The AAA+ chaperone Hsp104 mediates the reactivation of aggregated proteins in Saccharomyces cerevisiae and is crucial for cell survival after exposure to stress. Protein disaggregation depends on cooperation between Hsp104 and a cognate Hsp70 chaperone system. Hsp104 forms a hexameric ring with a narrow axial channel penetrating the centre of the complex. In Chapter 2, I show that conserved loops in each AAA+ module that line this channel are required for disaggregation and that the position of these loops is likely determined by the nucleotide bound state of Hsp104. This evidence supports a common protein remodeling mechanism among Hsp100 members in which proteins are unfolded and threaded along the axial channel. In Chapter 3, I use a peptide-based substrate mimetic to reveal other novel features of Hsp104’s disaggregation mechanism. An Hsp104-binding peptide selected from solid phase arrays recapitulated several properties of an authentic Hsp104 substrate. Inactivation of the pore loops in either AAA+ module prevented stable peptide or protein binding. However, when the loop in the first AAA+ was inactivated, stimulation of ATPase turnover in the second AAA+ module of this mutant was abolished. Drawing on these data, I propose a detailed mechanistic model of protein unfolding by Hsp104 in which an initial unstable interaction involving the loop in the first AAA+ module simultaneously promotes penetration of the substrate into the second axial channel binding site and activates ATP turnover in the second AAA+ module. In Chapter 4, I explore the recognition elements within a model Hsp104-binding peptide that are required for rapid binding to Hsp104. Removal of bulky hydrophobic residues and lysines abrogated the ability of this peptide to function as a peptide-based substrate mimetic for Hsp104. Furthermore, rapid binding of a model unfolded protein to Hsp104 required an intact N-terminal domain and ATP binding at the first AAA+ module. Taken together, I have defined numerous structural features within Hsp104 and its model substrates that are crucial for substrate binding and processing by Hsp104. This work provides a theoretical framework that will encourage research in other protein remodeling AAA+ ATPases.

Amilases microbianas têm aplicações biotecnológicas principalmente na conversão de amido para xaropes de maltose ou glicose. O objetivo deste trabalho foi estudar amilases produzidas pelo fungo Paecilomyces variotii, isolado de folhas de Psidium guajava. As melhores condições para produção enzimática foram padronizadas, usando o meio SR, pH 7,0, suplementado com farinha de aveia 1,5% como fonte de carbono, inoculado com 1,75x108 conídios/mL, a 30ºC, sem agitação, por 6 dias. O fungo desenvolveu-se em uma ampla faixa de pH (3-8), concentração osmótica (0?15% NaCl), e altas temperaturas (até 45ºC), o que o caracteriza como um microrganismo tolerante a ambientes extremos. As amilases foram purificadas por seqüencial eluição em cromatografias de DEAE-celulose e Sephadex G-100, sendo nesta última separada em duas formas amilolíticas (PI e PII). PI foi submetido em eletroeluição resultando em uma ?amilase com 16.1 vezes de purificação com 8,9% de recuperação. PII foi caracterizado como uma glucoamilase, com recuperação e fator de purificação de 6,2% e 47,9 vezes, respectivamente. Análises em PAGE confirmou o grau de homogeneidade e o caráter amilolítico (?-amilase e glucoamilase) após revelação dos géis com iodo ou glicoseoxidase. Esses resultados foram confirmados através de TLC, onde maltose e maltotriose foram os principais produtos de hidrólise do amido para PI e somente glicose para PII. ?-amilases e glucoamilase apresentaram massas moleculares (SDS-PAGE) de 75 kDa e 86,5 kDa, com pH e temperatura ótimos de 4,0 e 60ºC, e 5,0 e 55ºC, respectivamente. Em relação à estabilidade, ambas enzimas foram estáveis até 60ºC. Temperaturas superiores levaram a instabilidade e uma diminuição das atividades. As amilases foram estáveis em todos os pHs testados (2,5 ? 8,0), sendo maior em pH ácido para a glucoamilase e o contrário para a ?amilase. Amido favoreceu a estabilidade da glucoamilase, mas esta foi inibida por glicose, já para ?-amilase o amido atuou como protetor da enzima em períodos superiores a 40 minutos e esta não sofreu inibição pelo produto final. O ponto isoelétrico e o conteúdo de carboidratos foram de 3,5 e 27,5% para a glucoamilase e 4,5 e 23% para a ?-amilase, respectivamente. A glucoamilase e a ?-amilase de P. variotii hidrolisaram preferencialmente amido Reagen® e Sigma®. Constante de Michaelis e Vmáx foram determinados para vários substratos, podendo estimar uma eficiência catalítica favorável a amilopectina para a glucoamilase e amido Reagen® para a ?