Early studies on the genetics of insecticide resistance showed that single major semi-dominant genes were generally involved, and biochemical studies defined a limited number of enzymes and structural nerve proteins that were encoded by these genes. Recent advances in resistance detection now allow the measurement of genotype frequencies for some of these resistance mechanisms. Molecular studies are in progress for most of the major resistance genes, and the amplified esterase B1 gene in Culex quinquefasciatus has been cloned. Changes in resistance gene expression occur, with increasing age of the adult insect, by way of specific mechanisms, and amplified esterase-based resistance genes that are not expressed can occur in aphids. Here Janet Hemingway assesses current knowledge of the genetics of insecticide resistance in mosquitoes.
Insecticide resistance is an inherited characteristic involving changes in one or more insect gene. The molecular basis of these changes are only now being fully determined, aided by the availability of the Drosophila melanogaster and Anopheles gambiae genome sequences. This paper reviews what is currently known about insecticide resistance conferred by metabolic or target site changes in mosquitoes.
DOI : 10.1016/s0965-1748(04)00069-4
In response to intensive use of organophosphorus insecticides against mosquitoes, resistance in Culex pipiens from Lucca, Italy, has increased, largely thwarting control efforts. In 1985, the field population was segregating for both altered acetylcholinesterase and elevated nonspecific esterase-based resistance mechanisms. Strains colonized in the laboratory at this time were selected for resistance or susceptibility to organophosphorus and carbamate insecticides. Microtiter plate assay analysis of genotypes was used to identify when strains had reached homozygosity for the altered acetylcholinesterase (AChE) mechanism of insecticide resistance. In Italian C. pipiens, the alteration in the enzyme, AChE, associated with resistance had no significant effect on either the Km or Vmax values with the substrate acetylthiocholine iodide. In vitro measurement of bimolecular rate constants showed that the altered form of AChE was more insensitive to inhibition by certain insecticides, than the AChE variant in the line selected for susceptibility. The cross-resistance conferred by this mechanism to a number of organophosphorus and carbamate insecticides was determined by bioassay, the contribution of nonspecific esterase activity to insecticide resistance being quantified by use of a nonspecific esterase inhibitor. Altered AChE conferred greater resistance to carbamate than to organophosphorus insecticides, the contribution of nonspecific esterase activity being greater in resistance to the latter. Data indicated that a third resistance mechanism may be present in this population conferring moderate levels of resistance to pirimiphos methyl and fenitrothion.
Collection of field populations of Culex pipiens from Lucca, Italy in 1984 and 1985 showed consistently high levels of resistance to a range of organophosphorus (OP) insecticides. Bioassays on the 1985 collection revealed that this population also had high levels of resistance to carbamates, even though these compounds had not been used in the field for Culex control. Biochemical analysis showed that the 1985 population contained the high esterase, Est-3A, and altered acetylcholinesterase (AChE) resistance mechanisms. The first OP resistance mechanism selected in Italian Culex was the Est-3A, and the AChE was then selected subsequently. Since September 1984 the altered AChE gene has been selected to a very high frequency in the Lucca population (P = 0.98), while there has been a significant decline in the frequency of the Est 3A gene from 0.70 to 0.36. The reason for these changes in gene frequency are considered.
Organophosphorus (OP) resistance in a population of Culex quinquefasciatus from California is associated with elevated levels of an esterase (Est-2). Exposure of OP-resistant larvae to the synergists IBP (S-benzyl O,O-diisopropyl phosphorothioate), DEF (S,S,S-tributyl phosphorotrithioate), and TPP (triphenyl phosphate) decreased the esterase activity to α- and β-naphthyl acetate to or below the level found in the unsynergized susceptible strain. The OP-resistant strain was crossed to a homozygous susceptible strain and the F1 progeny were backcrossed to the susceptible parent. The progeny from this backcross were selected for insecticide resistance. This was repeated for seven generations of backcrosses with insecticide selection at the LC50 level, so that the genetic background of the resistant (R) strain became essentially equivalent to that of the S. The seventh selected generation was allowed to breed to establish a colony of known R gene frequency that was checked by esterase assays. Selection of this population over three generations with temephos, malathion, or IBP + temephos at the 80% mortality level increased the R gene frequency. But selection with IBP + malathion decreased the R gene frequency at a rate similar to that of the unselected population. Selection of homozygous-resistant individuals with IBP + temephos and IBP + malathion increased the levels of resistance to the selecting agents slightly (2- to 5-fold). This is associated with an increase in tolerance to IBP of 1.7-fold. If the selective disadvantage associated with the R gene under IBP + malathion selection pressure is also manifest in field populations then IBP + malathion mixtures may be of practical value.
