterase (AChE), a duplication of the AChE gene (Ccace2) (one of the copies bearing the mutation G328A), and metabolic resistance mediated by esterases (probably aliesterases). However, when the gene of the aliesterase CcE7 was sequenced in individuals from a malathion resistant strain of C. capitata, none of the known G137D and/or W251L/S/G mutations associated to resistance in other species were found, though other mutations were detected when compared with individuals from a susceptible strain. Noteworthy, a lambda-cyhalothrin resistant strain (W-1K) was obtained by selecting a field-derived malathion resistant strain (W-4Km) with lambda-cyhalothrin. Moreover, it has also been demonstrated the capacity of this species to develop resistance to spinosad by laboratory selection. The multiple resistance mechanisms identified highlight the potential of this species to develop resistance to insecticides with different modes of action. The objectives of this PhD Thesis are: 1) to assess the susceptibility of Spanish field populations of C. capitata to lambda-cyhalothrin and to elucidate the resistance mechanisms in the W-1Kλ strain; 2) to compare the inheritance, fitness cost and stability of the malathion resistance mediated by the G328A mutation and the duplication of the Ccace2 gene (with one of the copies bearing the mutation G328A); and 3) to investigate the role of the aliesterase CcαE7 mutations in malathion resistance. All these studies will be of use for devising proactive resistance management strategies that could prevent or delay resistance development and would increase the sustainability of the insecticides available for Medfly control. Our results indicate that Spanish field populations of C. capitata have developed resistance to lambda-cyhalothrin. The LC50 values estimated for populations collected at Comunidad Valenciana, Cataluña and Andalucía ranged from 129 ppm to 287 ppm, equaling or overpassing the recommended concentration for field treatments (125 ppm). These results contrast with those obtained with three different Tunisian field populations, whose susceptibility was similar to that of the control (C) strain. The lambda-cyhalothrin resistant W-1K strain has been further selected to achieve a 205-fold resistance compared to the C strain, being its LC50 (4,224 ppm) more than 30 times higher than the recommended concentration for field applications. This resistant strain showed high levels of cross-resistance to deltamethrin (150-fold) and etofenprox (240-fold), suggesting that the development of resistance to lambda-cyhalothrin may compromise the effectiveness of other pyrethroids for the control of this species. We have shown that the resistance of the W-1K strain to lambda-cyhalothrin was almost completely suppressed by the synergist PBO, indicating that P450 enzymes play a very important role in resistance to this insecticide. However, both susceptible C and resistant W-1K flies were knocked down after topical treatment with lambda-cyhalothrin, suggesting that kdr resistance mediated by alterations of the target site is not playing a major role. Metabolic resistance mediated by P450 was further analyzed by comparing the expression of 53 genes of the families CYP4, CYP6, CYP9 and CYP12 in adults flies from the resistant W-1K and the susceptible C strains. We found that the gene CYP6A51 (GenBank accession number XM_004534804) was overexpressed (13-18-fold) in the W-1K strain. Moreover, the expression of the CYP6A51 gene was induced when adults of the W-1K and C strains were treated with lambda-cyhalothrin. However, no significant differences were obtained between susceptible C and resistant W-1K strains for the quantity of P450 and for the activity of NADPH- cytochrome c reductase measured in microsomal fractions obtained from abdomens. Moreover, we failed to correlate the metabolism of deltamethrin, analyzed in vitro by incubating this insecticide with microsomal fractions, with the resistance level against this pyrethroid observed in bioassays with W-1K. The sequencing of the 5´UTR region of the CYP6A51 gene failed in finding an alteration in the promoter region (-500 bp from translation start site) that could explain overexpression in the W-1K strain. All data obtained suggest that resistance to lambda-cyhalothrin in the W- 1K strain is mediated by P450 and that overexpression of CYP6A51 may play a major role, although further evidences are needed to establish a direct association of resistance with this gene. We have studied the inheritance, fitness cost and stability of the malathion resistance mediated by the G328A mutation and the duplication of the Ccace2 gene (with one of the copies bearing the mutation G328A). The malathion-susceptible C strain where the G328A mutation is not found (S/S genotype) was crossed with two isolines established to represent unique genotypes corresponding to the two target-site resistance mechanisms: 1) the 267Y isoline (genotype R/R) was established from a couple bearing the mutation G328A in homozygosis; and 2) the 306TY isoline (genotype RS/RS) was established from a couple being homozygous for the duplication of the Ccace2 gene. Reciprocal crosses have not been performed, since in situ hybridization on polythene chromosomes showed that the AChE locus and the duplication (most probably in tandem) are placed at the autosomal chromosome 2L. Mortality responses to malathion of resistant isolines (R/R or RS/RS) and susceptible (S/S) genotypes, F1 crosses (S/R, S/RS, and R/RS), and the back-crosses indicated that resistance to malathion is inherited as a semi-dominant trait in both cases. However, our results were not conclusive about the monogenic nature of the resistance to malathion in these isolines. Thus, we can not discard that other genes contributing to resistance, in addition to the mutation G328A (isoline 267Y) and the duplication of the Ccace2 gene (isoline 306TY), may have been selected during the selection process of 267Y and 306TY. Several biological parameters