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Genome wide association studies (GWAS) have revolutionized our approach to mapping genetic determinants of complex human diseases. However, even with success from recent studies, we have typically been able to explain only a fraction of the trait heritability. GWAS are typically analysed by testing for the marginal effects of single variants. Consequently, it has been suggested that gene-gene interactions might contribute to the missing heritability of complex diseases. GWAS incorporating interaction effects have not been routinely applied because of statistical and computational challenges relating to the number of tests performed, genome-wide. To overcome this issue, I have developed novel methodology to allow rapid testing of pairwise interactions in GWAS of complex traits, implemented in the IntRapid software. Simulations demonstrated that the power of this approach was equivalent to computationally demanding exhaustive searches of the genome, but required only a fraction of the computing time. Application of IntRapid to GWAS of a range of complex human traits undertaken by the Wellcome Trust Case Control Consortium (WTCCC) identified several interaction effects at nominal significance, which warrant further investigation in independent studies. In an attempt to fine-map the identified interacting loci, I undertook imputation of the WTCCC genotype data up to the 1000 Genomes Project reference panel (Phase 1 integrated release, March 2012) in the neighbourhood of the lead SNPs. I modified the IntRapid software to take account of imputed genotypes, and identified stronger signals of interaction after imputation at the majority of loci, where the lead SNP often had moved by hundreds of kilobases. The X-chromosome is often overlooked in GWAS of complex human traits, primarily because of the difference in the distribution of genotypes in males and females. I have extended IntRapid to allow for interactions with the X chromosome by considering males and females separately, and combining effect estimates across the sexes in a fixed-effects meta-analysis. Application to genotype data from the WTCCC failed to identify any strong signals of association with the X-chromosome, despite known epidemiological differences between the sexes for the traits considered. The novel methods developed as part of this doctoral work enable a user friendly, computationally efficient and powerful way of implementing genome-wide gene-gene interaction studies. Further work would be required to allow for more complex interaction modelling and deal with the associated computational burden, particularly when using next-generation sequencing (NGS) data which includes a much larger set of SNPs. However, IntRapid is demonstrably efficient in exhaustively searching for pairwise interactions in GWAS of complex traits, potentially leading to novel insights into the genetic architecture and biology of human disease.

O uso da cocaína e do crack tornou-se um problema de saúde pública importante no Brasil por conta de prejuízos significativos do ponto de vista médico, psicológico e social que ele acarreta. Estudos de gêmeos e, em famílias, sugerem que a dependência de cocaína é uma doença complexa, com participação importante de fatores genéticos. Os estudos genéticos sobre usuários de cocaína são poucos e padecem de problemas metodológicos, tais como, amostras pequenas, com alto grau de miscigenação populacional e um número limitado de marcadores genéticos pesquisados. Além disto, há pouco sendo feito no sentido de verificar como os genes já associados à dependência de cocaína interagem entre si, ou seja, de investigações sobre a epistasia genética. Com o intuito de aprofundar a investigação dos aspectos biológicos da dependência de cocaína, nós estudamos, através de um estudo casocontrole, uma amostra de inicial de 746 pacientes dependentes de crack/cocaína hospitalizados em clínicas especializadas para o tratamento de dependência química na cidade de São Paulo, que foram comparados a 891 controles normais, sem história prévia de abuso ilegal de substâncias. Os objetivos desta tese foram: 1) verificar a associação de três polimorfismos (rs1803274, rs4263329, rs4680662) para o gene da butirilcolinesterase (BCHE), uma enzima envolvida na metabolização da cocaína no desenho do tipo gene candidato; 2) testar a hipótese de interação entre o marcador funcional Val158Met do gene para a enzima catecol-O-metiltransferase (COMT) e os marcadores do tipo VNTR das regiões 3´UTR e Intron8 do gene do transportador da dopamina (DAT1) e; 3) numa análise de caráter exploratório, verificar a interação gene-gene de 40 polimorfismos em 12 genes com plausibilidade biológica para a dependência da cocaína. A análise estatística fez uso de modelos de regressão logística para a interação de marcadores nos dois genes, COMT e DAT1 e, do programa Multifactor Dimensionality Reduction (MDR) para a análise multivariada. A análise envolvendo os marcadores para o gene BCHE não se mostraram associados ao fenótipo da dependência de cocaína porém, encontrou-se