Despite decades of research, the question of whether solutions and melts of highly entangled polymers exhibit shear banding as their steady state response to a steadily imposed shear flow remains controversial. From a theoretical viewpoint, an important unanswered question is whether the underlying constitutive curve of shear stress σ as a function of shear rate γ[over ˙] (for states of homogeneous shear) is monotonic, or has a region of negative slope, dσ/dγ[over ˙]<0, which would trigger banding. Attempts to settle the question experimentally via velocimetry of the flow field inside the fluid are often confounded by an instability of the free surface where the sample meets the outside air, known as "edge fracture." Here we show by numerical simulation that in fact even only very modest edge disturbances-which are the precursor of full edge fracture but might well, in themselves, go unnoticed experimentally-can cause strong secondary flows in the form of shear bands that invade deep into the fluid bulk. Crucially, this is true even when the underlying constitutive curve is monotonically increasing, precluding true bulk shear banding in the absence of edge effects.
Isostatic equilibrium is commonly defined as the state achieved when there are no lateral gradients in hydrostatic pressure, and thus no lateral flow, at depth within the lower viscosity mantle that underlies a planetary body's outer crust. In a constant-gravity Cartesian framework, this definition is equivalent to the requirement that columns of equal width contain equal masses. Here we show, however, that this equivalence breaks down when the spherical geometry of the problem is taken into account. Imposing the "equal masses" requirement in a spherical geometry, as is commonly done in the literature, leads to significant lateral pressure gradients along internal equipotential surfaces and thus corresponds to a state of disequilibrium. Compared with the "equal pressures" model we present here, the equal masses model always overestimates the compensation depth-by similar to 27% in the case of the lunar highlands and by nearly a factor of 2 in the case of Enceladus. Plain Language Summary "Isostasy" is the principle that, just as an iceberg floats on the water, crustal rocks effectively float on the underlying higher density mantle, which behaves essentially like a fluid on geologic timescales. Although there are subtle inconsistencies among the various ways isostasy can be defined, they have not been historically problematic for bodies like the Earth, where the crust is thin compared with the overall radius. When the thickness of the crust is a nonnegligible fraction of a planetary body's radius, however, it becomes important to take the spherical geometry into account. In this case, the inconsistencies in the definitions can lead to significant discrepancies. Here we argue that one of the most commonly used approaches, which requires equal width columns to contain equal masses, always results in overestimating the crustal thickness. In particular, we suggest that the lunar and Martian highlands crustal thickness may have been overestimated by similar to 27% and similar to 10%, respectively, and that the ice shell thickness for Saturn's small icy moon Enceladus may have been overestimated by nearly a factor of 2.
Its extraordinary level of geologic activity, its potential for habitability, and the prospects of returning samples from its plume of erupting water ice make Saturn's small (∼500 km diameter) moon Enceladus a high priority target for future exploration and a key to our developing understanding of icy ocean worlds. The structure of its outer ice shell is particularly important as it relates to the global heat budget, the global-scale response to tidal forces, and the nature of the ongoing eruptions. It is also diagnostic of how and where heat is dissipated internally. Here, using the most recent shape model and a new approach to modeling isostasy, we obtain a shell structure that simultaneously accommodates the shape, gravity, and libration observations and suggests that tidal dissipation near the base of the ice shell is likely an important mode of internal heating. The implied conductive heat loss is greater than the heat loss associated with the eruptions but is nevertheless compatible with the condition of steady state.
