NONRANDOM GENOTYPIC ASSOCIATIONS IN A LEGUME—BRADYRHIZOBIUM MUTUALISM

Genetically divergent lineages often coexist within populations of the annual legume Amphicarpaea bracteata. At one site dominated by two such lineages (termed biotypes “C” and “S”), isolates of root-nodule bacteria (Bradyrhizobium sp.) were sampled from both hosts and analyzed by enzyme electrophoresis. Symbiont populations on the two plant biotypes were highly distinct. Out of 15 bacterial multilocus genotypes detected (among 51 isolates analyzed), only one was shared in common by the two plant biotypes. Cluster analysis revealed three bacterial lineages (designated I, II, and III), with lineage I found exclusively on biotype C plants, and the two other lineages almost completely restricted to biotype S hosts. Laboratory inoculation tests indicated that lineage I bacteria were strictly specialized on biotype C hosts, forming few or no nodules on plants of the other host biotype. Bacterial lineages II and III were capable of forming nodules on both kinds of plants, but nodule numbers were often significantly higher on biotype S hosts. The nonrandom association between plant and bacterial lineages at this site implies that genetic diversity of hosts is an important factor in the maintenance of polymorphism within the symbiont population.     

Geminivirus transmission and biological characterisation of Bemisia tabaci (Gennadius) biotypes from different geographic regions

Eighteen populations of Bemisia tabaci, collected from different geographic locations (North & Central America, the Caribbean, Africa, the Middle East, Asia and Europe), were studied to identify and compare biological and genetic characteristics that can be used to differentiate biotypes. The morphology of the fourth instar/pupal stage and compound eye structures of adults were investigated using scanning electron microscopy and found to be typical of the species among all biotypes and populations studied. Setae and spines of B. tabaci larval scales from the same colony were highly variable depending on the host plant species or leaf surface characteristics. The location and the morphology of caudal setae, characteristic of all B. tabaci studied to date, were present in all colonies. However, differences in adult body lengths and in the ability to induce phy to toxic disorders in certain plant species were found between biotypes or populations. The recently identified “B” biotype, characterised by a diagnostic esterase banding pattern and by its ability to induce phytotoxic responses in squash, honeysuckle and nightshade was readily distinguished from non-“B” biotype populations. None of the non-“B” biotypes studied, were found to induce phytotoxic responses. Nine populations examined showed typical “B” biotype characteristics, regardless of country of origin. All tested populations, determined as “B” or “B”-like biotypes successfully mated with other “B” biotype colonies from different geographic areas. Non-“B” biotype colonies did not interbreed with other biotypes. The B. tabaci populations were tested for their ability to transmit 15 whitefly-transmitted geminiviruses (WTGs) from different geographic areas with a wide range of symptom types. All WTGs were transmitted by the “B” biotype colonies and by most non-“B” biotype colonies, with the exception of three viruses found in ornamental plants which were non-transmissible by any colony. Some non-“B” biotypes would not transmit certain geminiviruses and some geminiviruses were more efficiently transmitted than were others.     

Comparison of performance on different host plants between the B biotype and a non-B biotype of Bemisia tabaci from Zhejiang, China

The capacity of the B biotype of the whitefly, Bemisia tabaci (Gennadius) (Homoptera: Aleyrodidae), to invade has often been linked to its presumably wider host range than the non‐B indigenous biotypes. However, there are few experimental studies of the relative performance of the B biotype and non‐B biotypes on different host‐plant species. Here, we compared the performance of the B biotype and an indigenous non‐B biotype (China‐ZHJ‐1) of B. tabaci from Zhejiang, China on five commonly cultivated host plants, each from a different family: cotton, tobacco, cabbage, squash, and kidney bean. We also examined the effect of rearing host plants on the performance of the B biotype. Overall, the performance of the B biotype on the five species of plants was much better than that of the indigenous non‐B population. On tobacco, cabbage, and kidney bean, no individuals of ZHJ‐1 completed development to adulthood, whereas the B biotype developed successfully from egg to adult on all three plants. On squash, the B biotype survived better, developed to adulthood earlier and had a higher fecundity than ZHJ‐1. The two biotypes performed more equally on cotton, but even on this plant the B biotype female adults lived nearly twice as long as that of ZHJ‐1 and may have realized a higher life‐time fecundity. The B biotype also showed a substantial capacity to acclimatize to alternative host plants for improved survival and reproduction, on both highly suitable and marginally suitable host plants. We conclude that the host range of the B biotype of B. tabaci may be much wider than those of some indigenous biotypes, and this advantage of the B biotype over the non‐B biotypes may assist in its invasion and displacement of some indigenous biotypes in the field.     