uma associação do marcador funcional rs1803274 (p=0,001; OR=5,83; IC95%=2,10 - 16,16) nos usuários exclusivos de crack, a forma cheirada da cocaína quando comparados aos grupos de uso exclusivo da cocaína na forma cheirada ou, de uso das duas formas de administração. Os marcadores do tipo VNTR da DAT1 não interagiram em um modelo de regressão logística com o marcador Val158Met da COMT. Finalmente, os modelos construídos pelo programa MDR não forneceram interações gene-gene que tivessem uma previsibilidade além do acaso. Dentro de uma perspectiva genética, os estudos futuros para a dependência de cocaína devem aprimorar a caracterização fenotípica, por meio de subgrupos divididos por sintomas clínicos e pelo uso de fenótipos intermediários, fazer um rastreio minucioso dos marcadores ao longo dos genes de interesse e, usar de métodos analíticos para as interações gene-gene e gene-ambiente / The use of crack/cocaine has become a major public health problem in Brazil due to its manifold problems in the medical, psychological and social realms. Twin and family studies have documented the role played by genetic factors and environment in cocaine addiction. Genetic association studies in cocaine addiction are few and have methodological problems: small sample size, population stratification and a paucity of genetic markers have been studied so far. There is also a lack of knowledge on how the genes already shown to be associated with cocaine addiction interact, that is, genetic epistasis. In order to advance the knowledge of biological factors in cocaine addiction we investigated, by means of a case-control study, 746 patients with crack/cocaine dependence admitted to specialized clinics for the treatment of drug addiction in the city of São Paulo. They were compared to 891 control subjects with no previous history of illegal drug abuse. The objectives of this thesis were: 1) investigate the association of three SNPs (rs1803274, rs4263329, rs4680662) in the butirilcholinesterase gene (BCHE) that encodes an enzyme involved in cocaine metabolism; 2) test the hypothesis of an interaction between the functional marker Val158Met of the catechol-o-metiltransferase enzyme (COMT) gene and two VNTRs markers, 3´UTR and Intron8, of the dopamine transporter gene (DAT1) and, 3) in an exploratory analysis, investigate the gene-gene interaction of 40 polymorphisms in 12 genes with a biological plausibility for cocaine addiction. Logistic regression was used to assess COMT*DAT1 gene-gene interaction and, the Multifactor Dimensionality Reduction (MDR) program was used for the other multivariate analysis. Genetic variants for the BCHE gene were not associated with cocaine addiction but, an association was found between the functional marker rs1803274 (p=0,001; OR=5,83; IC95%=2,10 - 16,16) and crack users compared to those that snorted cocaine or used both forms of administration. The DAT1 VNTRs did not interact with the COMT Val158Met marker. Finally, the models generated by the MDR program did not provided any predictive gene-gene interaction better than chance. Future studies investigating genetic risk factors for cocaine dependence should improve phenotype characterization (clinically derived subgroups and use of endophenotypes) and should also make a thorough scan of the genetic markers along the genes of interest including gene-gene and gene-environment analysis

O uso da cocaína e do crack tornou-se um problema de saúde pública importante no Brasil por conta de prejuízos significativos do ponto de vista médico, psicológico e social que ele acarreta. Estudos de gêmeos e, em famílias, sugerem que a dependência de cocaína é uma doença complexa, com participação importante de fatores genéticos. Os estudos genéticos sobre usuários de cocaína são poucos e padecem de problemas metodológicos, tais como, amostras pequenas, com alto grau de miscigenação populacional e um número limitado de marcadores genéticos pesquisados. Além disto, há pouco sendo feito no sentido de verificar como os genes já associados à dependência de cocaína interagem entre si, ou seja, de investigações sobre a epistasia genética. Com o intuito de aprofundar a investigação dos aspectos biológicos da dependência de cocaína, nós estudamos, através de um estudo casocontrole, uma amostra de inicial de 746 pacientes dependentes de crack/cocaína hospitalizados em clínicas especializadas para o tratamento de dependência química na cidade de São Paulo, que foram comparados a 891 controles normais, sem história prévia de abuso ilegal de substâncias. Os objetivos desta tese foram: 1) verificar a associação de três polimorfismos (rs1803274, rs4263329, rs4680662) para o gene da butirilcolinesterase (BCHE), uma enzima envolvida na metabolização da cocaína no desenho do tipo gene candidato; 2) testar a hipótese de interação entre o marcador funcional Val158Met do gene para a enzima catecol-O-metiltransferase (COMT) e os marcadores do tipo VNTR das regiões 3´UTR e Intron8 do gene do transportador da dopamina (DAT1) e; 3) numa análise de caráter exploratório, verificar a interação gene-gene de 40 polimorfismos em 