Abstract In Culex mosquitoes (Diptera: Culicidae), the most common mechanism for resistance to organophosphorus (OP) insecticides involves amplification of one or more esterases. Two esterase loci are often involved, with different allelic forms co‐amplified. Estα31 is co‐amplified with estβ1 in a Colombian (COL) strain of Culex quinquefasciatus Say. These two alleles co‐migrate on acrylamide gels, often leading to misscoring of the phenotype as elevation of a single estβ enzyme. By sequencing COL genomic DNA, we determined the estα31 gene length is 1623 nucleotides. The open reading frame of estα31 encodes a 540 amino acid protein, as for estα21 in strain Pel RR from Sri Lanka. The intron/exon boundaries of estα31 are identical to those of estα21, suggesting that they are alleles of the same locus. The COL estα31 gene differs from estα32 in strain MRES from Cuba, although they have equivalent electrophoretic mobility, showing that these two strains contain distinct resistance‐associated amplicons. Twenty nucleotide differences were scored between the MRES partial 495 bp sequence and that in the COL strain, with two amino acid changes, demonstrating distinct estα enzymes. Our sequencing data show 95% identity between the three estα genes (each has six introns and seven exons) in OP‐resistant Cx. quinquefasciatus. Amplified estα31 and estβ1 are at least 10 kb apart in temephos‐selected COL and 2.7 kb apart in Pel RR, whereas these non‐amplified genes are only 1.7 kb apart in the non‐selected parental COL stock, as in Pel SS (susceptible Sri Lankan strain), demonstrating that this region of the genome is susceptible to expansion and contraction.
Biochemical comparisons were made between esterases of elevated and nonelevated activity from an organophosphate resistant strain of Italian Culex pipiens. Isoelectric focusing demonstrated that nonelevated A4 and elevated A2 and B2 esterases had similar isoelectric points (pI) ranging from 5.6 to 5.9. Elevated A1 had a slightly higher pI of 6.1–6.5. A1, A2, and A4 esterases had molecular weights of approximately 110 kDa, whereas B2 had a MW of 65 kDa. Soluble A esterases were partially purified for kinetic studies, which revealed that there was no qualitative difference in their ability to hydrolyse the substrate α-naphthyl acetate. This similarity at the active site suggests that there may be no significant differences between alleles to make one more effective at hydrolysing or sequenstering insecticides. Quantitative differences in A esterase activity between strains were evident, supporting the hypothesis that A esterases are overproduced in insecticide resistant strains.
An Italian organophosphate-resistant strain ofCulex pipiens (Lucca) was found to be polymorphic for elevated and nonelevated esterases. Selection for high esterase activity produced a strain homozygous for elevated esterases A2 and B2. Selection for low activity produced a strain homozygous for nonelevated esterases, A4i and B1i. Crossing experiments showed that A2 and B2 are coded by separate but closely linked genes, as are A4i and B1i. Results indicate that elevated A2 and nonelevated A4i are alleles of a single gene (Est-3 locus), as are elevated B2 and nonelevated B1i (Est-2 locus). Selection for electromorph variants gave four elevated A variants and three elevated B variants. These esterases were not selected in the field. In Lucca, A2 and B2 replaced A1, suggesting a selective advantage to the former over the latter in the presence of chlorpyrifos. It is hypothesized that the degree of amplification is an important factor in the selection of a particular esterase electromorph as a resistance mechanism and that migrating individuals with amplified genes could have an advantage when invading a population under selection.
Ancient shorelines on Mars must have formed before and during the emplacement of the Tharsis volcanic province, instead of afterwards as previously assumed, suggesting that oceans on Mars formed early. The observation of potential palaeo-shorelines around the northern plains of Mars points to the existence of an ancient Martian ocean. However, these features do not seem to follow an equipotential surface, which would argue against an oceanic origin. The varying elevation of these shorelines could be explained by true polar wander occurring after the formation of Tharsis, a volcanic province that dominates the gravity and topography of Mars. However, this would require Tharsis to have formed far from the equator, which is thought to be unlikely. Robert Citron and co-authors show that variations in shoreline topography can instead be explained if the shorelines formed before and during the emplacement of Tharsis, implying that oceans formed early on Mars, potentially in relation to the initiation and decline of Tharsis volcanism. Widespread evidence points to the existence of an ancient Martian ocean1,2,3,4,5,6,7,8. Most compelling are the putative ancient shorelines in the northern plains2,7. However, these shorelines fail to follow an equipotential surface, and this has been used to challenge the notion that they formed via an early ocean9 and hence to question the existence of such an ocean. The shorelines’ deviation from a constant elevation can be explained by true polar wander occurring after the formation of Tharsis10, a volcanic province that dominates the gravity and topography of Mars. However, surface loading from the oceans can drive polar wander only if Tharsis formed far from the equator10, and most evidence indicates that Tharsis formed near the equator11,12,13,14,15, meaning that there is no current explanation for the shorelines’ deviation from an equipotential that is consistent with our geophysical understanding of Mars. Here we show that variations in shoreline topography can be explained by deformation caused by the emplacement of Tharsis. We find that the shorelines must have formed before and during the emplacement of Tharsis, instead of afterwards, as previously assumed. Our results imply that oceans on Mars formed early, concurrent with the valley networks15, and point to a close relationship between the evolution of oceans on Mars and the initiation and decline of Tharsis volcanism, with broad implications for the geology, hydrological cycle and climate of early Mars.