Local genetic differentiation for disease resistance in a selfing annual

Electrophoretic analysis of a population of the annual legume Amphicarpaea bracteata revealed that plants in two areas 50 m apart were fixed or nearly fixed for different alleles at eight polymorphic allozyme loci. This multilocus differentiation was highly correlated with variation in disease resistance to the specialist pathogen Synchytrium decipiens : in both laboratory and field experiments, all plants from one area were susceptible, and all plants from the other area were resistant to local pathogen isolates. Analysis of F2 plants from a cross between the two plant biotypes suggested polygenic inheritance of disease resistance, with a minimum of 7–8 loci contributing to the difference among parental lines. The high degree of self-fertilization in A. bracteata , together with spatially restricted seed dispersal, may account for perpetuation of the two divergent plant lineages at this site with little introgression or mixing. Although pathogens could influence the relative frequency of the two host lineages in this habitat, the restricted diversity of local pathogens implies that there may be little selection on plants for outcrossing as a mechanism to reduce pathogen impact.     

微卫星位点BEM06与BEM23鉴别烟粉虱B型与Q型的有效性

烟粉虱Bemisia tabaci (Gennadius) B型与Q型是烟粉虱复合种中入侵性较强、分布较广的2种生物型, 当前在许多地区混合发生。这2种生物型的快速鉴别对其种群动态调查及入侵生态学研究具有重要价值。为了验证微卫星位点BEM06与BEM23鉴别B型与Q型烟粉虱的有效性, 本研究分析了这2个微卫星位点的等位基因在国内外17个B型、4个Q型、3个非B/Q型烟粉虱种群的分布特点。结果表明: 这2个微卫星位点的联合使用可鉴别B型与Q型烟粉虱, 但是无法有效地将B型、Q型与一些非B/Q型烟粉虱某些个体区分开来。结果提示: 利用微卫星位点BEM06与BEM23鉴别B型与Q型烟粉虱具有一定的局限性, 尤其是田间烟粉虱存在其他生物型时需要慎重使用这种鉴别方法。     

Allozyme variation among biotypes of the brown planthopperNilaparvata lugens in the Philippines

Allozyme variation was studied in threeNilaparvata lugens biotypes infesting specific rice varieties and a biotype infesting a weed grass,Leersia hexandra. Of the 20 enzymes inN. lugens for which activity was noted, 9 were polymorphic. Eleven enzyme loci were monomorphic for the same allele in all biotype populations; the rest were polymorphic for two or more alleles. The mean number of alleles per polymorphic locus was 2.3, while the mean number of alleles per locus was 1.5; heterozygosity ranged from 0.02 to 0.06 (biotype 1 > biotype 3 >Leersia-infesting biotype > biotype 2). Allelic frequency differences were observed in five loci among the four biotypes. However, the coefficient of genetic identity (I) of 0.99+ showed that the four biotype populations were genetically close relatives or merely populations ofN. lugens undergoing genetic differentiation. This work was partly supported by a financial grant received from the Directorate for Technical Cooperation and Humanitarian Aid, Switzerland.     