-amilase. A atividade glucoamilásica foi ensaiada com vários íons metálicos, mas somente Mn2+ 5mM aumentou a atividade em 80,7%. A ?-amilase foi ativada por 1mM de Ca2+ (65%) e Co2+ (60%). Análises de dicroísmo circular forneceram alguns dados sobre a estrutura dessas enzimas, sendo que, a glucoamilase mostrou-se rica em ?-hélices e a ?amilase uma mistura de ?-hélices e folhas ?. Através de análises em Western Blotting foi possível determinar pouca homologia entre as glucoamilases produzidas por Paecilomyces variotti e Scytalidium thermophilum, o que é condizente com os dados encontrados na literatura, uma vez que o gênero Paecilomyces encontra-se mais próximo, do ponto de vista evolucionário, de Talaromyces. Esses dados foram confirmados com a realização de espectrometria de massa para a glucoamilase de P. variotii, no qual um fragmento de aminoácidos da glucoamilase deste microrganismo apresentou homologia com a glucoamilase de T. emersonii. / Microbial amylases have biotechnological applications mainly for the conversion of starch to maltose or glucose syrup. The aim of this work was to study amylases produced by the fungus Paecilomyces variotii, isolated from Psidium guajava leaves. The best growth conditions for enzymatic production were standardized, using SR medium, pH 7.0, supplemented with 1.5% oat flour as carbon source, inoculated with 1.75x108 conidia/mL, at 30ºC, without agitation, for 6 days. The fungus developed in a wide range of pH (3-8), osmotic concentration (0-15% NaCl), and high temperatures (up to 45ºC), which characterized it as a tolerant microorganism at extreme environment. The amylases were purified by sequential elution in DEAE-cellulose and Sephadex G-100 chromatography, being in the last step resolved two amylolytic forms (PI and PII). PI was submitted to electroelution resulting in an ?-amylase purified 16.1-fold, with 8.9% recuperation. PII was characterized as a glucoamylase, with a recovery and purification factor of 6.2% and 47.9-fold, respectively. Analysis in PAGE showed that the enzyme was homogeneous and the amylolytic activities (?-amylase and glucoamylase) were confirmed after PAGE, revealing the gels for activity using both iodine and glucose-oxidase. Analysis by Thin Layer Chromatography confirmed the character of these enzymes, which released either maltose and maltotriose (PI), or only glucose (PII), as the main products of starch hydrolysis. ?-amylase and glucoamylase presented molecular masses in SDS-PAGE of 75 kDA and 86.5 kDa, respectively. Optimal pH and temperature were 4.0 and 60ºC for ?-amylase, and 5.0 and 55ºC for glucoamylase. Thermostability analysis showed that both enzymes were stable up to 60ºC. Higher temperatures decreased amylolytic activities. The amylases were stable in all pH tested (2.5-8.0), being higher in acid pH for glucoamylase, and the opposite for ?amylase. Addition of starch in the assay favored the stability of glucoamylase, but it was inhibited by addition of glucose (end product). In contrast, starch protected ?-amylase activity in periods higher 40 minutes and the enzyme was not inhibited by the end product. The isoelectric point and carbohydrate content were 3.5 and 27.5% for glucoamylase and 4.5 and 23% for ?-amylase, respectively. The glucoamylase and ?-amylase of P. variotii hydrolyzed preferentially starch Reagen® and Sigma®, respectively. Km and Vmax values were determined for several substrates, being estimated a catalytic efficiency favorable to amylopectin for glucoamylase and starch Reagen® for ?-amylase. The glucoamylase activity was tested with several metal ions, but only 5mM Mn2+ increased the activity in 80.7%. The ?-amylase was activated by 1mM Ca2+ (65%) and Co2+ (60%). Circular dichroism analysis supplied additional information about the glucoamylase structure, since the protein showed to be rich in ?-helix but ?-amylase showed to be a mixture of ?-helix and ?-sheet. By the analysis in Western Blotting was possible to determine some structural homology between the glucoamylases produced by Paecilomyces variotii and Scytalidium thermophilum, that is in agreement with published information, since the genera Paecilomyces is related, from the evolutionary viewpoint, with Talaromyces. This information were confirmed with the data from mass spectrometry for the glucoamylase of P. variotii, which a sequenced fragment of protein glucoamylase of this microorganism presented homology with the same enzyme produced by Talaromyces emersonii.

