Diazinon resistance, fluctuating asymmetry and fitness in the Australian sheep blowfly, lucilia cuprina

Genetic evidence suggests that the evolution of resistance to the insecticide diazinon in Lucilia cuprina initially produced an increase in asymmetry. At that time resistant flies were presumed to be at a selective disadvantage in the absence of diazinon. Subsequent evolution in natural populations selected modifiers to ameliorate these effects. The fitness and fluctuating asymmetry levels of resistant flies are currently similar to those of susceptibles. Previous genetic analyses have shown the fitness modifier to co-segregate with the region of chromosome III marked by the white eyes, w, locus, unlinked to the diazinon resistance locus, Rop-1, on chromosome IV. This study maps the asymmetry modifier to the same region, shows, as in the case of the fitness modifier, its effect to be dominant and presents data consistent with the fitness/asymmetry modifier being the same gene (gene complex). These results suggest changes in fluctuating asymmetry reflect changes in fitness.     

Polygenic and Single Gene Responses to Selection for Resistance to Diazinon in Lucilia Cuprina

Following mutagenesis with ethyl methanesulfonate, selection in a susceptible strain with a concentration of the insecticide diazinon (0.0004%, w/v) above that required to kill 100% of the susceptible strain, the LC100 of that strain, resulted in a single gene response. The resultant four mutant resistant strains have equivalent physiological, genetical and biochemical profiles to a diazinon-resistant strain derived from a natural population and homozygous for the Rop-1 allele. Modification of the microsomal esterase E3 is responsible for resistance in each case. The Rop-1 locus maps approximately 4.4 map units proximal to bu on chromosome IV. Selection within the susceptible distribution, at a concentration of diazinon [0.0001% (w/v)] less than the LC100, resulted in a similar phenotypic response irrespective of whether the base population had been mutagenized. The responses were polygenically based, unique to each selection line and independent of Rop-1. The relevance of the results to selection for insecticide resistance in laboratory and natural populations is discussed.     

Modification of developmental instability and fitness: Malathion-resistance in the Australian sheep blowfly,Lucilia cuprina

The evolution of resistance to malathion byLucilia cuprina initially results in an increase in fluctuating asymmetry. Resistant flies are at a selective disadvantage, relative to susceptibles, in the absence of the insecticide. A fitness/asymmetry modifier of diazinon-resistant phenotypes ameliorates these effects resulting in malathion-resistant phenotypes of relative fitness and asymmetry similar to susceptibles. For the nine genotypic combinations of the modifier and malathion-resistance alleles, developmental time increases linearly with increasing asymmetry. Percentage egg hatch decreases linearly with increasing asymmetry. The initially disruptive effect of the malathion-resistant allele was partially dominant, the effect of the modifier dominant. The results are discussed in terms of developmental perturbation, asymmetry estimation and relative fitness to consider whether it is adequate to use changes in fluctuating asymmetry alone as measures of developmental instability. It is suggested that in some circumstances antisymmetry may indicate developmental instability and that the diazinon/malathion-resistance systems inL. cuprina may allow the relative importance of genetical and/or environmental developmental perturbations to be ascertained.     

The nematode-resistance gene,<Emphasis Type="Italic"> Mi-1</Emphasis>, is associated with an inverted chromosomal segment in susceptible compared to resistant tomato

The gene Mi-1 confers effective resistance in tomato (Lycopersicon esculentum) against root-knot nematodes and some isolates of potato aphid. This locus was introgressed from L. peruvianum into the corresponding region on chromosome 6 in tomato. In nematode-resistant tomato, Mi-1 and six homologs are grouped into two clusters separated by 300 kb. Analysis of BAC clones revealed that the Mi-1 locus from susceptible tomato carried the same number and distribution of Mi-1 homologs, as did the resistant locus. Molecular markers flanking the resistant and susceptible loci were in the same relative orientation, but markers between the two clusters were in an inverse orientation. The simplest explanation for these observations is that there is an inversion between the two clusters of homologs when comparing the Mi-1 loci from L. esculentum and L. peruvianum. Such an inversion may explain previous observations of severe recombination suppression in the region. Two Mi-1 homologs identified from the BAC library derived from susceptible tomato are not linked to the chromosome 6 locus, but map to chromosome 5 in regions known to contain resistance gene loci in other solanaceous species.Communicated by J.S. Heslop-Harrison     