Distribution patterns of the Q and B biotypes of Bemisia tabaci in the Mediterranean Basin based on microsatellite variation

At least five of the biotypes described in the Bemisia tabaci (Gennadius) (Homoptera: Aleyrodidae) complex are known to be present in the Mediterranean Basin area. Only two of them, however, are economically relevant, that is, biotypes B and Q. Biological and genetic differences between the two biotypes have been well studied, but less is known about their patterns of genetic variation and population structure. To address these issues, a study was undertaken based on variation at six microsatellite loci among a subset of nine B. tabaci populations (five belonging to the Q and four to the B biotype). The data obtained show that (i) these loci showed considerable polymorphism in the Q and B biotypes populations although the presence of null alleles can obscure the picture; (ii) the Iberian‐Q, Canarian‐Q, and Egyptian‐B populations exhibit heterozygosity excess as a result of bottleneck events; (iii) the low genetic differentiation between the Israeli, Iberian Peninsula, and Italian populations suggest that these populations share a common gene pool; (iv) the genetic distances between the Canarian‐Q population and the geographically close population from Morocco indicates spatial isolation and a limited gene flow; and finally (v) the microsatellite data for the B populations indicate that the whiteflies from Egypt and Israel have a close phylogenetic relationship, but the source of these biotype B invasions into the Mediterranean area remains unknown.     

Multiple origins of pyrethroid resistance in sympatric biotypes of Bemisia tabaci (Hemiptera: Aleyrodidae)

The two most damaging biotypes of Bemisia tabaci, B and Q, are sympatric in the Mediterranean basin and show high resistance to pyrethroids synergized by organophosphates. Previous work showed that in the B biotype, this resistance is associated with the L925I mutation in the para-type voltage gated sodium channel. Here we identified two mutations in the para-type voltage gated sodium channel associated with resistance to pyrethroids synergized by organophosphates in the Q biotype: the L925I mutation that occurs in the B biotype, and substitution of threonine to valine, at position 929 (T929V). To determine if the L925I and T929V mutations have single or multiple origins, we sequenced the DNA regions flanking the mutations from 13 B and Q strains collected worldwide. The survey identified five resistant alleles and five susceptible alleles. In the resistant alleles, the nucleotide diversity was low within biotypes (0.001), but high between biotypes (0.033). Nucleotide diversity in susceptible alleles was high between the two biotypes (0.028). These observations are consistent with multiple independent origins of resistance. Although the B and Q biotypes coexist in several regions of the Mediterranean basin, divergence in their DNA sequences at the para-type voltage gated sodium channel locus suggests gene flow between these biotypes is low or nil.     

The mechanism of resistance to paraquat is strongly temperature dependent in resistant Hordeum leporinum Link and H. glaucum Steud.

Paraquat-resistant biotypes of the closely-related weed species Hordeum leporinum Link and H. glaucum Steud. are highly resistant to paraquat when grown during the normal winter growing season. However, when grown and treated with paraquat in summer, these biotypes are markedly less resistant to paraquat. This reduced resistance to paraquat in summer is primarily a result of increased temperature following herbicide treatment. The mechanism governing this decrease in resistance at high temperature was examined in H. leporinum. No differences were observed between susceptible and resistant biotypes in the interaction of paraquat with isolated thylakoids when assayed at 15, 25, or 35 °C. About 98 and 65% of applied paraquat was absorbed through the leaf cuticle of both biotypes at 15 and 30 °C, respectively. Following application to leaves, more herbicide was translocated in a basipetal direction in the susceptible biotype compared to the resistant biotype at 15 °C. However, at 30 °C more paraquat was translocated in a basipetal direction in the resistant biotype. Photosynthetic activity of young leaf tissue from within the leaf sheath which had not been directly exposed to paraquat was measured 24 h after treatment of plants with para. quat. This activity was inhibited in the susceptible biotype when plants were maintained at either 15 °C or 30 °C after treatment. In contrast, photosynthetic activity of such tissue of the resistant biotype was not inhibited when plants were maintained at 15 °C after treatment, but was inhibited at 30 °C. The mechanism of resistance in this biotype of H. leporinum correlates with decreased translocation of paraquat and decreased penetration to the active site. This mechanism is temperature sensitive and breaks down at higher temperatures.We are grateful to Zeneca Agrochemicals, Jealotts Hill, Berkshire, UK who provided [¹⁴C]paraquat. E.P. was supported through a Ph.D. scholarship from the Australian International Development Assistance Bureau and C.P. was the recipient of an Australian Research Council Postdoctoral Fellowship.     