were evaluated to determine if these two malathion resistance mechanisms impose a fitness cost for resistant genotypes. Individuals with genotype R/R have a reduced fitness in terms of developmental time from egg to pupa, pupal weight and adult longevity, when compared to susceptible individuals (genotype S/S). Interestingly, the fitness cost was substantially diminished in individuals with genotype RS/RS. These differences in fitness may be related to the reduction of the catalytic efficiency of mutated AChE in individuals R/R, and the compensatory effect that the non-mutated copy of the gene has on individuals RS/RS bearing the duplication. The stability of malathion reistance associated with the mutation G328A or the duplication was analyzed by following these resistant traits in the progeny of the back-crosses S/RS x RS/RS and S/R x R/R over consecutive generations in the absence of insecticide selection pressure. Our results show that the frequency of the allele bearing the mutation G328A decreased from 67.5% at the first generation of the back-cross S/R x R/R (75% expected, assuming Mendelian segregation and that there are only two alleles: one mutated and the other non-mutated) to 12% after 10 generations. By contrast, the frequency of the duplication only declined from 75% at the first generation of the back-cross S/RS x RS/RS (75% expected, assuming Mendelian segregation and that the duplication segregates as an unique allele) to 50% in the same period, indicating that the duplication is more stable than the mutation. The presence of the mutation and the duplication was analyzed in field populations collected in six localities in 2004-2007, when malathion was still used, and compared to populations collected in the same fields in 2010, one year after the prohibition of malathion in the European Union. The average frequency of the susceptible genotype (S/S) increased from 55.9% in the period 2004-2007 to 70.8% in 2010, whereas the frequency of those genotypes carrying the mutation in homozygosis or heterozygosis (R/R and S/R) declined from 30.4 to 9.2%, those carrying the duplication in homozygosis or heterozygosis (RS/RS and S/RS) increased slightly from 12.8 to 13.3%, and those carrying both the mutation and the duplication (R/RS) also increased from 1 to 6.7%. These results are consistent with the duplication of the Ccace2 gene (with one of the copies bearing the mutation G328A and the other copy non-mutated) being more advantageous than the G328A mutation alone by maintaining resistance while restoring part of the fitness. In order to investigate the association of malathion resistance with mutations previously found in the aliesterase CcE7, we have generated isolines bearing specific mutations selected by their putative location near the upper part of the active site gorge of the enzyme. The isoline Sm2 (originating from a female heterozygous for the mutation V96L and a male homozygous for the non-mutated allele) kept high levels of resistance to malathion, even after 30 generations without selection pressure. On the contrary, the isoline 267Y (composed by individuals homozygous for the mutation L267Y) and the strain 306TY (composed by homozygous for the double mutation R306T-N307Y) showed a significant reduction in the levels of resistance. We have found also that resistance to malathion in the Sm2 isoline was partially reverted by DEF and TPP, and that Sm2 showed a significant reduction in MTB activity, as reported for other species showing malathion-specific resistance mediated by aliesterases. Besides, it was possible to associate the presence of the mutation V96L in individuals from the Sm2 isoline with both survival to a discriminating concentration of malathion (5,000 ppm) and low MTB activity. Our results point out to a possible connection betwen the mutation V96L in the aliesterase CcE7 and resistance to malathion, though we can not yet conclude that the resistance is caused by the mutation, being needed further work to understand its contribution to resistance. In conclusion, resistance to lambda-cyhalothrin has been found for the first time in field populations of C. capitata, and metabolic resistance mediated by P450 appears to be the main resistance mechanism in the resistant strain W-1K. These findings add to the previously reported case of field resistance to malathion, associated to the G328A mutation and the duplication of the Ccace2 gene (with one of the copies bearing the mutation G328A) and to metabolic resistance mediated by esterases. Our results also indicate that altered target site AChE appears to be responsible for a certain level of resistance to malathion in C. capitata, that can be estimated as about 25-40-fold for the mutation G328A and 40-60-fold for the duplication; whereas metabolic resistance mediated by esterases and associated in this study with the mutation V96L in CcE7 may confer a multiplicative effect (by a factor of 5 to10) increasing malathion resistance to 200-400-fold. Moreover, we have shown that resistant insects carrying the duplication have better fitness and exhibit a higher stability than those with the mutation G328A in the absence of insecticide pressure. This represents a scenario where genotypes with the duplication will remain in the field at low to moderate frequencies, but could be rapidly selected if malathion or other insecticides showing cross-resistance are used. These findings have important implications for resistance management programs, as the repertoire of effective insecticides for C. capitata control is becoming very limited. Besides, multiple resistance mechanisms coexisting in field populations provide the potential to develop resistance to other available insecticides for the control of this pest. Appropriate resistance management strategies based on the alternation of insecticides with different modes of action, and their combination with other control methods, must then be implemented to avoid the evolution of resistance in the field.