12 genes com plausibilidade biológica para a dependência da cocaína. A análise estatística fez uso de modelos de regressão logística para a interação de marcadores nos dois genes, COMT e DAT1 e, do programa Multifactor Dimensionality Reduction (MDR) para a análise multivariada. A análise envolvendo os marcadores para o gene BCHE não se mostraram associados ao fenótipo da dependência de cocaína porém, encontrou-se uma associação do marcador funcional rs1803274 (p=0,001; OR=5,83; IC95%=2,10 - 16,16) nos usuários exclusivos de crack, a forma cheirada da cocaína quando comparados aos grupos de uso exclusivo da cocaína na forma cheirada ou, de uso das duas formas de administração. Os marcadores do tipo VNTR da DAT1 não interagiram em um modelo de regressão logística com o marcador Val158Met da COMT. Finalmente, os modelos construídos pelo programa MDR não forneceram interações gene-gene que tivessem uma previsibilidade além do acaso. Dentro de uma perspectiva genética, os estudos futuros para a dependência de cocaína devem aprimorar a caracterização fenotípica, por meio de subgrupos divididos por sintomas clínicos e pelo uso de fenótipos intermediários, fazer um rastreio minucioso dos marcadores ao longo dos genes de interesse e, usar de métodos analíticos para as interações gene-gene e gene-ambiente / The use of crack/cocaine has become a major public health problem in Brazil due to its manifold problems in the medical, psychological and social realms. Twin and family studies have documented the role played by genetic factors and environment in cocaine addiction. Genetic association studies in cocaine addiction are few and have methodological problems: small sample size, population stratification and a paucity of genetic markers have been studied so far. There is also a lack of knowledge on how the genes already shown to be associated with cocaine addiction interact, that is, genetic epistasis. In order to advance the knowledge of biological factors in cocaine addiction we investigated, by means of a case-control study, 746 patients with crack/cocaine dependence admitted to specialized clinics for the treatment of drug addiction in the city of São Paulo. They were compared to 891 control subjects with no previous history of illegal drug abuse. The objectives of this thesis were: 1) investigate the association of three SNPs (rs1803274, rs4263329, rs4680662) in the butirilcholinesterase gene (BCHE) that encodes an enzyme involved in cocaine metabolism; 2) test the hypothesis of an interaction between the functional marker Val158Met of the catechol-o-metiltransferase enzyme (COMT) gene and two VNTRs markers, 3´UTR and Intron8, of the dopamine transporter gene (DAT1) and, 3) in an exploratory analysis, investigate the gene-gene interaction of 40 polymorphisms in 12 genes with a biological plausibility for cocaine addiction. Logistic regression was used to assess COMT*DAT1 gene-gene interaction and, the Multifactor Dimensionality Reduction (MDR) program was used for the other multivariate analysis. Genetic variants for the BCHE gene were not associated with cocaine addiction but, an association was found between the functional marker rs1803274 (p=0,001; OR=5,83; IC95%=2,10 - 16,16) and crack users compared to those that snorted cocaine or used both forms of administration. The DAT1 VNTRs did not interact with the COMT Val158Met marker. Finally, the models generated by the MDR program did not provided any predictive gene-gene interaction better than chance. Future studies investigating genetic risk factors for cocaine dependence should improve phenotype characterization (clinically derived subgroups and use of endophenotypes) and should also make a thorough scan of the genetic markers along the genes of interest including gene-gene and gene-environment analysis

Sympatric speciation has received much attention both empirically and theoretically. However, the contribution of sympatric speciation to biodiversity remains unclear. One piece missing from the speciation puzzle is the plausibility of sympatric ecological divergence of species through adaptation in polygenic traits. I consider an environment consisting of two niches, where one value of the trait is advantageous in only one niche, and vice versa. The selection regime is described by a trade-off in viabilities between the niches. These polygenic traits can, and often do, involve epistatic interactions among and between loci, so that the contribution of the alleles to viability deviates from additivity. Epistasis then also affects the curvature of the trade-offs: predominant less-than-additive epistasis turns the curve towards concavity and predominant more-than-additive towards convexity. The curvature of the trade-off plays a crucial role in the evolution of populations. With a convex trade- off, extreme values of the trait are favored and the population tends to diverge, but relatively stringent symmetry in strength of selection within the niches and the niche proportions is necessary to maintain polymorphism. In this study I use two and three- locus haploid versions of Levene's model to...