Effects of host,temperature and relative humidity on competitive displacement of two invasive Bemisia tabaci biotypes [Q and B]

Abstract Bemisia tabaci shifted unexpectedly in China from a predominance of B biotype to Q biotype during 2005–2008. This observation stimulated an interest in investigating whether environmental factors, including host, temperature and relative humidity (RH) could possibly explain the observed shift in biotypes distribution. Results indicated that all three parameters examined influenced biotype survivability. The percentage of B biotype, when reared together on pepper plants with the Q biotype, decreased significantly from 66.7% in the founder population, to 13.6% and 3.7% in the first and second generations, respectively. When the B (founder at 66.7%) and Q (founder at 33.3%) biotypes were reared together on eggplant alone, or on pepper‐plus‐eggplant combination, the population size of the B biotype either remained constant, or increased somewhat in the first and second generations. On eggplant, the effects of RH and temperature on the competitiveness between the Q and B biotypes (3 pairs of Q and 6 pairs of B) were not significant.     

MtDNA variability in whitefly (Bemisia tabaci) populations in Brazil

Bemisia tabaci (Hemiptera: Aleyrodidae) consists of a complex of morphologically indistinct biotypes that vary mainly in their capacity to transmit plant viruses and to induce physiological disorders in plants of economic importance. The adaptability of B. tabaci to many regions of the world has fostered the appearance of various biotypes and has resulted in a broad spectrum of host plants. Our goal was to identify which biotypes were present in four B. tabaci populations in Brazil. We quantified genetic variability between and within populations. Three individuals were collected from three host plant species: two populations on soybean (Campinas and Rondonópolis), one on pumpkin (Barreiras) and one on tomato (Cruz das Almas) in three States of Brazil (S?o Paulo, Mato Grosso, and Bahia). We chose one sequence of the B biotype, obtained from GenBank; the Campinas population, which had been previously characterized as biotype B, was used as a control for this biotype. We also included one sequence of the Q biotype, obtained from GenBank, as an outgroup. The COI region of the mtDNA gene was partially amplified with the CI-J-2195 and L2-N-3014 pair of primers, and the reaction products were sequenced. Based on distance-based algorithm analyses, we found that all haplotypes belong to biotype B, which was confirmed by the haplotype network. Genetic structure analyses showed that the host plant species does not influence population structuring of this pest; only the geographic location mattered.     

Soybean aphid intrabiotype variability based on colonization of specific soybean genotypes

The soybean aphid, Aphis glycines Matsumura (Hemiptera: Aphididae), is one of the most destructive insect pests on soybeans in the United States. One method for managing this pest is through host plant resistance. Since its arrival in 2000, 4 aphid biotypes have been identified that are able to overcome soybean aphid resistance (Rag) genes. A soybean aphid isolate collected from Moline, Illinois readily colonized soybean plants with the soybean aphid resistance gene Rag2, unlike biotypes 1 and 2, but similar to soybean aphid biotype 3. Two no‐choice experiments compared the virulence of the Moline isolate with biotype 3. In both experiments, differences in aphid population counts were not significant (P > 0.05) on soybean genotypes LD08–12957a (Rag2) and LD11–5413a (Rag2), but the aphid counts for the Moline isolate were significantly (P < 0.05) lower than the aphid counts for the biotype 3 isolate on the soybean genotypes Dowling (Rag1), LD05–16611 (Rag1), LD11–4576a (Rag1), and PI 567598B (rag1b and rag3). The Moline isolate was a variant of aphid biotype 3, which is the first report showing that soybean aphid isolates classified as the same biotype, based on virulence against specific Rag genes, can differ in aggressiveness or ability to colonize specific host genotypes.     