Isolation and characterization of rice mutants compromised in<Emphasis Type="Italic"> Xa21</Emphasis>-mediated resistance to<Emphasis Type="Italic"> X. oryzae</Emphasis> pv.<Emphasis Type="Italic"> oryzae</Emphasis>

The rice gene, Xa21, confers resistance to diverse races of Xanthomonas oryzae pv. oryzae (Xoo) and encodes a receptor-like kinase with leucine-rich repeats in the extra-cellular domain. To identify genes essential for the function of the Xa21 gene, 4,500 IRBB21 (Xa21 isogenic line in IR24 background) mutants, induced by diepoxybutane and fast neutrons, were screened against Philippine race six (PR6) Xoo for a change from resistance to susceptibility. From two greenhouse screens, 23 mutants were identified that had changed from resistant to fully (6) or partially (17) susceptible to PR6. All fully susceptible mutants carried rearrangements at the Xa21 locus as detected by PCR and Southern hybridization. For the partially susceptible mutants, no changes were detected at the Xa21 locus based on Southern and PCR analyses. However, two of these mutants were confirmed via genetic analysis to have mutations at the Xa21 locus. Partially susceptible mutants exhibited variation in level of susceptibility to different Xoo strains, suggesting that they may carry different mutations required for the Xa21-mediated resistance. The mutants identified in this study provide useful materials for dissecting the Xa21-mediated resistance pathway in rice.Communicated by D.J. Mackill     

Environmental and Genetic Effects on the Asymmetry Phenotype: Diazinon Resistance in the Australian Sheep Blowfly, Lucilia Cuprina

The asymmetry phenotype of diazinon-resistant flies lacking a fitness/asymmetry Modifier (+/+; R/-) was dominant and independent of developmental temperature, larval density and diazinon concentration. Asymmetry score, pooled over three bristle characters, was ~50% greater for these phenotypes than for those of modified genotypes (M/-; -/-) and unmodified susceptibles (+/+; S/S) reared under standard laboratory conditions. Modified and susceptible phenotypes showed increased asymmetry score for temperatures and larval densities above and below standard rearing conditions; a positive correlation was observed between diazinon concentration and asymmetry score. Single and multiple environmental stresses resulted in similar scores that approached, but never exceeded, those of unmodified resistant phenotypes. Irrespective of the developmental conditions anti-symmetry and fluctuating asymmetry were typically observed for each bristle character of unmodified resistant and the modified and susceptible phenotypes, respectively. Thus while similar asymmetry scores could arise from genetic or environmental effects, asymmetry pattern was genetically based. Population cage analyses at different temperatures and larval densities showed a negative association between mean asymmetry and relative fitness.     

Conversion of an RFLP marker for the barley stem rust resistance geneRpg1 to a specific PCR-amplifiable polymorphism

TheRpg1 gene in barley has provided satisfactory levels of stem rust resistance for the last 50 years. The appearance of a new race of stem rust that is virulent toRpg1 has resulted in efforts to incorporate new stem rust resistance genes into barley. Marker-assisted selection may provide the only means of combining this useful gene with resistance genes for which no virulent races have been identified. Several RFLP markers have been identified as linked to theRpg1 locus. One of these, ABG704 was converted into a post-amplification restriction polymorphism. To generate a specific PCR-amplifiable polymorphism the sequence of the ABG704 locus from four barley cultivars was determined. Primers were developed that can detect a single-base difference between resistant and susceptible cultivars. The successful conversion of an RFLP marker to an allele-specific PCR-based marker not only demonstrates that this type of conversion is possible for cereals, but also results in an immediately useful marker for application to plant breeding programmes.     