L'ingénierie enzymatique est un outil très avantageux dans l'industrie biotechnologique. Elle permet d'adapter les enzymes à une activité ou à une condition de réaction spécifique. En outre, elle peut permettre de déchiffrer les éléments clés qui ont facilité leur modification. Bien que l'ingénierie enzymatique soit largement pratiquée, elle comporte encore plusieurs goulets d'étranglement. Certains de ces goulets d'étranglement sont techniques, comme le développement de méthodologies pour la création de banques de mutations ciblées ou la réalisation de criblages à haut débit, et d'autres sont conceptuels, comme le déchiffrage des caractéristiques clés pertinentes d'une protéine cible pour la réussite d'un projet d'ingénierie. Parmi ces défis, l'épistasie intra-génique, ou la non-additivité des effets phénotypiques des mutations, est une caractéristique qui entrave grandement la prévisibilité. L'amélioration de l'ingénierie enzymatique nécessite une approche multidisciplinaire qui inclut une meilleure compréhension des relations structure-fonction-évolution. Cette thèse vise à contribuer à l'avancement de l'ingénierie enzymatique en étudiant deux systèmes modèles. Premièrement, des variantes dynamiques de la ß-lactamase TEM-1 ont été choisies pour étudier le lien entre la dynamique des protéines et l'évolution. La ß-lactamase TEM-1 a été largement caractérisée dans la littérature, ce qui s'est traduit par des connaissances approfondies sur son mécanisme de réaction, ses caractéristiques structurelles et son évolution. Les variantes de la ß-lactamase TEM-1 utilisées comme système modèle dans cette thèse ont été largement caractérisées, montrant une dynamique accrue à l'échelle temporelle pertinente pour la catalyse (µs à ms) mais maintenant la reconnaissance du substrat. Dans cette thèse, l'évolution in vitro de ces variantes dynamiques a été réalisée par des cycles itératifs de mutagenèse et de sélection aléatoires pour permettre une exploration impartiale du paysage de ‘fitness’. Nous démontrons que la présence de ces mouvements particuliers au début de l'évolution a permis d'accéder à des voies de mutations connues. De plus, des interactions épistatiques connues ont été introduites dans les variantes dynamiques. Leur caractérisation in silico et cinétique a révélé que les mouvements supplémentaires sur l'échelle de temps de la catalyse ont permis d'accéder à des conformations conduisant à une fonction améliorée, comme dans le TEM-1 natif. Dans l'ensemble, nous démontrons que l'évolution de la b-lactamase TEM-1 vers une nouvelle fonction est compatible avec divers mouvements à l'échelle de temps µs à ms. Il reste à savoir si cela peut se traduire par d'autres enzymes ayant un potentiel biotechnologique. Deuxièmement, la lipase Cal-A, pertinente sur le plan industriel, a été choisie pour identifier les caractéristiques qui pourraient faciliter son ingénierie. La lipase Cal-A présente des caractéristiques telles que la polyvalence du substrat et une grande stabilité thermique et réactivité qui la rendent attrayante pour la modification des triglycérides ou la synthèse de molécules pertinentes dans les industries alimentaire et pharmaceutique. Contrairement à TEM-1, la plupart des études d'évolution in vitro de la lipase Cal-A ont été réalisées dans un but industriel, avec une exploration limitée de l'espace de mutation. Par conséquent, les caractéristiques qui définissent la fonction de la lipase Cal-A restent insaisissables. Dans cette thèse, nous faisons état de la mutagenèse ciblée de la lipase Cal-A, confirmant l'existence d'une région clé pour la reconnaissance du substrat. Cela a été fait en combinant une nouvelle méthodologie de création de bibliothèque basée sur l'assemblage Golden-gate avec une visualisation structurelle basée sur des scripts pour identifier et cartographier les mutations sélectionnées dans la structure 3D. La caractérisation et la déconvolution de deux des plus aptes ont révélé l'existence d'une épistasie dans l'évolution de la lipase