Biomedical subjects

T Lenormand

Publications and source records attributed to T Lenormand.

5 recordsLinked to original sources

PubMed1998

An overview of the evolution of overproduced esterases in the mosquito Culex pipiens.

Insecticide resistance genes have developed in a wide variety of insects in response to heavy chemical application. Few of these examples of adaptation in response to rapid environmental change have been studied both at the population level and at the gene level. One of these is the evolution of the overproduced esterases that are involved in resistance to organophosphate insecticides in the mosquito Culex pipiens. At the gene level, two genetic mechanisms are involved in esterase overproduction, namely gene amplification and gene regulation. At the population level, the co-occurrence of the same amplified allele in distinct geographic areas is best explained by the importance of passive transportation at the worldwide scale. The long-term monitoring of a population of mosquitoes in southern France has enabled a detailed study to be made of the evolution of resistance genes on a local scale, and has shown that a resistance gene with a lower cost has replaced a former resistance allele with a higher cost.

Alleles↗

PubMed1998

Evaluating gene flow using selected markers: a case study.

The extent to which an organism is locally adapted in an environmental pocket depends on the selection intensities inside and outside the pocket, on migration, and on the size of the pocket. When two or more loci are involved in this local adaptation, measuring their frequency gradients and their linkage disequilbria allows one to disentangle the forces-migration and selection-acting on the system. We apply this method to the case of a local adaptation to organophosphate insecticides in the mosquito Culex pipiens pipiens in southern France. The study of two different resistance loci allowed us to estimate with support limits gene flow as well as selection pressure on insecticide resistance and the fitness costs associated with each locus. These estimates permit us to pinpoint the conditions for the maintenance of this pocket of adaptation as well as the effect of the interaction between the two resistance loci.

Animals↗

PubMed1997

Variation of dominance of newly arisen adaptive genes.

Newly arisen adaptive alleles such as insecticide resistance genes represent a good opportunity to investigate the theories put forth to explain the molecular basis of dominance and its possible evolution. Dominance levels of insecticide resistance conferred by insensitive alleles of the acetylcholinesterase gene were analyzed in five resistant strains of the mosquito Culex pipiens. Dominance levels were found to differ between strains, varying from partial recessivity to complete dominance. This variation was not explained by differences in catalytic properties of the enzyme, since four of the five resistant strains had identical inhibition properties for the insensitive acetylcholinesterase. Among these four laboratory strains and in individuals collected from natural populations, we found a correlation between increased acetylcholinesterase activities and higher dominance levels. We propose a molecular explanation for how variation in acetylcholinesterase activity may result in variation of dominance level. We also conjecture that the four resistant strains did not differ in their amino acid sequence in the catalytically active regions of acetylcholinesterase, but that the expression of the gene was regulated by either neighboring or distant sites, thereby modifying the dominance level. Under this interpretation, dominance levels may evolve in this system, since heritable variation in acetylcholinesterase activity was found.

Acetylcholinesterase↗

PubMed1997

Chromosomal transmission bias in laboratory hybrids between wild strains of the two European subspecies of house mice.

Laboratory crosses between wild strains of the two European house mouse subspecies Mus musculus domesticus (2n = 34) and M. m. musculus (2n = 40) were performed to analyze the selective processes involved in the non-introgression of centromeric regions of Robertsonian (Rb) fusions in the Danish hybrid zone. The chromosomal analysis of 226 backcross progeny from 22 reciprocal crosses showed that the segregation of the three Rb fusions present did not significantly differ from Mendelian expectations. However, a significant negative correlation was found between Rb transmission rates and the average litter sizes of the F1 pairs. Among the different models of selection discussed, the most likely one supported the existence of two opposing selective factors resulting in an overall compensation of chromosomal types in the backcross progeny. A two-phase selective process involving embryo competition was postulated with non-Rb carriers being favored during pre-implantation but disadvantaged after implantation. Such balanced selective pressures acting on musculus non-Rb centromeres are compatible with the steep slope and off-centered position of the chromosomal cline observed in the Danish hybrid zone. These results suggested that these selective factors may be more related to centromere origin (musculus or domesticus) than to centromere structure (Rb or non-Rb).

Animals↗

PubMed1996

Genomic incompatibilities in the hybrid zone between house mice in Denmark: evidence from steep and non-coincident chromosomal clines for Robertsonian fusions.

The pattern of chromosomal variation is investigated in house mice from the Danish hybrid zone between the translocation-prone Mus musculus domesticus and the chromosomally conservative M. m. musculus. The cytogenetic analysis confirmed the non-introgression of three pairs of Robertsonian (Rb) fusions from M. m. domesticus into the M. m. musculus genome. The geographic distribution of two of these Rb fusions was shown to follow staggered chromosomal clines which increased in steepness the closer they were to the centre of the hybrid zone as defined by allozymes. Analysis of alternate hypotheses suggests that chromosomal differentiation of the Danish domesticus occurred after contact was established with musculus. The staggering of the clines would reflect the order of arrival of the Rb fusions into the hybrid zone. Several models with different processes of underdominance of the chromosomal heterozygotes are discussed to account for the difference in width between clines. A selective model with increasing levels of genomic underdominance due to interaction with a progressively enriched musculus genome provides the best fit for the observed pattern. Selection against Rb fusions with little effect on the recombination of linked allozyme markers supports the view that no reduction in gene flow due to chromosomal heterozygosity is yet apparent through the hybrid zone and that only the centromeric segments of the Rb fusions are incompatible with the musculus genome.

Animals↗