Fitness cost due to herbicide resistance may trigger genetic background evolution

This article investigates the possible existence of mechanisms counterbalancing the negative pleiotropic effects on development and reproduction that are conferred by alleles responsible for herbicide resistance in the weed Alopecurus myosuroides. We considered three herbicide-resistant, mutant acetyl-coenzyme A carboxylase (ACCase) alleles, Leu1781, Asn2041, and Gly2078, found in eight resistant populations. Of these, Gly2078 is the only allele with a known fitness cost. We compared plants homozygous for wild-type ACCase alleles that were siblings of plants carrying a given mutant resistant ACCase allele with plants from three populations where resistance did not evolve. In each of two series of experiments, we measured germination dynamics, seedling vigor, plant height, vegetative biomass, and seed production. The wild-type siblings of plants carrying Gly2078 performed better in the field, on average, than wild-type plants that were sibling of plants carrying other mutant ACCase alleles, and particularly those carrying Leu1781. We propose that rapid evolution of the genetic background of plants from the populations where the Gly2078 allele originally arose could partially counterbalance Gly2078 fitness cost, enhancing the spread of the resistant genotypes.     

Mate choice, frequency dependence, and the maintenance of resistance to parasitism in a simultaneous hermaphrodite

Biomphalaria glabrata are simultaneous hermaphroditic freshwatersnails that act as intermediate hosts for the macroparasitictrematode Schistosoma mansoni, a causative agent of schistosomiasis.Heritability and strain-specificity of both snail resistanceand susceptibility to schistosome infection have been demonstrated,genetic variability for which is maintained, in part, throughtrade-offs between high fitness costs associated with infectionand those associated with resistance. However, despite sucha high cost of resistance and a low prevalence of infectionin natural snail populations, genes for resistance are maintainedwithin snail populations over successive generations, includingin the complete absence of parasite pressure in laboratory populations.This may be indicative of alternative benefits of resistancegenes, in addition to parasite defense, such as differentialmating success between genotypes. Here we examined the mateand gender choice of snails across a multi-factorial range ofpotential partner combinations. These included host-resistanceor susceptibility genotype, host genotype frequency within thepopulation, current parasite infection status, and parasitegenotype. We demonstrate recognition and discrimination by hostsnails depending on host and/or parasite genotype for each ofthese factors. In particular, our results suggest that a raremating advantage to resistant genotypes may be a potential explanationfor the maintenance of highly costly resistance genes withinintermediate host populations under conditions of low or zeroparasite pressure.     

Impact of the age of Biomphalaria alexandrina snails on Schistosoma mansoni transmission: modulation of the genetic outcome and the internal defence system of the snail

Of the approximately 34 identified Biomphalaria species,Biomphalaria alexandrina represents the intermediate host of Schistosoma mansoni in Egypt. Using parasitological and SOD1 enzyme assay, this study aimed to elucidate the impact of the age of B. alexandrina snails on their genetic variability and internal defence against S. mansoni infection. Susceptible and resistant snails were reared individually for self-reproduction; four subgroups of their progeny were used in experiment. The young susceptible subgroup showed the highest infection rate, the shortest pre-patent period, the highest total cercarial production, the highest mortality rate and the lowest SOD1 activity. Among the young and adult susceptible subgroups, 8% and 26% were found to be resistant, indicating the inheritance of resistance alleles from parents. The adult resistant subgroup, however, contained only resistant snails and showed the highest enzyme activity. The complex interaction between snail age, genetic background and internal defence resulted in great variability in compatibility patterns, with the highest significant difference between young susceptible and adult resistant snails. The results demonstrate that resistance alleles function to a greater degree in adults, with higher SOD1 activity and provide potential implications for Biomphalaria control. The identification of the most susceptible snail age enables determination of the best timing for applying molluscicides. Moreover, adult resistant snails could be beneficial in biological snail control.     