Cal-A vers une nouvelle fonction. Dans l'ensemble, nous démontrons que l’identification d'une variété de propriétés suite à la mutagenèse ciblée peut grandement améliorer la connaissance d'une enzyme. Cette information peut être appliquée pour améliorer l'efficacité de l'ingénierie dirigée. / Enzyme engineering is a tool with great utility in the biotechnological industry. It allows to tailor enzymes to a specific activity or reaction condition. In addition, it can allow to decipher key elements that facilitated their modification. While enzyme engineering is extensively practised, it still entails several bottlenecks. Some of these bottlenecks are technical such as the development of methodologies for creating targeted mutational libraries or performing high-throughput screening and some are conceptual such as deciphering the key relevant features in a target protein for a successful engineering project. Among these challenges, intragenic epistasis, or the non-additivity of the phenotypic effects of mutations, is a feature that greatly hinders predictability. Improving enzyme engineering needs a multidisciplinary approach that includes gaining a better understanding of structure-function-evolution relations. This thesis seeks to contribute in the advancement of enzyme engineering by investigating two model systems. First, dynamic variants of TEM-1 ß-lactamase were chosen to investigate the link between protein dynamics and evolution. TEM-1 ß-lactamase has been extensively characterized in the literature, which has translated into extensive knowledge on its reaction mechanism, structural features and evolution. The variants of TEM-1 ß-lactamase used as model system in this thesis had been extensively characterized, showing increased dynamics at the timescale relevant to catalysis (µs to ms) but maintaining substrate recognition. In this thesis, in vitro evolution of these dynamic variants was done by iterative rounds of random mutagenesis and selection to allow an unbiased exploration of the fitness landscape. We demonstrate that the presence of these particular motions at the outset of evolution allowed access to known mutational pathways. In addition, known epistatic interactions were introduced in the dynamic variants. Their in silico and kinetic characterization revealed that the additional motions on the timescale of catalysis allowed access to conformations leading to enhanced function, as in native TEM-1. Overall, we demonstrate that the evolution of TEM-1 b-lactamase toward new function is compatible with diverse motions at the µs to ms timescale. Whether this can be translated to other enzymes with biotechnological potential remains to be explored. Secondly, the industrially relevant Cal-A lipase was chosen to identify features that could facilitate its engineering. Cal-A lipase presents characteristics such as substrate versatility and high thermal stability and reactivity that make it attractive for modification of triglycerides or synthesis of relevant molecules in the food and pharmaceutical industries. Contrary to TEM-1, most in vitro evolution studies of Cal-A lipase have been done towards an industrially-specified goal, with limited exploration of mutational space. As a result, features that define function in Cal-A lipase remain elusive. In this thesis, we report on focused mutagenesis of Cal-A lipase, confirming the existence of a key region for substrate recognition. This was done by combining a novel library creation methodology based on Golden-gate assembly with script-based structural visualization to identify and map the selected mutations into the 3D structure. The characterization and deconvolution of two of the fittest revealed the existence of epistasis in the evolution of Cal-A lipase towards new function. Overall, we demonstrate that mapping a variety of properties following mutagenesis targeted to specific regions can greatly improve knowledge of an enzyme that can be applied to improve the efficiency of directed engineering.