Tomato resistance to Alternaria stem canker: localization in host genotypes and functional expression compared to non-host resistance

Summary The Alternaria stem canker resistance locus (Asc-locus), involved in resistance to the fungal pathogen Alternaria alternata f. sp. lycopersici and in insensitivity to host-specific toxins (AAL-toxins) produced by the pathogen, was genetically mapped on the tomato genome. Susceptibility and resistance were assayed by testing a segregating F₂ population for sensitivity to AAL-toxins in leaf bioassays. Linkage was observed to phenotypic markers solanifolium and sunny, both on chromosome 3. For the Asc-locus, a distance of 18 centiMorgan to solanifolium was calculated, corresponding to position 93 on chromosome 3. This map position of the resistance locus turned out to be the same in three different resistant tomato accessions, one Dutch and two American, that are at least 40 years apart. AAL-toxin sensitivity in susceptible and resistant tomato genotypes was compared with AAL-toxin sensitivity in a non-host Nicotiana tabacum during different levels of plant cell development. In susceptible and resistant tomato genotypes, inhibitory effects were demonstrated at all levels, except for leaves of resistant genotypes. However, during pollen and root development, inhibitory effects on susceptible genotypes were larger than on resistant genotypes. In the non-host Nicotiana tabacum, hardly any effects of AAL-toxins were demonstrated. Apparently, a cellular target site is present in tomato, but not in Nicotiana tabacum. It was concluded that three levels of AAL-toxin sensitivity exist: (1) a susceptible host sensitivity, (2) a resistant host sensitivity, (3) a non-host sensitivity, and that the resistance mechanism operating in tomato is different from that operating in Nicotiana tabacum.     

Biochemistry of Esterases Associated with Organophosphate Resistance in Lucilia cuprina with Comparisons to Putative Orthologues in Other Diptera

Esterase activities associated with organophosphate insecticide resistance in the Australian sheep blowfly, Lucilia cuprina, are compared with similar activities in other Diptera. The enzymes making the major contribution to methyl butyrate hydrolysis (ali-esterase) in L. cuprina, M. domestica, and D. melanogaster comigrate during electrophoresis. The enzymes in L. cuprina and D. melanogaster correspond to the naphthyl acetate hydrolyzing E3 and EST23 isozymes of those species. These and previously published data suggest that the ali-esterases of all three species are orthologous. Strains of L. cuprina fall into four groups on the basis of quantitative determinations of their ali-estesterase, OP hydrolase, and malathion carboxylesterase activities and these groups correspond to their status with respect to two types of OP resistance. Strains susceptible to OPs have high ali-esterase, low OP hydrolase, and intermediate MCE activities; those resistant to malathion but not diazinon have low ali-esterase, intermediate OP hydrolase, and high MCE activities; those resistant to diazinon but not malathion have low ali-esterase, high OP hydrolase, and low MCE activities; those resistant to both OPs have low ali-esterase, high OP hydrolase, and high MCE activities. The correlated changes among the three biochemical and two resistance phenotypes suggest that they are all properties of one gene/enzyme system; three major allelic variants of that system explain OP susceptibility and the two types of OP resistance. Models are proposed to explain the joint contribution of OP hydrolase and MCE activities to malathion resistance and the invariant association of low ali-esterase and elevated OP hydrolase activities in either type of resistance.     

Identification of AFLP molecular markers for resistance against Melampsora larici-populina in Populus

We have identified AFLP markers tightly linked to the locus conferring resistance to the leaf rust Melampsora larici-populina in Populus. The study was carried out using a hybrid progeny derived from an inter-specific, controlled cross between a resistant Populus deltoides female and a susceptible P. nigra male. The segregation ratio of resistant to susceptible plants suggested that a single, dominant locus defined this resistance. This locus, which we have designated Melampsora resistance (Mer), confers resistance against E1, E2, and E3, three different races of Melampsora larici-populina. In order to identify molecular markers linked to the Mer locus we decided to combine two different techniques: (1) the high-density marker technology, AFLP, which allows the analysis of thousands of markers in a relatively short time, and (2) the Bulked Segregant Analysis (BSA), a method which facilitates the identification of markers that are tightly linked to the locus of interest. We analyzed approximately 11,500 selectively amplified DNA fragments using 144 primer combinations and identified three markers tightly linked to the Mer locus. The markers can be useful in current breeding programs and are the basis for future cloning of the resistance gene.     