Genetic variability and chromosome-length polymorphisms of the witches' broom pathogen Crinipellis perniciosa from various plant hosts in South America

Crinipellis perniciosa has been classified into at least four known biotypes associated with members of unrelated plant families. In this study, genetic variability is shown for 27 C (Cacao), 4 S (Solanum), and 7 L biotype (Liana) isolates of C. perniciosa collected from different regions of Brazil and South America. The objective was to investigate the genetic variability of the pathogen in the cacao-producing region of Bahia, Brazil, and elsewhere, through microsatellite analysis, and attempt to identify possible correlations between host specificity and electrophoretic karyotypes. The PCR-banding patterns were found to vary both within and between the different biotypes, and a correlation was established between the PCR-banding patterns and the chromosomal-banding patterns of each isolate. Microsatellite and chromosomal patterns among all of the L and S biotype isolates were distinctly different from the C biotypes analysed. A higher degree of genetic and chromosomal variability was found among C biotype isolates from the Amazon in comparison with C biotype isolates from Bahia, which seems to be comprised of only two main genotypes. This finding has important implications to the current cacao-breeding programme in Brazil.     

Balancing selection and wild gene pool contribute to resistance in global rice germplasm against planthopper

Interactions and co-evolution between plants and herbivorous insects are critically important in agriculture. Brown planthopper (BPH) is the most severe insect of rice, and the biotypes adapt to feed on different rice genotypes. Here, we present genomics analyses on 1,520 global rice germplasms for resistance to three BPH biotypes. Genome-wide association studies identified 3,502 single nucleotide polymorphisms (SNPs) and 59 loci associated with BPH resistance in rice. We cloned a previously unidentified gene Bph37 that confers resistance to BPH. The associated loci showed high nucleotide diversity. Genome-wide scans for trans-species polymorphisms revealed ancient balancing selection at the loci. The secondarily evolved insect biotypes II and III exhibited significantly higher virulence and overcame more rice varieties than the primary biotype I. In response, more SNPs and loci evolved in rice for resistance to biotypes II and III. Notably, three exceptional large regions with high SNP density and resistance-associated loci on chromosomes 4 and 6 appear distinct between the resistant and susceptible rice varieties. Surprisingly, these regions in resistant rice might have been retained from wild species Oryza nivara. Our findings expand the understanding of long-term interactions between rice and BPH and provide resistance genes and germplasm resources for breeding durable BPH-resistant rice varieties.     

Functional significance of triazine-herbicide resistance in defence of Senecio vulgaris against a rust fungus

We investigated the functional significance of plant performance (dry mass, photosynthesis) in plant defence (resistance and tolerance) against pathogen infection, and potential negative cross-resistance between herbicide resistance and plant defence against disease. We compared isonuclear triazine-herbicide-resistant (TR) and -susceptible (TS) biotypes of Senecio vulgaris, in the presence and absence of infection by the rust Puccinia lagenophorae. In a growth chamber study with two reduced irradiance levels, rust infection had a severe effect on plant performance with infected plants having 55% less dry mass and 54% reduced whole-plant photosynthesis than non-infected plants. The TR biotype was more susceptible (reduced resistance) to the pathogen, but the biotypes did not differ in their ability to compensate for rust infection (tolerance). TR plants were less productive than TS plants when grown non-shaded (ca. 10% full sunlight) but not when shaded (ca. 5% full sunlight). This is especially important for situations, where S. vulgaris grows under the crop canopy (e.g. in maize). Here, very low light levels might contribute to a numerical increase of TR relative to TS plants even when only occasionally treated with triazine. Whole-plant photosynthesis was reduced by 21% in TR plants as compared to the TS biotype, and by 59% in plants grown in the shaded as compared to the non-shaded treatment. When whole-plant photosynthesis values were corrected for the estimated leaf area of plants, we found no significant variation between biotypes, shade treatments or rust treatments. In experimental mixed TR:TS field populations, the proportion of TR plants decreased more rapidly in rust-infected populations than uninfected. This finding, together with the lower resistance in the TR than the TS biotype to the rust fungus observed in the growth chamber experiment, may indicate negative cross-resistance, which is a potential tool in the management of herbicide-resistant weeds.     