Biomphalaria tenagophila: dynamics of populations of resistant and susceptible strains to Schistosoma mansoni, with or without pressure of the parasite

Resistant (Taim, RS) and susceptible albino (Joinville, SC) Biomphalaria tenagophila populations were kept together, at different proportions, throughout a 18-month-period. Some of the snail groups were submitted to Schistosoma mansoni infection. The targets of this study were (a) to analyze the populational dynamics among resistant and susceptible individuals to S. mansoni; (b) to study the resistance phenotype in descendants of cross-breeding; (c) to observe whether the parasite could exert any kind of selection in those snail populations. Throughout the experiment it could be observed that the susceptible B. tenagophila strain (Joinville) underwent a selective pressure of the parasite that was negative, since the individuals showed a high mortality rate. Although B. tenagophila (Taim) population presented a higher mortality rate without pressure of the parasite, this event was compensated by a reproductive capacity. B. tenagophila Taim was more fecund than B. tenagophila Joinville and was able to transmit the resistance character to their descendants. F1 generation obtained by cross-breeding between resistant and susceptible lineages was completely resistant to S. mansoni infection, irrespective of the Taim proportion. Moreover, less than 5% of F2 progeny were susceptible to S. mansoni infection.     

Allele-specific effects of ecSOD on asbestos-induced fibroproliferative lung disease in mice

Previous work by others suggests that there is a strain-dependent variation in the susceptibility to inflammatory lung injury in mice. Specifically, the 129/J mice appear to be more resistant to asbestos-induced pulmonary fibrosis than the C57BL/6 strain. A separate line of evidence suggests that extracellular superoxide dismutase (ecSOD) may play an important role in protecting the lung from such injuries. We have recently reported that the 129/J strain of mice has an ecSOD genotype and phenotype distinctly different from those of the C57BL/6 mice. In order to identify ecSOD as a potential "asbestos-injury resistance" gene, we bred congenic mice, on the C57BL/6 background, carrying the wild type (sod3wt) or the 129/J (sod3129) allele for ecSOD. This allowed us to examine the role of ecSOD polymorphism in susceptibility to lung injury in an otherwise identical genetic background. Interestingly, asbestos treatment induces a significant (~40%) increase in plasma ecSOD activity in the sod3129 mice, but not in the sod3wt mice. Asbestos administration results in a loss of ecSOD activity and protein from lung tissue of both congenic strains, but the lung ecSOD activity remains significantly higher in sod3129 mice. As expected, asbestos treatment results in a significant recovery of ecSOD protein in bronchoalveolar lavage fluid (BALF). The BALF of sod3129 mice also have significantly lower levels of proteins and inflammatory cells, especially neutrophils, accompanied by a significantly lower extent of lung injury, as measured by a pathology index score or hydroxyproline content. Immunohistochemistry reveals a significant loss of ecSOD from the tips of the respiratory epithelial cells in response to asbestos treatment and that the loss of immunodetectable ecSOD is compensated for by enzyme expression by infiltrating cells, especially in the sod3wt mice. Our studies thus identify ecSOD as an important anti-inflammatory gene, responsible for most, if not all of the resistance to asbestos-induced lung injury reported for the 129/J strain of mice. The data further suggest allele-specific differences in the regulation of ecSOD expression. These congenic mice therefore represent a very useful model to study the role of this enzyme in all inflammatory diseases. Polymorphisms in human ecSOD have also been reported and it appears logical to assume that such variations may have a profound effect on disease susceptibility.     

Genotyping natural infections of Schistosoma mansoni in Biomphalaria alexandrina from Damietta, Egypt, with comparisons to natural snail infections from Kenya

The distribution of Schistosoma genotypes among individuals in snail populations provides insights regarding the dynamics of transmission and compatibility between schistosome and snail hosts. A survey of Biomphalaria alexandrina from Damietta (Nile Delta, Egypt), an area subjected to persistent schistosomiasis control efforts, provided only 17 snails infected with Schistosoma mansoni (6.1% overall prevalence), each shown by microsatellite analysis to have a single genotype infection. By contrast, recent studies of uncontrolled S. mansoni transmission foci in Kenya revealed that 4.3% Biomphalaria pfeifferi and 20-25% Biomphalaria sudanica snails had multiple genotype infections. Compared with the 3 Kenyan populations, the Egyptian population of S. mansoni also showed a lesser degree of genetic variability and was genetically differentiated from them. We suggest that tracking of genotype diversity in infected snails could be further developed to serve as an additional and valuable independent indicator of efficacy of schistosomiasis control in Egypt and elsewhere.     