Cloning and characterization of the histidine kinase gene<Emphasis Type="Italic"> Dic1</Emphasis> from<Emphasis Type="Italic"> Cochliobolus heterostrophus</Emphasis> that confers dicarboximide resistance and osmotic adaptation

A gene for a putative two-component histidine kinase, which is homologous to os-1 from Neurospora crassa, was cloned and sequenced from the plant-pathogenic fungus Cochliobolus heterostrophus. The predicted protein possessed the conserved histidine kinase domain, the response regulator domain, and six tandem repeats of 92-amino-acids at the N-terminal end that are found in histidine kinases from other filamentous fungi. Introduction of the histidine kinase gene complemented the deficiency of the C. heterostrophus dic1 mutant, suggesting that the Dic1 gene product is a histidine kinase. Dic1 mutants are resistant to dicarboximide and phenylpyrrole fungicides, and they are sensitive to osmotic stress. We previously classified dic1 alleles into three types, based on their phenotypes. To explain the phenotypic differences among the dic1 mutant alleles, we cloned and sequenced the mutant dic1 genes and compared their sequences with that of the wild-type strain. Null mutants for Dic1, and mutants with a deletion or point mutation in the N-terminal repeat region, were highly sensitive to osmotic stress and highly resistant to both fungicides. A single amino acid change within the kinase domain or the regulator domain altered the sensitivity to osmotic stress and conferred moderate resistance to the fungicides. These results suggest that this predicted protein, especially its repeat region, has an important function in osmotic adaptation and fungicide resistance.Communicated by C. A. M. J. J. van den Hondel     

Ecological fitness of a glyphosate-resistant Lolium rigidum population: Growth and seed production along a competition gradient

Repeated use of glyphosate has resulted in evolution of glyphosate-resistant Lolium rigidum populations in Australia. The relative growth, competitiveness and reproductive output of glyphosate resistant (R) and susceptible (S) L. rigidum phenotypes from a single population were compared in competition with wheat. Vegetative growth of R and S individuals was studied in response surface experiments in the glasshouse and seed production was measured using an additive neighbourhood design experiment conducted in pots outside during the normal growing season of L. rigidum in Australia. There were no significant differences in vegetative growth or competitiveness of the R and the S phenotypes. The mean weight of seeds produced by R plants was significantly greater than that produced by S individuals. In the absence of wheat and at low wheat densities, S plants produced more seeds than R plants. However, at higher crop densities, differences in seed production were not significant. This is the first study to compare components of fitness at key life history stages for glyphosate R and S phenotypes isolated from a single weed population. The results presented indicate important differences in resource allocation during the reproductive stage for R and S phenotypes. Subtle differences in life history strategies may be manipulated by agronomic management to exploit the potential ecological fitness costs of the R phenotype. Further studies are required to provide a greater understanding of the occurrence and extent of fitness costs associated with glyphosate resistance. This knowledge can then be incorporated into models that simulate resistance evolution to design management strategies to prevent and/or contain the spread of glyphosate resistance.     

Mapping and diagnostic marker development for Soil-borne cereal mosaic virus resistance in bread wheat

Monogenically-inherited resistance to Soil-borne cereal mosaic virus (SBCMV) in hexaploid bread wheat cultivars ‘Tremie’ and ‘Claire’ was mapped on chromosome 5D. The two closest flanking markers identified in the Claire-derived mapping population, Xgwm469-5D and E37M49, are linked to the resistance locus at distances of 1 and 9 cm, respectively. Xgwm469-5D co-segregated with the SBCMV resistance in the Tremie-derived population and with the recently identified Sbm1 locus in the cv. Cadenza. This suggested that Tremie and Claire carry a resistance gene allelic to Sbm1, or one closely linked to it. The diagnostic value of Xgwm469-5D was assessed using a collection of SBCMV resistant and susceptible cultivars. Importantly, all susceptible genotypes carried a null allele of Xgwm469-5D, whereas resistant genotypes presumably related to either Claire and Tremie or Cadenza revealed a 152 or 154 bp allele of Xgwm469-5D, respectively. Therefore, Xgwm469-5D is well suited for marker assisted selection for SBCMV resistance.     