Differential indirect effects of two plant viruses on an invasive and an indigenous whitefly vector: implications for competitive displacement

The role of vector–begomovirus–plant interactions in the widespread invasion by some members of the whitefly species complex Bemisia tabaci is poorly understood. The invasive B biotype of B. tabaci entered China in the late 1990s and had become the predominant or only biotype of the whitefly in many regions of the country by 2005–2006. Meanwhile epidemics of begomoviruses have been observed in many crops including tomato for which Tomato yellow leaf curl China virus (TYLCCNV) and Tomato yellow leaf curl virus (TYLCV) have been identified as two major disease-causing agents. Here, we conducted laboratory experiments to compare the performance of the invasive B and indigenous ZHJ1 whitefly biotypes on uninfected, TYLCCNV-infected and TYLCV-infected plants of tomato cv. Hezuo903, a cultivar that has been widely cultivated in many regions of China. The infection of tomato plants by either of the viruses had no or only marginal effects on the development, survival and fecundity of the B biotype. In contrast, survival and fecundity of the ZHJ1 biotype were significantly reduced on virus-infected plants compared to those on uninfected plants. Populations of the B biotype on uninfected and TYLCCNV-infected plants increased at similar rates, whereas population increase of the ZHJ1 biotype on TYLCCNV-infected plants was affected adversely. These asymmetric responses to virus infection of tomato plants between the B and ZHJ1 biotypes are likely to offer advantages to the B biotype in its invasion and displacement of the indigenous biotype.     

Resistant Acetyl-CoA Carboxylase is a Mechanism of Herbicide Resistance in a Biotype of Avena sterilis ssp. ludoviciana

A biotype of Avena sterilis ssp. ludoviciana is highly resistantto a range of herbicides which inhibit a key enzyme in fattyacid synthesis, acetyl-CoA carboxylase (ACCase). Possible mechanismsof herbicide resistance were investigated in this biotype. Acetyl-CoAcarboxylase from the resistant biotype is less sensitive toinhibition by herbicides to which resistance is expressed. I₅₀values for herbicide inhibition of ACCase were 52 to 6 timesgreater in the resistant biotype than in the susceptible biotype.This was the only major difference found between the resistantand susceptible biotypes. The amount of ACCase in the meristemsof the resistant and susceptible is similar during ontogenyand no difference was found in distribution of ACCase betweenthe two biotypes. Uptake, translocation and metabolism of [¹⁴C]diclofop-methylwere not different between the two biotypes. In vivo, ACCaseactivity in the meristems of the susceptible biotype was greatlyinhibited by herbicide application whereas only 25% inhibitionoccurred in the resistant biotype. Depolarisation of plasmamembrane potential by 50 µM diclofop acid was observedin both biotypes and neither biotype showed recovery of themembrane potential following removal of the herbicide. Hence,a modified form of ACCase appears to be the major determinantof resistance in this resistant wild oat biotype. (Received February 10, 1994; Accepted March 11, 1994)     

浙江B型与一非B型(China-ZHJ-1)烟粉虱形态学和生物学特性的比较研究

本文对新入侵我国的B型与浙江一非B型（China-ZHJ-1种群）烟粉虱之间的生殖亲和性进行了测定,并对两个生物型在形态特征、发育历期、诱导西葫芦银叶反应及寄主植物适应性几方面进行了比较研究。结果表明,当将B型与非B型ZHJ-1种群进行杂交组合时,两生物型之间虽然有求偶行为发生,但从不发生交配,后代全部为孤雌生殖产下的雄虫;而B型、非B型各自内部羽化后0～72 h的平均交配次数分别为4.4次和1.0次, 所产后代的雌雄性比分别为2.8∶1、1.0∶1,前者性比显著高于后者。两生物型之间伪蛹形态存在明显差异,非B型ZHJ-1种群的前、后蜡缘饰宽度分别为B型的2.5～2.7倍、1.8～2.1倍。在棉花上,除B型1龄若虫历期较非B型的略长外,两生物型之间其他各虫态的发育历期均无显著差异;从2龄若虫起,非B型各虫态的体长明显长于B型,并且同一生物型内,4龄雌性若虫个体明显大于同龄雄性若虫。B型在多种寄主植物上的存活力比非B型的显著要强。B型与非B型ZHJ-1种群生物学特性的比较,为对我国烟粉虱这两个生物型进行深入的比较研究、探讨B型烟粉虱的入侵生物学提供了基础。     