The E. coli ArsA is involved in arsenic detoxification but the role of yArsA (ArsA homologue of Saccharomyces cerevisiae, encoded by YDL100c ORF) in yeast is still undefined. Disruption of YDL100c ORF is not lethal but the disrupted strain (KO) shows decreased thermotolerance. To study the role of yArsA in thermotolerance, wild type (WT) and KO were grown at 25¢Jand 37¢J, and assayed for the intracellular levels of trehalose accumulation and molecular oxidation, and the biosynthesis of heat shock proteins. The results show that molecular oxidation is higher and trehalose accumulation is lower in KO compared with WT grown at 37¢J, suggesting that increased ROS and decreased trehalose content are the cause of cell death. Further analysis of the expression of ROS defense mechanisms show that there is no significant difference in TSL1 and SOD1 expression in WT and KO grown at 25¢J or 37¢J but the CTT1 expression in KO was much less than WT grown at 37¢J. These observations are consistent with the assays of enzymatic activity of catalase and antioxidant GSH contents. Loss of catalase activity, decreased trehalose contents and Hsp104p expression suggest a deficiency in activation of general but not specific stress response in KO when grown at 37¢J. Therefore, yArsA was involved in signaling the general stress response in stress tolerance network.

The most universal abiotic influence is temperature, and thus, thermotolerance, adaptations and response to thermal variation, is a fundamental factor shaping evolution. Prokaryotic life may have an upper thermal limit near \(150^{\circ}C\); however, eukaryotic survival is limited to \(50^{\circ}C\) – the thermal maximum for sustained biosynthesis and homeostasis. My research focuses on understanding the physiological and biochemical factors that limit eukaryotic thermotolerance, by studying an organism near the upper limit of all eukaryotes: Paralvinella sulfincola. P. sulfincola, a hydrothermal vent polychaete, has the broadest known thermal range of any metazoan: \((5-48^{\circ}C)\). This species, along with the mesotolerant congener with Paralvinella palmiformis, is found at vents along the Juan de Fuca Ridge, Washington, USA. Making an ideal study system, both species are found in similar habitats, genetically comparable, and amenable to recovery and shipboard experimentation. Here, I present data from a series of high pressure in vivo experiments that investigate stress response to variations in temperature, pH, sulfide concentration, and duration. Field work was coupled with a suite of biomolecular techniques including pyrosequencing, comparative proteomics, enzyme assays, and quantitative PCR. From this research, the first to quantify global protein and antioxidant responses to temperature in an extremely thermotolerant eukaryote, three primary conclusions can be reached. 1) Pronounced thermal tolerance in P. sulfincola is likely enabled by its constitutive expression of heat shock proteins and limited by its ability to quickly and appropriately respond to the commensurate increase in oxidative stress. 2) Thermal tolerance limits are likely negatively affected by synergistic multistress effects. 3) Antioxidant gene expression response differs significantly between chronically and acutely stressed treatments, supporting the theory that oxidative stress is limiting in this system.