Fitness consequences of malathion-specific resistance in red flour beetle (Coleoptera: Tenebrionidae) and selection for resistance in the absence of malathion

Malathion resistance in the red flour beetle, Tribolium castaneum (Herbst), is a worldwide problem and is very stable once it becomes widespread in natural populations. In the absence of insecticide the proportion of resistant phenotypes may rapidly decline but the development of resistance does not always involve reduced fitness. Malathion-specific resistance in T. castaneum seems not to involve any loss of fitness in laboratory or field conditions. Susceptible beetles were in competition with resistant beetles at different initial frequencies and modifications of susceptible gene frequency were estimated in these laboratory populations over 10 generations. A significant decrease in susceptible gene frequency was observed in Tribolium populations over time. The selection coefficient of the susceptible allele was estimated and the fitness of susceptible alleles in all tests was observed to range from 0.89 to 0.93 compared with the fitness of resistant genotypes, which was assumed to be 1. Data provided evidence that the resistant strains exhibited fitness advantages in the absence of malathion. We also compared the biotic potential (fecundity and developmental time) of the susceptible strain, the homozygous malathion-specific resistant strain, and their hybrids. Malathion-specific resistant strains showed an 8 -23% increase in biotic potential relative to the susceptible strain. These findings are consistent with those of malathion-specific resistance in T. castaneum; the fitness of the insects seems independent of the genetic background and the fitness of the resistant insects is not affected by this resistance mechanism.     

Respiratory burst of Biomphalaria glabrata hemocytes: Schistosoma mansoni-resistant snails produce more extracellular H2O2 than susceptible snails

The production of reactive oxygen species by hemocytes from the gastropod Biomphalaria glabrata has been linked to their ability to kill the trematode parasite Schistosoma mansoni. For 2 laboratory strains of the snail, 1 resistant (13-16-R1) and 1 susceptible (MO) to the PR1 strain of S. mansoni, we compared hemocyte production of extracellular hydrogen peroxide when stimulated with the protein kinase C agonist phorbol myristate acetate (PMA). The time course of the PMA-induced response is similar in both strains with respect to onset, peak production, and termination of the respiratory burst. However, the magnitude of the response differs between strains, in that hemocytes from resistant snails generate significantly more hydrogen peroxide. These findings suggest that the capacity to produce hydrogen peroxide could be critical in determining susceptibility or resistance to S. mansoni.     

EVOLUTIONARY REVERSALS OF ANTIBIOTIC RESISTANCE IN EXPERIMENTAL POPULATIONS OF PSEUDOMONAS AERUGINOSA

Antibiotic resistance mutations are accompanied by a fitness cost, and two mechanisms allow bacteria to adapt to this cost once antibiotic use is halted. First, it is possible for resistance to revert; second, it is possible for bacteria to adapt to the cost of resistance by compensatory mutations. Unfortunately, reversion to antibiotic sensitivity is rare, but the underlying factors that prevent reversion remain obscure. Here, we directly study the evolutionary dynamics of reversion by experimentally mimicking reversion mutations—sensitives—in populations of rifampicin‐resistant Pseudomonas aeruginosa. We show that, in our populations, most sensitives are lost due to genetic drift when they are rare. However, clonal interference from lineages carrying compensatory mutations causes a dramatic increase in the time to fixation of sensitives that escape genetic drift, and mutations surpassing the sensitives’ fitness are capable of driving transiently common sensitive lineages to extinction. Crucially, we show that the constraints on reversion arising from clonal interference are determined by the potential for compensatory adaptation of the resistant population. Although the cost of resistance provides the incentive for reversion, our study demonstrates that both the cost of resistance and the intrinsic evolvability of resistant populations interact to determine the rate and likelihood of reversion.     

Genetic diversity and recruitment pattern of Schistosoma mansoni in a Biomphalaria glabrata snail population: a field study using random-amplified polymorphic DNA markers.

