Improved resistance to bacterial soft rot by protoplast fusion between Brassica rapa and B. oleracea

Erwinia soft rot is a destructive disease of Brassica rapa vegetables. Reliable sources of resistance and control methods are limited, so development of highly resistant breeding lines is desirable. Protoplasts from B. rapa and B. oleracea genotypes selected for resistance to soft rot were fused in order to combine different sources of resistance. Twelve somatic hybrids (synthetic B. napus) were obtained and confirmed by morphology, nuclear DNA content, and RAPD analysis. They were normal looking plants that easily set seeds following self-pollination and backcrossing to B. rapa. Assays of detached leaves or seedlings inoculated in a mist-chamber showed that most somatic hybrids had lower disease severity ratings than the B. rapa fusion partner and a commercial variety of B. napus. Some progeny from selfing or backcrossing of somatic hybrids to B. rapa showed much more resistance than either fusion partner. The offspring populations of the somatic hybrids (F₁–S₁ and F₁–BC₁) clearly moved to the resistant direction compared to the parents; the percentage of resistant plants increased from 21% (average of parents) to 36% (F₁–S₁) and 48% (F₁–BC₁). These results suggest that it may be possible to obtain highly resistant B. rapa lines by further backcrossing and selection. Received: June 1999 / Accepted: 29 July 1999     

Opportunities for gene transfer from transgenic oilseed rape (Brassica napus) to related species

Before novel transgenic plant genotypes are grown outside containment facilities and evaluated under field conditions, it is necessary to complete a risk assessment to consider the possible consequences of that release. An important aspect of risk assessment is to consider the likelihood and consequences of the transgene being transferred by cross-pollination to related species, including other crops, weeds and ruderal populations. The purpose of this report is to review the literature to assess the ease with whichBrassica napus can hybridize with related species. The evidence for hybridization is considered at three levels: a) by open pollination, b) by hand pollination and c) by the use ofin vitro ovule and embryo rescue techniques; and also examines the fertility and vigour of the F₁, F₂ and backcross generations. Four species are reported to hybridize withB. napus by open pollination:B. rapa andB. juncea using fully fertile parents; andB. adpressa andR. raphanistrum using a male-sterileB. napus parent. Seventeen species are reported to form hybrids (including the four species above) withB. napus when pollination is carried out manually. At least 12 of these species were unable to form F₂ progeny, and eight were unable to produce progeny when the F₁ was backcrossed to one of the parental species. Many factors will influence the success of hybridization under field conditions, including: distance between the parents, synchrony of flowering, method of pollen spread, specific parental genotypes used, direction of the cross and the environmental conditions. Even where there is a possibility of hybridization betweenB. napus and a related species growing in the vicinity of a release, poor vigour and high sterility in the hybrids will generally mean that hybrids and their progeny will not survive in either an agricultural or natural habitat.     

Intersubgenomic heterosis in seed yield potential observed in a new type of Brassica napus introgressed with partial Brassica rapa genome

This paper reports the observation on the intersubgenomic heterosis for seed yield among hybrids between natural Brassica napus (AⁿAⁿCⁿCⁿ) and a new type of B. napus with introgressions of genomic components of Brassica rapa (A^rA^r). This B. napus was selected from the progeny of B. napus × B. rapa and (B. napus × B. rapa) × B. rapa based on extensive phenotypic and cytological observation. Among the 129 studied partial intersubgenomic hybrids, which were obtained by randomly crossing 13 lines of the new type of B. napus in F₃ or BC₁F₃ to 27 cultivars of B. napus from different regions as tester lines, about 90% of combinations exceeded the yield of their respective tester lines, whereas about 75% and 25% of combinations surpassed two elite Chinese cultivars, respectively. This strong heterosis was further confirmed by reevaluating 2 out of the 129 combinations in a successive year and by surveying hybrids between 20 lines of the new type of B. napus in BC₁F₅ and its parental B. napus in two locations. Some DNA segments from B. rapa were identified with significant effects on seed yield and yield components of the new type of B. napus in BC₁F₅ and intersubgenomic hybrids in positive or negative direction. It seems that the genomic components introgressed from B. rapa contributed to improvement of seed yield of rapeseed.     

Within-population variation in hybridisation and transgene transfer between wild Brassica rapa and Brassica napus in the UK

Several studies in Europe and North America have shown that cultivated Brassica napus will readily hybridise with wild Brassica rapa but at widely different frequencies. To understand the implications of this phenomenon with regard to transgene flow, we examined the rate at which cultivated B. napus cv. Westar containing a capsid (coat protein, CP)‐coding sequence from Turnip mosaic virus (Potyvirus) hybridised under glasshouse conditions with wild B. rapa from Culham, in Oxfordshire, UK. We found that the hybridisation rate, as judged using simple sequence repeat (SSR)‐PCR and primer oligonucleotides specific for either the C or the A genomes in progeny from individual crosses varied from 5% to 100%. In hybrids (F₁ progeny), transgene transfer was always observed (inferred by SSR‐PCR) when hybrids were detected. Our observations revealed a hitherto unrecorded source of variability in transgene flow to wild UK B. rapa.     