The AAA+ chaperone Hsp104 mediates the reactivation of aggregated proteins in Saccharomyces cerevisiae and is crucial for cell survival after exposure to stress. Protein disaggregation depends on cooperation between Hsp104 and a cognate Hsp70 chaperone system. Hsp104 forms a hexameric ring with a narrow axial channel penetrating the centre of the complex. In Chapter 2, I show that conserved loops in each AAA+ module that line this channel are required for disaggregation and that the position of these loops is likely determined by the nucleotide bound state of Hsp104. This evidence supports a common protein remodeling mechanism among Hsp100 members in which proteins are unfolded and threaded along the axial channel. In Chapter 3, I use a peptide-based substrate mimetic to reveal other novel features of Hsp104’s disaggregation mechanism. An Hsp104-binding peptide selected from solid phase arrays recapitulated several properties of an authentic Hsp104 substrate. Inactivation of the pore loops in either AAA+ module prevented stable peptide or protein binding. However, when the loop in the first AAA+ was inactivated, stimulation of ATPase turnover in the second AAA+ module of this mutant was abolished. Drawing on these data, I propose a detailed mechanistic model of protein unfolding by Hsp104 in which an initial unstable interaction involving the loop in the first AAA+ module simultaneously promotes penetration of the substrate into the second axial channel binding site and activates ATP turnover in the second AAA+ module. In Chapter 4, I explore the recognition elements within a model Hsp104-binding peptide that are required for rapid binding to Hsp104. Removal of bulky hydrophobic residues and lysines abrogated the ability of this peptide to function as a peptide-based substrate mimetic for Hsp104. Furthermore, rapid binding of a model unfolded protein to Hsp104 required an intact N-terminal domain and ATP binding at the first AAA+ module. Taken together, I have defined numerous structural features within Hsp104 and its model substrates that are crucial for substrate binding and processing by Hsp104. This work provides a theoretical framework that will encourage research in other protein remodeling AAA+ ATPases.

Amilases microbianas têm aplicações biotecnológicas principalmente na conversão de amido para xaropes de maltose ou glicose. O objetivo deste trabalho foi estudar amilases produzidas pelo fungo Paecilomyces variotii, isolado de folhas de Psidium guajava. As melhores condições para produção enzimática foram padronizadas, usando o meio SR, pH 7,0, suplementado com farinha de aveia 1,5% como fonte de carbono, inoculado com 1,75x108 conídios/mL, a 30ºC, sem agitação, por 6 dias. O fungo desenvolveu-se em uma ampla faixa de pH (3-8), concentração osmótica (0?15% NaCl), e altas temperaturas (até 45ºC), o que o caracteriza como um microrganismo tolerante a ambientes extremos. As amilases foram purificadas por seqüencial eluição em cromatografias de DEAE-celulose e Sephadex G-100, sendo nesta última separada em duas formas amilolíticas (PI e PII). PI foi submetido em eletroeluição resultando em uma ?amilase com 16.1 vezes de purificação com 8,9% de recuperação. PII foi caracterizado como uma glucoamilase, com recuperação e fator de purificação de 6,2% e 47,9 vezes, respectivamente. Análises em PAGE confirmou o grau de homogeneidade e o caráter amilolítico (?-amilase e glucoamilase) após revelação dos géis com iodo ou glicoseoxidase. Esses resultados foram confirmados através de TLC, onde maltose e maltotriose foram os principais produtos de hidrólise do amido para PI e somente glicose para PII. ?-amilases e glucoamilase apresentaram massas moleculares (SDS-PAGE) de 75 kDa e 86,5 kDa, com pH e temperatura ótimos de 4,0 e 60ºC, e 5,0 e 55ºC, respectivamente. Em relação à estabilidade, ambas enzimas foram estáveis até 60ºC. Temperaturas superiores levaram a instabilidade e uma diminuição das atividades. As amilases foram estáveis em todos os pHs testados (2,5 ? 8,0), sendo maior em pH ácido para a glucoamilase e o contrário para a ?amilase. Amido favoreceu a estabilidade da glucoamilase, mas esta foi inibida por glicose, já para ?-amilase o amido atuou como protetor da enzima em períodos superiores a 40 minutos e esta não sofreu inibição pelo produto final. O ponto isoelétrico e o conteúdo de carboidratos foram de 3,5 e 27,5% para a glucoamilase e 4,5 e 23% para a ?-amilase, respectivamente. A glucoamilase e a ?-amilase de P. variotii hidrolisaram preferencialmente amido Reagen® e Sigma®. Constante de Michaelis e Vmáx foram determinados para vários substratos, podendo estimar uma eficiência catalítica favorável a amilopectina para a glucoamilase e amido Reagen® para a ?