A mechanism of chlorotoluron resistance in Lolium rigidum

Many biotypes of Lolium rigidum Gaud, (annual ryegrass) have developed resistance to herbicides; however, few have developed resistance to phenylurea herbicides. Two biotypes with different histories of herbicide selection pressure were six to eight times less sensitive to the phenylurea herbicide, chlorotoluron, than a susceptible biotype. Resistance was not due to differences in the herbicide target site as oxygen evolution by thylakoids isolated from resistant and susceptible biotypes was similarly inhibited by diuron and chlorotoluron. There was no difference in the uptake and distribution of chlorotoluron into resistant and susceptible plants. There was a twofold greater rate of chlorotoluron detoxification in resistant plants with N-demethylation being a major detoxification reaction. Resistant plants treated with a 3-h pulse of 120 M chlorotoluron recovered net carbon fixation after 42 h, half the time taken by susceptible plants. The mixed-function oxidase inhibitor 1-aminobenzotriazole (70 M) intensified the effects of chlorotoluron in resistant plants when applied in combination with the herbicide for 7 d. 1-Aminobenzotriazole also inhibited the metabolism of chlorotoluron in both resistant and susceptible plants. The cytochrome P-₄₅₀ inhibitor, piperonyl butoxide piperonyl butoxide, interacted with chlorotoluron when applied to plants growing in soil. Chlorotoluron applied with reduced plant dry weight to a greater extent than chlorotoluron alone. It appears, therefore, that enhanced detoxification is the major mechanism of resistance to chlorotoluron in the resistant biotypes studied.Abbreviations ABT 1-aminobenzotriazole - VLR1 Victorian L. rigidum biotype 1 — herbicide susceptible - VLR69 Victorian L. rigidum biotype 69 — herbicide resistant - WLR2 Western Australian L. rigidum biotype 2 — herbicide resistant M.W.M.B, was supported by an Australian Postgraduate Research Award and a supplementary scholarship from the Grains Research and Development Corporation. We are very grateful to Dr. E. Ebert, Ciba Geigy, Basal, Switzerland for providing [¹⁴C]chlorotoluron and standards of chlorotoluron metabolites. We express our gratitude to Dr. John Huppatz of the CSIRO Division of Plant Industry for providing ABT. We also thank Ciba Geigy Australia for providing technical-grade chlorotoluron and formulated phenylurea herbicides.     

Identification and mapping of two brown planthopper resistance genes in rice

The brown planthopper (BPH) is one of the most serious insect pests of rice. In this study, we conducted a molecular marker-based genetic analysis of the BPH resistance of ’B5’, a highly resistant line that derived its resistant genes from the wild rice Oryza officinalis. Insect resistance was evaluated using 250 F₃ families from a cross between ’B5’ and ’Minghui 63’, based on which the resistance of each F₂ plant was inferred. Two bulks were made by mixing, respectively, DNA samples from highly resistant plants and highly susceptible plants selected from the F₂ population. The bulks were surveyed for restriction fragment length polymorphism using probes representing all 12 chromosomes at regular intervals. The survey revealed two genomic regions on chromosome 3 and chromosome 4 respectively that contained genes for BPH resistance. The existence of the two loci were further assessed by QTL (quantitative trait locus) analysis, which resolved these two loci to a 14.3-cM interval on chromosome 3 and a 0.4-cM interval on chromosome 4. Comparison of the chromosomal locations and reactions to BPH biotypes indicated that these two genes are different from at least nine of the ten previously identified BPH resistance genes. Both of the genes had large effects on BPH resistance and the two loci acted essentially independent of each other in determining t he resistance. These two genes may be a useful BPH resistance resource for rice breeding programs. Received: 6 March 2000 / Accepted: 28 July 2000