Random-amplified polymorphic DNA markers have been used to assess the amount and the distribution of the genetic diversity of Schistosoma mansoni within a natural population of Biomphalaria glabrata at a transmission site of the murine schistosomiasis focus of Guadeloupe. Despite high infection rate and heavy schistosome load within the definitive hosts (Ratus rattus), prevalences within intermediate snails ranged from 0.2 to 4.8%. Whatever the transmission season may be (rainy vs. dry), most of the infected snails were spatially aggregated and 88.4% of them harbored a single parasite genotype indicative of a monomiracidial infection; 4.7% had dual sex infections and a parasite intensity not exceeding 3 miracidia per snail. A substantial resistance level toward the parasite and recruitment regulatory process within snails may explain in part the observed low parasite prevalences and intensities. Considering such a distribution pattern of larval S. mansoni genetic diversity among B. glabrata, mobility of the definitive hosts, or rapid turnover of infected snails, or both, are required to maintain genetic heterogeneity within adult schistosome populations.     

Enhancing Fusarium crown rot resistance by pyramiding large-effect QTL in common wheat (<Emphasis Type="Italic">Triticum aestivum</Emphasis> L.)

Fusarium crown rot (FCR) has become one of the most damaging cereal diseases in semi-arid regions worldwide. Targeting three large-effect QTL (located on the chromosome arm 3BL, 5DS and 2DL, respectively), we investigated the feasibility of enhancing FCR resistance by gene pyramiding. Significant effects were detected for each of the three QTL in both populations assessed. Lines with any combination of two resistant alleles gave significantly better resistance than those with a single resistant allele only and those without any allele, and lines possessing all three resistant alleles showed the best resistance. These results demonstrated that gene pyramiding can be an effective approach in improving FCR resistance. Those lines with resistant alleles from all three QTL could be valuable genetic stocks for breeding programs.     

Genetics of Biomphalaria glabrata: Linkage analysis of genes for pigmentation,enzymes, and resistance to Schistosoma mansoni

The snail, Biomphalaria glabrata, is a major intermediate host of the human blood fluke, Schistosoma mansoni, in the Americas. The inheritance and linkage relationships of a gene enabling adult snails to resist infection by a Puerto Rican strain of the parasite were analyzed using two laboratory stocks that differed in susceptibility, pigmentation, and five electrophoretically detectable enzyme markers. Segregation ratios in second-generation intraspecific hybrids between susceptible (M-stock) and resistant (10-R2-stock) snails indicate that the susceptibility gene is not linked to the enzyme (ACON-1, ACP, EST-2, PEP-2, PGD) or pigmentation loci studied. These seven loci assort independently of one another. Observed rates of infection among F1 and F2 progeny are consistent with Richards' finding that adult susceptibility to the PR-1 strain of S. mansoni is controlled by a single locus with resistance dominant. No association between allozymes of acid phosphatase and snail susceptibility to PR-1 was seen in the snail-parasite combinations studied.     

Whole-body enantiomorphy and maternal inheritance of chiral reversal in the pond snail Lymnaea stagnalis

Sinistral and dextral snails have repeatedly evolved by left-right reversal of bilateral asymmetry as well as coiling direction. However, in most snail species, populations are fixed for either enantiomorph and laboratory breeding is difficult even if chiral variants are found. Thus, only few experimental models of chiral variation within species have been available to study the evolution of the primary asymmetry. We have established laboratory lines of enantiomorphs of the pond snail Lymnaea stagnalis starting from a wild population. Crossing experiments demonstrated that the primary asymmetry of L. stagnalis is determined by the maternal genotype at a single nuclear locus where the dextral allele is dominant to the sinistral allele. Field surveys revealed that the sinistral allele has persisted for at least 10 years, that is, about 10 generations. The frequency of the sinistral allele showed large fluctuations, reaching as frequent as 0.156 in estimate under the assumption of Hardy-Weinberg equilibrium. The frequency shifts suggest that selection against chiral reversal was not strong enough to counterbalance genetic drift in an ephemeral small pond. Because of the advantages as a model animal, enantiomorphs of L. stagnalis can be a unique system to study aspects of chirality in diverse biological disciplines.     