-amilase. A atividade glucoamilásica foi ensaiada com vários íons metálicos, mas somente Mn2+ 5mM aumentou a atividade em 80,7%. A ?-amilase foi ativada por 1mM de Ca2+ (65%) e Co2+ (60%). Análises de dicroísmo circular forneceram alguns dados sobre a estrutura dessas enzimas, sendo que, a glucoamilase mostrou-se rica em ?-hélices e a ?amilase uma mistura de ?-hélices e folhas ?. Através de análises em Western Blotting foi possível determinar pouca homologia entre as glucoamilases produzidas por Paecilomyces variotti e Scytalidium thermophilum, o que é condizente com os dados encontrados na literatura, uma vez que o gênero Paecilomyces encontra-se mais próximo, do ponto de vista evolucionário, de Talaromyces. Esses dados foram confirmados com a realização de espectrometria de massa para a glucoamilase de P. variotii, no qual um fragmento de aminoácidos da glucoamilase deste microrganismo apresentou homologia com a glucoamilase de T. emersonii. / Microbial amylases have biotechnological applications mainly for the conversion of starch to maltose or glucose syrup. The aim of this work was to study amylases produced by the fungus Paecilomyces variotii, isolated from Psidium guajava leaves. The best growth conditions for enzymatic production were standardized, using SR medium, pH 7.0, supplemented with 1.5% oat flour as carbon source, inoculated with 1.75x108 conidia/mL, at 30ºC, without agitation, for 6 days. The fungus developed in a wide range of pH (3-8), osmotic concentration (0-15% NaCl), and high temperatures (up to 45ºC), which characterized it as a tolerant microorganism at extreme environment. The amylases were purified by sequential elution in DEAE-cellulose and Sephadex G-100 chromatography, being in the last step resolved two amylolytic forms (PI and PII). PI was submitted to electroelution resulting in an ?-amylase purified 16.1-fold, with 8.9% recuperation. PII was characterized as a glucoamylase, with a recovery and purification factor of 6.2% and 47.9-fold, respectively. Analysis in PAGE showed that the enzyme was homogeneous and the amylolytic activities (?-amylase and glucoamylase) were confirmed after PAGE, revealing the gels for activity using both iodine and glucose-oxidase. Analysis by Thin Layer Chromatography confirmed the character of these enzymes, which released either maltose and maltotriose (PI), or only glucose (PII), as the main products of starch hydrolysis. ?-amylase and glucoamylase presented molecular masses in SDS-PAGE of 75 kDA and 86.5 kDa, respectively. Optimal pH and temperature were 4.0 and 60ºC for ?-amylase, and 5.0 and 55ºC for glucoamylase. Thermostability analysis showed that both enzymes were stable up to 60ºC. Higher temperatures decreased amylolytic activities. The amylases were stable in all pH tested (2.5-8.0), being higher in acid pH for glucoamylase, and the opposite for ?amylase. Addition of starch in the assay favored the stability of glucoamylase, but it was inhibited by addition of glucose (end product). In contrast, starch protected ?-amylase activity in periods higher 40 minutes and the enzyme was not inhibited by the end product. The isoelectric point and carbohydrate content were 3.5 and 27.5% for glucoamylase and 4.5 and 23% for ?-amylase, respectively. The glucoamylase and ?-amylase of P. variotii hydrolyzed preferentially starch Reagen® and Sigma®, respectively. Km and Vmax values were determined for several substrates, being estimated a catalytic efficiency favorable to amylopectin for glucoamylase and starch Reagen® for ?-amylase. The glucoamylase activity was tested with several metal ions, but only 5mM Mn2+ increased the activity in 80.7%. The ?-amylase was activated by 1mM Ca2+ (65%) and Co2+ (60%). Circular dichroism analysis supplied additional information about the glucoamylase structure, since the protein showed to be rich in ?-helix but ?-amylase showed to be a mixture of ?-helix and ?-sheet. By the analysis in Western Blotting was possible to determine some structural homology between the glucoamylases produced by Paecilomyces variotii and Scytalidium thermophilum, that is in agreement with published information, since the genera Paecilomyces is related, from the evolutionary viewpoint, with Talaromyces. This information were confirmed with the data from mass spectrometry for the glucoamylase of P. variotii, which a sequenced fragment of protein glucoamylase of this microorganism presented homology with the same enzyme produced by Talaromyces emersonii.