Localization of a novel recessive powdery mildew resistance gene from common wheat line RD30 in the terminal region of chromosome 7AL

Segregation analysis of resistance to powdery mildew in a F₂ progeny from the cross Chinese Spring (CS) × TA2682c revealed the inheritance of a dominant and a recessive powdery mildew resistance gene. Selfing of susceptible F₂ individuals allowed the establishment of a mapping population segregating exclusively for the recessive resistance gene. The extracted resistant derivative showing full resistance to each of 11 wheat powdery mildew isolates was designated RD30. Amplified fragment length polymorphism (AFLP) analysis of bulked segregants from F₃s showing the homozygous susceptible and resistant phenotypes revealed an AFLP marker that was associated with the recessive resistance gene in repulsion phase. Following the assignment of this AFLP marker to wheat chromosome 7A by means of CS nullitetrasomics, an inspection of simple sequence repeat (SSR) loci evenly spaced along chromosome 7A showed that the recessive resistance gene maps to the distal region of chromosome 7AL. On the basis of its close linkage to the Pm1 locus, as inferred from connecting partial genetic maps of 7AL of populations CS × TA2682c and CS × Virest (Pm1e), and its unique disease response pattern, the recessive resistance gene in RD30 was considered to be novel and tentatively designated mlRD30.Communicated by C. Möllers     

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.     

Dieldrin and diazinon resistance in populations of the Australian sheep blowfly, Lucilia cuprina, from sheep-grazing areas and rubbish tips

Populations of L. cuprina collected from adjacent sheep-grazing areas and rubbish tips in Victoria (Mansfield and Warrnambool) and New South Wales (Lismore) were tested for resistance to the insecticides diazinon and dieldrin. Populations from sheep-grazing areas had a significantly higher diazinon Rop-1 allele frequency than those from adjacent tips with the Victorian populations being more resistant than those from Lismore. Victorian sheep and tip populations had similar gene frequencies at the dieldrin resistance locus, but the Rdl allele frequency was significantly greater in the population at the tip than in the population from the sheep-grazing area at Lismore. The Rdl allele is at a higher frequency in flies from the Lismore area than in Victorian populations. The results at both loci are explained by a balance of selection and gene flow between sheep and tip populations and by selective differences between geographical areas. The exceptionally high frequency of the dieldrin Rdl allele in populations at the Lismore tip may be partially explained by the use of dichlorvos for fly control. Dosage mortality curve and genetic analyses suggest that dichlorvos (an organophosphorus compound) may select at the dieldrin resistance locus. Possible mechanisms for this are discussed. The consequences of genetic differentiation between L. cuprina populations within a region for an autocidal control program are considered.     

Identification of two loci for resistance to clubroot (<Emphasis Type="Italic">Plasmodiophora brassicae</Emphasis> Woronin) in <Emphasis Type="Italic">Brassica rapa</Emphasis> L.

In an analysis of 114 F₂ individuals from a cross between clubroot-resistant and susceptible lines of Brassica rapa L., 'G004' and 'Hakusai Chukanbohon Nou 7' (A9709), respectively, we identified two loci, Crr1 and Crr2, for clubroot (caused by Plasmodiophora brassicae Woronin) resistance. Each locus segregated independently among the F₂ population, indicating that the loci reside on a different region of chromosomes or on different chromosomes. Genetic analysis showed that each locus had little effect on clubroot resistance by itself, indicating that these two loci are complementary for clubroot resistance. The resistance to clubroot was much stronger when both loci were homozygous for resistant alleles than when they were heterozygous. These results indicate that clubroot resistance in B. rapa is under oligogenic control and at least two loci are necessary for resistance.Communicated by H.C. Becker     

The recessive potyvirus resistance gene <Emphasis Type="Italic">pot-1</Emphasis> is the tomato orthologue of the pepper <Emphasis Type="Italic">pvr2-eIF4E</Emphasis> gene