Long-term genetic stability and population dynamics of laboratory strains of Schistosoma mansoni

Measures of genetic differentiation between populations are useful tools for understanding the long-term dynamics of parasite communities. We followed the allele frequencies of microsatellite markers in samples taken over a period of 16 yr from the Case Western Reserve University-Naval Medical Research Institute (CWRU-NMRI) laboratory strain of Schistosoma mansoni. DNA was isolated from pooled samples of adults, eggs, or cercariae collected at 46 time points and genotyped for 14 tri- or tetranucleotide microsatellite markers. For comparison, 2 S. mansoni reference strains (Biomedical Research Institute-NMRI, which has a common origin with the CWRU line, and PR-1) were analyzed over shorter periods of time. We observed that the long-term allele frequencies are generally stable in large laboratory populations of this parasite, and a high degree of similarity was observed between the allele frequencies of consecutive samples from different developmental stages. The CWRU strain, however, showed 2 periods of marked deviation from stability as demonstrated using genetic differentiation measures. The first period corresponds to an admixture event with the BRI strain in which a new equilibrium was established as the "migrants" became blended into the existing CWRU population, consistent with 23% admixture from BRI. The second corresponds to a period of genetic drift when the CWRU population size was greatly reduced with an accompanying loss in genetic diversity. Having demonstrated the utility of pooled samples for the genetic analysis of population dynamics in laboratory strains of schistosomes, this approach will be useful for analyzing field samples to determine the impact of schistosomiasis control programs on parasite population structure. Accounting only for the intensity or prevalence of parasite populations may fail to register significant changes in population structure that could have implications for resistance, morbidity, and the design of control measures.     

Identification of a genetic marker associated with the resistance to Schistosoma mansoni infection using random amplified polymorphic DNA analysis

In schistosomiasis, the host/parasite interaction remains not completely understood. Many questions related to the susceptibility of snails to infection by respective trematode still remain unanswered. The control of schistosomiasis requires a good understanding of the host/parasite association. In this work, the susceptibility/resistance to Schistosoma mansoni infection within Biomphalaria alexandrina snails were studied starting one month post infection and continuing thereafter weekly up to 10 weeks after miracidia exposure. Genetic variations between susceptible and resistant strains to Schistosoma infection within B. alexandrina snails using random amplified polymorphic DNA analysis technique were also carried out. The results showed that 39.8% of the examined field snails were resistant, while 60.2% of these snails showed high infection rates.In the resistant genotype snails, OPA-02 primer produced a major low molecular weight marker 430 bp. Among the two snail strains there were interpopulational variations, while the individual specimens from the same snail strain, either susceptible or resistant, record semi-identical genetic bands. Also, the resistant character was ascendant in contrast to a decline in the susceptibility of snails from one generation to the next.     

Widespread pyrethroid resistance in Australian diamondback moth, Plutella xylostella (L.), is related to multiple mutations in the para sodium channel gene

Populations of Plutella xylostella, extending over 3800 km in southern Australia, show no genetic structure as assessed by microsatellite markers; yet outbreaks of pyrethroid resistance occur sporadically in cropping areas. Since mutations in the para voltage-gated sodium channel gene have been implicated in pyrethroid resistance, we looked for DNA sequence variation at this target among Australian moths. We found two resistance mutations previously reported for this species (L1014F and T929I), as well as a novel substitution (F1020S). Of the eight possible haplotypes formed by combinations of these three biallelic polymorphisms, only four were found in Australian populations: the wild-type allele (w), the kdr mutation allele (kdr) with only L1014F, the super-kdr-like combination of L1014F and T929I (skdrl), and the crashdown allele with only F1020S (cdr). Comparison of genotype frequencies among survivors of permethrin assays with those from untreated controls identified three resistant genotypes: skdrl homozygotes, cdr homozygotes and the corresponding heterozygote, cdr/skrdl - the heterozygote being at least as resistant as either homozygote. Spatial heterogeneity of allele frequencies was conspicuous, both across the continent and among local collections, consistent with reported spatial heterogeneity of pyrethroid resistance. Further, high resistance samples were sometimes associated with high frequency of cdr, sometimes high frequency of skdrl, or sometimes with a high combined cdr+skdrl frequency. The skdrl and cdr alleles explain a high proportion of the Australia-wide resistance variation. These data add to evidence that nerve insensitivity by mutations in the para-sodium channel gene is a common pyrethroid resistance mechanism in P. xylostella.     