Fenitrothion metabolism studies on a multiple organophosphorus resistant strain of Anopheles subpictus from Sri Lanka indicated that oxidases and glutathione S-transferases play a major role in the detoxication of this insecticide. The oxidases, as well as producing detoxication products, also produce two highly active acetylcholinesterase inhibitors, fenitrooxon and 3-hydroxymethyl fenitrooxon. Further metabolism studies on fenitrooxon indicated that the glutathione S-transferases in A. subpictus were able to detoxify these activation products. DDT resistance in the field population of A. subpictus has increased since 1983, although there has been no direct DDT selection pressure during this period. DDT resistance in this species is due to an increase in DDT dehydrochlorination which is correlated with an increase in glutathione S-transferase activity. It is possible that the same enzyme is responsible for both DDT and secondary organophosphate (fenitrothion) metabolism. If so, organophosphorus insecticide selection pressure will actively maintain, or select for, DDT resistance in this population. Linkage disequilibrium between the oxidase and glutathione S-transferase-based resistance genes would be expected if the same glutathione S-transferase fulfils both functions, and this is now being investigated in the field population in Sri Lanka.
The esterase enzymes Est A′ and Est B are both quantitatively increased in organophosphorus-resistant Culex quinquefasciatus from Monrovia. An enzyme fraction containing Est B and traces of another unidentified esterase, but no Est A′, was produced by centrifugation and size exclusion chromatography. SDS-electrophoresis revealed five protein bands in this fraction. Antibodies raised in Balb-C mice to this fraction gave high levels of cross reactivity, in a peroxidase-linked ELISA test, to both organophosphorus-resistant and susceptible C. quinquefasciatus in fractions containing either Est B or Est A′. Only low levels of reactivity were obtained to a number of organophosphorus-resistant and susceptible Anopheles strains. This suggests that the family of esterases quantitatively changed in Culex are not present in the anophelines tested.
Data from recreationally caught adult paddlefish from North Dakota and Oklahoma, USA, was used in an initial attempt to explain variation in lipid content in adult wild paddlefish Polyodon spathula (Walbaum, 1792) by stock and by sex. The objectives of this study were to describe the relative importance of three distinct paddlefish tissues [white muscle, red muscle, and gonadal fat body (GFB)] for lipid storage, and compare lipid content of these tissues between stock and sex. It was hypothesized that lipid content of the three tissue types would be related to each other within individual fish; and that, consistent with previously documented life history differences, lipid concentrations for all tissues would be sex‐ and stock‐specific. Paddlefish were sampled in April and May 2012 from the Grand Lake, Oklahoma, USA (GL) and Yellowstone‐Sakakawea, North Dakota (SAK) stocks. Three tissue types were collected from SAK fish, including white muscle (N = 137; 83M, 54F), red muscle (N = 74; 36M, 38F), and GFB (N = 164; 105M, 59F). Samples from GL fish included white muscle (N = 60; 36M, 24F), and GFB (N = 42; 41M, 1F). Proximate analysis was used to quantify the percentage of lipid, water, ash, and protein in tissue samples. The lipid component of proximate analysis was completed using the ANKOM method of lipid extraction which uses heated petroleum ether to extract lipid. Also investigated were lipid accumulation and storage in adult paddlefish in relation to tissue type: white muscle, red muscle, gonadal fat bodies (GFB) within individual fish, between the sexes, and between the two different stocks. White muscle was significantly correlated with both red muscle and GFB lipid, but red muscle lipid was not significantly correlated with GFB lipid. White muscle lipid content differed by stock and sex, with the more northerly stock having higher lipid content than the more southerly stock. Females had higher white muscle lipid content (SAK F = 17.18, M = 9.09; GL F = 6.74, M = 4.99) and red muscle (SAK F = 47.72, M = 30.93) than males. In contrast, GFB lipid differed by sex but not stock, with females having lower lipid content (SAK = 79.79, GL = 89.08) than males (SAK = 89.08; GL = 91.72). Life history differences, growing season, and the role of metabolism may help explain the differences in concentrations of lipids observed between stocks and between the sexes for all three tissues. It was hypothesized that different tissues may be used for different metabolic processes and that although metabolism likely strongly influences muscle tissue lipid, GFB lipid is probably linked more closely to reproductive demands associated with gonad development.