Lignocellulosic biomass is a potential feedstock for production of sugars, which can be fermented into ethanol. The work presented in this thesis proposes some solutions to overcome problems with suboptimal process performance due to elevated cultivation temperatures and inhibitors present during ethanol production from lignocellulosic materials. In particular, continuous processes operated at high dilution rates with high sugar utilisation are attractive for ethanol fermentation, as this can result in higher ethanol productivity. Both encapsulation and membrane bioreactors were studied and developed to achieve rapid fermentation at high yeast cell density. My studies showed that encapsulated yeast is more thermotolerant than suspended yeast. The encapsulated yeast could successfully ferment all glucose during five consecutive batches, 12 h each at 42 °C. In contrast, freely suspended yeast was inactivated already in the second or third batch. One problem with encapsulation is, however, the mechanical robustness of the capsule membrane. If the capsules are exposed to e.g. high shear forces, the capsule membrane may break. Therefore, a method was developed to produce more robust capsules by treating alginate-chitosan-alginate (ACA) capsules with 3-aminopropyltriethoxysilane (APTES) to get polysiloxane-ACA capsules. Of the ACA-capsules treated with 1.5% APTES, only 0–2% of the capsules broke, while 25% of the untreated capsules ruptured within 6 h in a shear test. In this thesis membrane bioreactors (MBR), using either a cross-flow or a submerged membrane, could successfully be applied to retain the yeast inside the reactor. The cross-flow membrane was operated at a dilution rate of 0.5 h-1 whereas the submerged membrane was tested at several dilution rates, from 0.2 up to 0.8 h-1. Cultivations at high cell densities demonstrated an efficient in situ detoxification of very high furfural levels of up to 17 g L-1 in the feed medium when using a MBR. The maximum yeast density achieved in the MBR was more than 200 g L-1. Additionally, ethanol fermentation of nondetoxified spruce hydrolysate was possible at a high feeding rate of 0.8 h-1 by applying a submerged membrane bioreactor, resulting in ethanol productivities of up to 8 g L-1 h-1. In conclusion, this study suggests methods for rapid continuous ethanol production even at stressful elevated cultivation temperatures or inhibitory conditions by using encapsulation or membrane bioreactors and high cell density cultivations. / <p>Akademisk avhandling som för avläggande av teknologie doktorsexamen vid Chalmers tekniska högskola försvaras vid offentlig disputation den 4 april 2014, klockan 9:30 i KE-salen, Kemigården 4, Göteborg.</p>

Doctor of Philosophy / Division of Biology / Theodore J. Morgan / Thermal stress impacts animals around the globe and understanding how organisms adapt to changes in temperature is of particular interest under current climate change predictions. My research focuses on the evolutionary genetics involved in cold tolerance and plasticity of cold tolerance using both artificially selected and naturally segregating populations, while tying the genes of interest to their physiological components. First I address cross-tolerance of stress traits following artificial selection to a non-lethal cold tolerance metric, chill-coma recovery. Using these artificial selection populations, we found that stress traits such as desiccation tolerance, starvation tolerance, acclimation, and chronic and acute cold tolerance do not correlate with level of cold tolerance as defined by chill-coma recovery time. We next assessed lifetime fitness of these different cold tolerance lines and found that only at low temperatures did fitness differ among cold tolerance levels. We then analyzed gene expression differences between resistant and susceptible populations at three time points to understand where selection pressures are hypothesized to act on genomic variation. Our gene expression analyses found many differences between resistant and susceptible lines, primarily manifesting themselves in the recovery period following cold exposure. We next utilized a community resource, the Drosophila melanogaster reference panel, to identify naturally segregating variation in genes associated with cold acclimation and fitness. We specifically asked if long- and short-term acclimation ability had overlapping genetic regions and if plasticity values from constant rearing environments were associated with demographic parameters in fluctuating environments. We found that long- and short-term acclimation are under unique genetic control and functionally tested several genes for acclimation ability. We also found that acclimation ability in constant environments and fitness in fluctuating environments do not correlate, but that genotypes are constrained in their fitness abilities between a warm and cool environment. Our analyses describe several novel genes associated with cold tolerance selection and long- and short-term acclimation expanding our knowledge of the complex relationship between demographic components and survivorship as well as a unique investigation of the change in gene expression during cold exposure.