Self‐fertilization,long‐distance flash invasion and biogeography shape the population structure of Pseudosuccinea columella at the worldwide scale

Population genetic studies are efficient for inferring the invasion history based on a comparison of native and invasive populations, especially when conducted at species scale. An expected outcome in invasive populations is variability loss, and this is especially true in self‐fertilizing species. We here focus on the self‐fertilizing Pseudosuccinea columella, an invasive hermaphroditic freshwater snail that has greatly expanded its geographic distribution and that acts as intermediate host of Fasciola hepatica, the causative agent of human and veterinary fasciolosis. We evaluated the distribution of genetic diversity at the largest geographic scale analysed to date in this species by surveying 80 populations collected during 16 years from 14 countries, using eight nuclear microsatellites and two mitochondrial genes. As expected, populations from North America, the putative origin area, were strongly structured by selfing and history and harboured much more genetic variability than invasive populations. We found high selfing rates (when it was possible to infer it), none‐to‐low genetic variability and strong population structure in most invasive populations. Strikingly, we found a unique genotype/haplotype in populations from eight invaded regions sampled all over the world. Moreover, snail populations resistant to infection by the parasite are genetically distinct from susceptible populations. Our results are compatible with repeated introductions in South America and flash worldwide invasion by this unique genotype/haplotype. Our study illustrates the population genetic consequences of biological invasion in a highly selfing species at very large geographic scale. We discuss how such a large‐scale flash invasion may affect the spread of fasciolosis.     

Analysis of wild-derived mice for Fv-1 and Fv-2 murine leukemia virus restriction loci: a novel wild mouse Fv-1 allele responsible for lack of host range restriction.

Wild-derived mice originally obtained from Asia, Africa, North America, and Europe were typed for in vitro sensitivity to ecotropic murine leukemia viruses and for susceptibility to Friend virus-induced disease. Cell cultures established from some wild mouse populations were generally less sensitive to exogenous virus than were cell cultures from laboratory mice. Wild mice also differed from inbred strains in their in vitro sensitivity to the host range subgroups defined by restriction at the Fv-1 locus. None of the wild mice showed the Fv-1n or Fv-1b restriction patterns characteristic of most inbred strains, several mice resembled the few inbred strains carrying Fv-1nr, and most differed from laboratory mice in that they did not restrict either N- or B-tropic murine leukemia viruses. Analysis of genetic crosses of Mus spretus and Mus musculus praetextus demonstrated that the nonrestrictive phenotype is controlled by a novel allele at the Fv-1 locus, designated Fv-10. The wild mice were also tested for sensitivity to Friend virus complex-induced erythroblastosis to type for Fv-2. Only M. spretus was resistant to virus-induced splenomegaly and did not restrict replication of Friend virus helper murine leukemia virus. Genetic studies confirmed that this mouse carries the resistance allele at Fv-2.     

Isolations of Cry2Ab resistance in Australian populations of Helicoverpa armigera (Lepidoptera: Noctuidae) are allelic

Alleles conferring resistance to Cry2Ab toxin occur at a frequency of 0.0033 in Australian populations of Helicoverpa armigera (Hübner) (Lepidoptera: Noctuidae), and it is evident that detectable levels of resistance predated the introduction of transgenic cotton expressing this toxin. From 2002 until 2006, 10 such resistant alleles were scored. Here, we examine colonies established from five of the 10 isolates by using complementation tests to determine their genetic relationships. The results demonstrate that the resistance in each colony is due to alleles at the same locus and that for each allele the resistance is recessive. This latter finding is in conflict with the frequency of apparently resistant individuals occurring in the initial F2 tests that were used to identify alleles that confer resistance. These frequencies were variable (range 6.7-35.6%, mean 16.2%), but they generally indicated a measure of dominance (i.e., were >6.25% expected for recessive resistance). We hypothesize that this conflict is the result of differences in the genetic background of the laboratory adapted resistant colonies and the initial field isolations.