[EN] Rice (Oryza sativa L) is the main staple food for over 3 billion people (almost half the world population), and rice blast, caused by the fungus Magnaporthe oryzae, is the major threat for this crop worldwide, triggering also important losses in Spain some years. Some resistant cultivars have been released, mainly with complete resistance genes (Pi genes), but their resistance has not lasted long, due to the emergence of new virulent isolates. Therefore, the efforts for obtaining effective and durable resistance are focused nowadays on the use of partial resistance or a combination of partial and complete resistance. Partial resistance is a quantitative character, controlled by numerous genes of small effects (QTLs), that can interact among them, and usually also strongly with the environment. For a better knowledge of the population of M. oryzae in the Albufera region, and of the resistance genes and QTLs that might be effective in various Spanish rice growing regions, we tested along four years 31 differential varieties (isogenic lines carrying one different Pi gene each); these studies showed that fungus population can significantly change from one year to the other. On the other hand, we analyzed the genetics of resistance to M. oryzae in two populations derived from crosses between local and well adapted, although moderately susceptible, varieties (Sivert, JSendra), and allegedly resistant but poorly adapted varieties (CAN-6159, Gigante Vercelly. In F3 lines of both populations, the leaf and panicle susceptibility was determined in field trials under conditions that favour the infection: SixCNA lines were tested in a plot in the Albufera region, and those of JSxGV in four locations of Valencia, the Ebro River Delta, and Seville; inoculations in controlled conditions were also carried out in the second population. We compare the different methodologies for assessing susceptibility, and we discuss the environmental influence on it. 22 QTLs were detected in SIxCNA, and 61 in JSxGV, most of them in only one location. All four parents displayed partial resistance QTLs; but some QTLs showed small additive effects and, often, high dominance. At the same time, most of these QTLs exhibit significant interactions with other QTLs; and many of them co-localize with QTLs found in other studies. We have discovered substantial coincidences between QTLs that control incidence in panicle and those that determine leaf severity, thus supporting the hypothesis that there are common defence mechanisms in both organs. Chromosomal regions of interest for marker assisted breeding of resistant genotypes have been identified. / [ES] El arroz (Oryza sativa L.) es la principal fuente de alimentación para más de 3.000 millones de personas, casi la mitad de la población mundial, y la "piriculariosis", causada por el hongo Magnaporthe oryzae Couch, es la enfermedad que más pérdidas causa en este cultivo a escala mundial; también en España ha causado pérdidas importantes algunos años. Se han obtenido diversas variedades resistentes, principalmente al incorporar genes de resistencia completa (genes Pi), pero la mayoría se han vuelto susceptibles en pocos años al aparecer nuevos aislados más virulentos. Actualmente, para conseguir una resistencia efectiva y duradera, se busca incorporar resistencia parcial, o una combinación de ambos tipos de resistencia. La resistencia parcial es un carácter cuantitativo, controlado por numerosos genes de efecto pequeño (o QTLs), que pueden interaccionar entre ellos y que, con frecuencia, también lo hacen intensamente con el ambiente. Para conocer mejor la población del patógeno presente en la zona de la Albufera, y qué genes y QTLs de resistencia pueden ser efectivos en ésta y otras zonas arroceras de España, por un lado se ensayaron durante varios años 31 variedades diferenciales (líneas con un gen Pi diferente cada una); esto demostró que la estructura de la población del hongo puede variar significativamente con los años. Por otro lado, realizamos estudios genéticos sobre la resistencia a M. oryzae en dos poblaciones, procedentes del cruzamiento entre variedades locales, bien adaptadas pero moderadamente susceptibles (Sivert y JSendra) y variedades resistentes, pero no adaptadas (CNA-6159 y Gigante Vercelli). En líneas F3 de