Thermotolerance limit in aquatic organism is set by the ability to sustain aerobic scope to sudden temperature shifts. This study tested the genetic and plastic differences in thermotolerance of Daphnia that can be explained by the differences in the ability to retain mitochondrial integrity at high temperatures. Five genotypes with different biogeographic origins were acclimated to 18ᵒC and 25ᵒC. We developed a rhodamine 123 in-vivo assay to measure mitochondrial membrane potential and observed higher fluorescent in heat damaged tissues as the disruption of the mitochondrial membrane potential. Significant effects on temperature tolerance were observed with CCCP and DNP but not with NaN3. Effects of toxins were significant in temperature sensitive genotype and high concentration of lactate was observed in 18ᵒC acclimated genotype only. We conclude that genetic and physiological differences are intricately linked to the ability of sustaining aerobic respiration at high temperatures which sets limit to the thermotolerance.

Un grand nombre de paramètres environnementaux affectent les insectes, et l’identification des éléments impliqués dans l’adaptation aux stress est cruciale. La nutrition est une clé de cette adaptation : elle contrôle la santé des organismes, et par conséquent affecte leur tolérance au stress. Le microbiote intestinal module lui aussi les processus physiologiques des organismes, notamment en agissant sur le phénotype nutritionnel de l’hôte. Cependant, nous ignorons encore la capacité de ce microbiote à modifier la tolérance au stress de l’hôte. A travers la manipulation du microbiote et de la nutrition, nous avons testé les effets de l’interaction microbiote-nutrition chez Drosophila melanogaster en nous concentrant sur la tolérance au stress. Nous avons montré une nette dépendance des mouches à l’accès aux levures pour maintenir un développement et un métabolisme normal.A l’inverse, l’altération du développement générée par les substrats nutritionnellement pauvres ou par la surpopulation larvaire était bénéfique pour la tolérance au stress. Nous avons démontré que le microbiote peut partiellement compenser les effets d’une privation en levures, mais n’a pas d’effet quand elles sont en excès. Pour la thermotolérance, nous avons constaté que les effets du microbiote étaient très variables, et avons attribué cette variabilité à la présence ou l’absence des levures. Les assemblages de microorganismes environnementaux étaient modifiées par la composition du substrat, mais le microbiote intestinal était étonnamment stable. Ces résultats témoignent de l’importance des facteurs nutritionnels et du microbiote à moduler le développement et le phénotype des Drosophiles. / A large number of environmental parameters constrain insect species, and identifying the elements involved in the adaptation to stressful conditions is crucial. Nutrition is a key factor of this adaptation: it largely controls health of organisms and therefore their stress tolerance. In addition, it is now established that microbiota (i.e. the whole microbial community associated with a host) is much involved in physiological processes of organisms. In particular, gut microbiota is able to modulate host’s nutritional phenotype, resulting in complex dietary-dependent effects. Whether or not gut microbiota can act on stress tolerance is not clear. Thus, using experimental manipulation of microbiota and nutrition, we investigated the outcome of the microbiota-nutrition relationship on Drosophila melanogaster, with a special focus on stress tolerance. We showed a clear dependency of flies on yeasts availability to maintain a normal development, metabolism, and size. Conversely, impaired development resulting from artificial poor diets or from larval crowding was often beneficial for stress tolerance. We demonstrated that microbiota could partially compensate for the absence of yeast resource, but no effects were found in rich food contexts. We observed variable importance of microbiota on thermal tolerance, and attributed this variation to the presence or absence of yeasts. Environmental microorganisms communities were modified by diet composition but gut microbiota was surprisingly stable. Together, these findings highlight the importance of nutritional factors and microbiota in developmental and phenotypical plasticity of fruit flies.