The translation initiation factor 4E (eIF4E) has been implicated in naturally occurring resistance to Potato virus Y (PVY) determined by the pvr2 locus in pepper (Capsicum annuum). Here, the molecular basis of the recessive resistance to PVY and Tobacco etch virus (TEV) controlled by the pot-1 locus in tomato (Lycopersicon esculentum; now Solanum lycopersicum) was investigated. On the basis of genetic mapping data that indicated that pot-1 and pvr2 are located in syntenic regions of the tomato and pepper genomes, the possible involvement of eIF4E in pot-1-mediated resistance was assessed. Genetic mapping of members of the eIF4E multigenic family in tomato introgression lines revealed that an eIF4E locus indeed maps in the same genomic region as pot-1. By comparing eIF4E coding sequences between resistant and susceptible Lycopersicon genotypes, a small number of polymorphisms that co-segregate with the pot-1 locus were identified, suggesting that this gene could be involved in resistance to potyviruses. Functional complementation experiments using Potato virus X-mediated transient expression of eIF4E from a susceptible genotype in a resistant pepper genotype confirmed that a small number of amino acid substitutions in the eIF4E protein indeed account for resistance/susceptibility to both the PVY and TEV, and consequently that pot-1 and pvr2 are orthologues. Taken together, these results support the role of this eIF4E gene as a key component of recessive resistance to potyviruses, and validate the comparative genomic approach for the molecular characterization of recessive resistance genes.     

Segregation at the SCN resistance locus rhg1 in soybean is distorted by an association between the resistance allele and reduced field emergence

Segregation distortion has been reported repeatedly in soybean (Glycine max [L.] Merr.) inbred line populations segregating for the soybean cyst nematode (SCN) (Heterodera glycines Ichinohe) resistance gene rhg1. In each reported case, the frequency of the SCN resistance allele at the rhg1 locus was lower than expected. Segregation distortion was studied in 51 F₄ populations by counting the number of plants predicted to be homozygous resistant, susceptible, and heterozygous for rhg1 based on the genetic markers Satt309, CTA, or TMA5. Significant (P<0.05) segregation distortion was observed in 44 out of the 51 F₄ populations. When the heterozygotes were ignored, there were significantly fewer homozygous-resistant plants than expected in 33 populations. To study whether differential field emergence was a cause of the segregation distortion, three near isogenic line (NIL) populations segregating at the rhg1 locus for SCN resistance from plant introduction 88788 were tested. Population sizes ranged from 32 to 44 NILs and emergence was determined in field experiments in three environments. In each population, SCN-resistant NILs had significantly (P<0.05) less field emergence than susceptible NILs. In the population with the greatest effect, field emergence of resistant NILs was 6% less than susceptible NILs, with the entire population having an average emergence rate of 46%. Equations were derived to describe the effect of selection on segregation ratios over generations of population development and the observed emergence rates were transformed into fitness factors. Depending on assumptions of gene action, it was predicted from these fitness factors that segregation distortions were in the range of those reported previously for the rhg1 locus and were similar to what was observed on average across the 51 F₄ populations. While other factors might also be involved, the results suggest that reduced field emergence associated with the SCN resistance allele contributes to previously reported segregation distortion at the rhg1 locus.     

Transgenic broccoli expressing aBacillus thuringiensis insecticidal crystal protein: Implications for pest resistance management strategies

We usedAgrobacterium tumefaciens to transform flowering stalk explants of five genotypes of broccoli with a construct containing the neomycin phosphotransferase gene and aBacillus thuringiensis (Bt) gene [CryIA(c) type] optimized for plant expression. Overall transformation efficiency was 6.4%; 181 kanamycin-resistant plants were recovered. Of the 162 kanamycin-resistant plants tested, 112 (69%) caused 100% morality of 1st-instar larvae of aBt-susceptible diamondback moth strain. Southern blots of some resistant transformants confirmed presence of theBt gene. Selected plants that gave 100% mortality of susceptible larvae allowed survival of a strain of diamondback moth that had evolved resistance toBt in the field. F₁ hybrids between resistant and susceptible insects did not survive. Analysis of progeny from 26 resistant transgenic lines showed 16 that gave segregation ratios consistent with a single T-DNA integration. Southern analysis was used to verify those plants possessing a single T-DNA integration. Because these transgenic plants kill susceptible larvae and F₁ larvae, but serve as a suitable host for resistant ones, they provide an excellent model for tests ofBt resistance management strategies.