Active eruptions from the south polar region of Saturn’s ~500-km-diameter moon Enceladus are concentrated along a series of lineaments known as the ‘tiger stripes’1,2, thought to be partially open fissures that connect to the liquid water ocean beneath the ice shell3,4. To date, no study simultaneously explains why the tiger stripes should be located only at the south pole, why there are multiple approximately parallel and regularly spaced fractures, what accounts for their spacing of about 35 km, and why similarly active fissures have not been observed on other icy bodies. Here we propose that secular cooling, which leads to a thickening of the ice shell and building of global tensile stresses5,6, causes the first fracture to form at one of the poles, where the ice shell is thinnest owing to tidal heating7. The tensile stresses are thereby relieved, preventing a similar failure at the opposite pole. The steadily erupting water ice loads the flanks of the open fissure, causing bending in the surrounding elastic plate and further tensile failure in bands parallel to the first fracture—a process that may be unique to Enceladus, where the gravity is too weak for compressive stresses to prevent fracture propagation through the thin ice shell. The sequence of fissures then cascades outwards until the loading becomes too weak or the background shell thickness becomes too great to permit through-going fractures. Enceladus’s tiger stripes at the south pole formed in cascade and spaced equally after the first fracture—probably Baghdad Sulcus—was created by the release of accumulated tensile stress, caused in turn by secular cooling.
Abstract. Broad spectrum organophosphate resistance in Culex quinquefasciatus Say from Saudi Arabia is inherited as a semi‐dominant characteristic. The resistance has a metabolic basis and confers cross‐resistance against the carbamate propoxur. Organophosphate‐selected strains contain two elevated esterases with the same electrophoretic mobilities as those in resistant Cx quinquefasciatus from Sri Lanka and a range of African locations. Alteration in the sensitivity of acetylcholinesterase to insecticide inhibition does not play a major role in resistance.
The objective of this study was to investigate lipid accumulation and storage in age‐0 and age‐1 paddlefish Polyodon spathula (Walbaum, 1792) in relation to age, stock, year, and growth. Juvenile paddlefish were collected from three locations in North Dakota and Montana, USA, during July and August of 2011 and 2012 and proximate analysis was used to determine lipid content. RNA/DNA ratios were used as an index of growth rates. Differences in age‐based lipid accumulation and storage in juvenile paddlefish suggest a split allocation between growth and lipid storage, with growth being the highest initial priority and emphasis on energy storage occurring at a larger size, later in life. Differences in lipid allocation between stocks indicate that allocation is influenced by hatchery/wild rearing conditions. Differences within and between year‐classes are consistent with field evidence observed in 2012 of a strong 2011 year‐class, and indicate that during productive times, paddlefish may allocate energy to both body growth and lipid reserves, and that allocation differs among years. The lack of a relationship between RNA/DNA ratio and lipid does not support a physiologically exclusive allocation strategy between growth and lipid. Evidence from this and other studies suggests rather that an emphasis on growth, some energy storage, and a large rostrum size in relation to overall fish length in age‐0 and age‐1 fish, may be adaptive in avoiding predation while accruing necessary energy reserves for overwintering. Although this study also provides reference information regarding proximate composition of wild and hatchery origin juvenile paddlefish, much more study is needed into the relationships among growth, low and high lipid groups, lipid allocation in juvenile paddlefish as well as the existence and timing of allocation changes between growth and storage. To aid in understanding paddlefish survival and year‐class strengths, these relationships also need to be linked to inter‐annual differences in early rearing environments for age‐0 and age‐1 fish.
The chloride shift of respiration which occurs between plasma and erythrocytes when CO2 enters or leaves the blood has been studied with the Hartridge-Roughton rapid reaction method adapted for filtration. It has been found that after mixing erythrocytes with glucose-Ringer-Locke solution exposed to one atmosphere of CO2 the chloride in the extra-cellular fluid decreased irregularly due to the chloride shift. In some instances a transitory elevation of chloride (reversal of shift) occurs indicating a second reaction superimposed on the shift. An estimate of the half-time of the shift is 138 msec. Acetazolamide reduces these reactions. The membrane ionic exchange of chloride with bicarbonate ions has a rapid and slowly diffusing phase.