ambas poblaciones se determinó la resistencia en campo, en condiciones favorables para el ataque del hongo, en hojas y órganos productivos: en SixCNA, en una parcela de la Albufera, y en JSxGV, en 4 localidades de Valencia, el Delta del Ebro y Sevilla; en JSxGV, además, se realizaron inoculaciones en condiciones controladas. Se comparan los diferentes sistemas de medida de la susceptibilidad, y se discute la influencia ambiental en ella. Se detectaron 22 QTLs en SixCNA, y 61 QTLs en JSxGV, la mayoría de los cuales sólo se expresan en una localidad. Todos los parentales aportan alelos de resistencia; pero la mayoría de los QTLs identificados son de efectos pequeños y, a menudo, con un notable componente dominante. Al mismo tiempo, gran parte de estos QTLs presentan interacciones significativas con otros QTLs; y muchos de ellos co-localizan con QTLs identificados en otros estudios. Hemos encontrado bastantes coincidencias entre QTLs que controlan la incidencia en panículas y los que determinan la severidad en las hojas, apoyando la hipótesis de que existen mecanismos de defensa comunes en ambos órganos. Se han localizado regiones cromosómicas de interés que podrían ser utilizadas para la selección de genotipos resistentes. / [CA] L'arròs (Oryza sativa L.) és la principal font d'alimentació per a més de 3.000 millions de persones, quasi la meitat de la població mundial, i la "piriculariosi", causada pel fong Magnaporthe oryzae Couch, és la malaltia que més pèrdues origina en aquest conreu a escala mundial; també a Espanya ha causat pèrdues importants alguns anys. S'han obtès diverses varietats resistents, principalment en incorporar gens de resistència completa (gens Pi), però la majoria s'han tornat susceptibles en pocs anys en aparèixer nous aïllats més virulents. Actualment, per aconseguir una resistència efectiva i duradora, es cerca incorporar resistència parcial, o una combinació de tots dos tipus de resistència. La resistència parcial és un caràcter quantitatiu, controlat per nombrosos gens d'efecte reduït (o QTLs), els quals poden interaccionar entre sí i que, sovint, també ho fan intensament amb l'ambient. Per conéixer millor la població del patògen present a la zona de l'Albufera, i quins gens i QTLs de resistència poden ser efectius a aquesta i a d'altres zones arrosseres d'Espanya, per una banda s'assajaren durant diversos anys 31 varietats diferencials (línies amb un gen Pi diferent cadascuna); això demostrà que l'estructura de la població del fong pot variejar significativament amb els anys. Per altra banda, realitzàrem estudis genètics sobre la resistència a M. oryzae en dues poblacions, procedents del encreuament entre varietats locals, adaptades però moderadament susceptibles (Sivert i JSendra) i varietats resistents però no adaptades (CNA-6159 i Gigante Vercelli). En línies F3 d'ambdues poblacions es determinà la resistència en camp, en condicions favorables per a l'atac del fong, en fulles i òrgans productius: en SixCNA, en una parcel·la de l'Albufera, i en JSxGV en quatre localitats de València, el Delta de l'Ebre i Sevilla; en JSxGV, a més a més, es realitzaren inoculacions en condicions controlades. Es comparen els diversos sistemes de mesura de la susceptibilidad, i es discuteix la influència ambiental en aquesta. Es detectaren 22 QTLs en SixCNA i 61 QTLs en JSxGV, la major part dels quals solament s'expressen a una localitat. Tots els parentals aporten al·lels de resistència; però alguns dels QTLs identificats són d'efectes reduïts i, freqüentment, amb un notable component dominant. Paral·lelament, gran part d'aquestos QTLs presenten interaccions significatives amb altres QTLs; i molts d'aquestos co-localitzen amb QTLs identificats en altres estudis. Hem trobat bastants coincidències entre QTLs que controlen la incidència en panícules i els que determinen la severitat a les fulles, recolzant la hipòtesi que existeixen mecanismes de defensa comuns en ambdós òrgans. S'han localitzat regions cromosòmiques d'interès que podrien ser emprades per a la selecció de genotipus resistents / Pérez Lotz, J. (2016). Análisis genético de la resistencia parcial a Magnaporthe oryzae en arroz (Oryza sativa) en varias poblaciones y ambientes [Tesis doctoral]. Universitat Politècnica de València. https://doi.org/10.4995/Thesis/10251/62582