RESUMO
Alleles conferring a higher adaptive value in one environment may have a detrimental impact on fitness in another environment. Alleles conferring resistance to pesticides and drugs provide textbook examples of this trade-off as, in addition to conferring resistance to these molecules, they frequently decrease fitness in pesticide/drug-free environments. We show here that resistance to chlorpyrifos, an organophosphate (OP), in Chinese populations of the diamondback moth, Plutella xylostella, is conferred by two mutations of ace1 - the gene encoding the acetylcholinesterase enzyme targeted by OPs - affecting the amino acid sequence of the corresponding protein. These mutations were always linked, consistent with the segregation of a single resistance allele, ace1R, carrying both mutations, in the populations studied. We monitored the frequency of ace1R (by genotyping more than 20 000 adults) and the level of resistance (through bioassays on more than 50 000 individuals) over several generations. We found that the ace1R resistance allele was costly in the absence of insecticide and that this cost was likely recessive. This fitness costs involved a decrease in fecundity: females from resistant strains laid 20% fewer eggs, on average, than females from susceptible strains. Finally, we found that the fitness costs associated with the ace1R allele were greater at high temperatures. At least two life history traits were involved: longevity and fecundity. The relative longevity of resistant individuals was affected only at high temperatures and the relative fecundity of resistant females - which was already affected at temperatures optimal for development - decreased further at high temperatures. The implications of these findings for resistance management are discussed.
Assuntos
Aptidão Genética , Resistência a Inseticidas , Mariposas/genética , Temperatura , Acetilcolinesterase/genética , Alelos , Animais , Clorpirifos , Feminino , Frequência do Gene , Genótipo , Dados de Sequência Molecular , Mariposas/enzimologia , Mutação , Fenótipo , Estresse FisiológicoRESUMO
The evolution of resistance in insect pests will imperil the efficiency of transgenic insect-resistant crops. The currently advised strategy to delay resistance evolution is to plant non-toxic crops (refuges) in close proximity to plants engineered to express the toxic protein of the bacterium Bacillus thuringiensis (Bt). We seek answers to the question of how to induce growers to plant non-toxic crops. A first strategy, applied in the United States, is to require Bt growers to plant non-Bt refuges and control their compliance with requirements. We suggest that an alternative strategy is to make Bt seed more expensive by instituting a user fee, and we compare both strategies by integrating economic processes into a spatially explicit, population genetics model. Our results indicate that although both strategies may allow the sustainable management of the common pool of Bt-susceptibility alleles in pest populations, for the European corn borer (Ostrinia nubilalis) one of the most serious pests in the US corn belt, the fee strategy is less efficient than refuge requirements.
Assuntos
Proteínas de Bactérias/genética , Toxinas Bacterianas/genética , Produtos Agrícolas/economia , Endotoxinas/genética , Proteínas Hemolisinas/genética , Controle de Insetos/métodos , Modelos Econômicos , Controle Biológico de Vetores/economia , Plantas Geneticamente Modificadas/parasitologia , Zea mays , Animais , Toxinas de Bacillus thuringiensis , Evolução Molecular , Controle de Insetos/economia , Modelos Genéticos , Mariposas/genética , Doenças das Plantas/parasitologia , Plantas Geneticamente Modificadas/genética , Sementes/genética , Estados Unidos , Zea mays/economia , Zea mays/genética , Zea mays/parasitologiaRESUMO
Genetic changes conferring adaptation to a new environment may induce a fitness cost in the previous environment. Although this prediction has been verified in laboratory conditions, few studies have tried to document this cost directly in natural populations. Here, we evaluated the pleiotropic effects of insecticide resistance on putative fitness components of the mosquito Culex pipiens. Experiments using different larval densities were performed during the summer in two natural breeding sites. Two loci that possess alleles conferring organophosphate (OP) resistance were considered: ace-1 coding for an acetylcholinesterase (AChE1, the OP target) and Ester, a ''super locus" including two closely linked loci coding for esterases A and B. Resistance ace-1 alleles coding for a modified AChE1 were associated with a longer development time and shorter wing length. The pleiotropic effects of two resistance alleles Ester1 and Ester4 coding for the overproduced esterases A1 and A4-B4, respectively, were more variable. Both A1 and A4-B4 reduced wing length, although only A1 was associated with a longer preimaginal stage. The fluctuating asymmetry (FA) of the wing did not respond to the presence or to the interaction of resistance alleles at the two loci at any of the density levels tested. Conversely, the FA of one wing section decreased when larval density increased. This may be the consequence of selection against less developmentally stable individuals. The results are discussed in relation to the local evolution of insecticide resistance genes.