The genetics of adult-plant blackleg (Leptosphaeria maculans) resistance from Brassica juncea in B. napus

The genetic control of adult-plant blackleg (Leptosphaeria maculans) resistance in a Brassica napus line (579NO48-109-DG-1589), designated R13 possessing Brassica juncea-like resistance (JR), was elucidated by the analysis of segregation ratios in F₂ and F₃ populations from a cross between R13 and the highly blackleg-susceptible B. napus cultivar Tower. The F₂ segregration ratios were bimodal, demonstrating that blackleg resistance in R13 was controlled by major genes. Analysis of the segregation ratios for 13 F₃ families indicated that blackleg resistance in these families was controlled by three nuclear genes, which exhibited a complex interaction. Randomly sampled plants of F₃ progeny all had the normal diploid somatic chromosome number for B. napus. The similarities between the action of the three genes found in this study with those controlling blackleg resistance in B. juncea is discussed.     

Transfer of bacterial blight and blast resistance from the tetraploid wild rice Oryza minuta to cultivated rice,Oryza sativa

Summary Oryza minuta J. S. Presl ex C. B. Presl is a tetraploid wild rice with resistance to several insects and diseases, including blast (caused by Pyricularia grisea) and bacterial blight (caused by Xanthomonas oryzae pv. oryzae). To transfer resistance from the wild species into the genome of cultivated rice (Oryza sativa L.), backcross progeny (BC₁, BC₂, and BC₃) were produced from interspecific hybrids of O. sativa cv IR31917-45-3-2 (2n=24, AA genome) and O. minuta Acc. 101141 (2n=48, BBCC genomes) by backcrossing to the O. sativa parent followed by embryo rescue. The chromosome numbers ranged from 44 to 47 in the BC₁ progeny and from 24 to 37 in the BC₂ progeny. All F₁ hybrids were resistant to both blast and bacterial blight. One BC₁ plant was moderately susceptible to blast while the rest were resistant. Thirteen of the 16 BC₂ progeny tested were resistant to blast; 1 blast-resistant BC₂, plant 75-1, had 24 chromosomes. A 3 resistant: 1 susceptible segregation ratio, consistent with the action of a major, dominant gene, was observed in the BC₂F₂ and BC₂F₃ generations. Five of the BC₁ plants tested were resistant to bacterial blight. Ten of the 21 BC₂ progeny tested were resistant to Philippine races 2, 3, and 6 of the bacterial blight pathogen. One resistant BC₂, plant 78-1, had 24 chromosomes. The segregation of reactions of the BC₂F₂, BC₂F₃, and BC₂F₄ progenies of plant 78-1 suggested that the same or closely linked gene(s) conferred resistance to races 2, 3, 5, and 6 of the bacterial blight pathogen from the Philippines.     

The transfer of pyrethroid resistance resulting from crosses between resistant German cockroaches and susceptible Asian cockroaches

Crosses were made between the Asian cockroach,Blattella asahinai Mizukubo, and resistant strains of the German cockroach,B. germanica (L.), to assess the transfer of pyrethroid resistance to the progeny and to study the inheritance mechanism(s) involved. It was shown that the strain of Asian cockroaches studied was susceptible to four pyrethroids. F₁ progeny were essentially susceptible to the same compounds. Tests with F₂ progeny and those from backcrosses to the resistant parent indicated that the data for each pyrethroid fit an hypothesis of simple, autosomal, nearly completely recessive inheritance. The results are discussed from the standpoint of the impact of the Asian genome on the inheritance mechanism(s).     

Doubled haploids of novel trigenomic Brassica derived from various interspecific crosses

To develop doubled haploid (DH) mapping populations of hexaploid Brassica, 10 F₁ hybrids derived from crosses between allohexaploid Brassica parents were evaluated in this study. The allohexaploid Brassica parents were selfed progenies of unique interspecific crosses between Brassica rapa (genome AA) × B. carinata (BBCC), B. nigra (BB) × B. napus (AACC), and a complex cross between B. juncea (AABB), B. napus and B. carinata, with relatively stable chromosome number (2n = 54). Hexaploid status and chromosome behavior during meiosis I in four promising F₁ hybrids were assessed using microscopy and flow cytometry, and progeny were obtained following microspore culture. Hybrids H11-2 and H16-1 demonstrated higher amenability for embryo generation, plantlet regeneration, and frequency of production of DH microspore-derived progeny of hexaploid DNA content (6x) compared to hybrids H08-1 and H24-1. A total of 370 6x DH progeny were selected out of 693 plantlets from H11-2, 241/436 from H16-1, 23/54 from H08-1, and 21/56 from H24-1. DH progenies of hybrids H11-2 and H16-1 were then designated as promising mapping populations of a new hexaploid Brassica species.     

Loss of C-genome-specific markers during transgene introgression from Brassica napus to wild Brassica juncea

Transgene flow from engineered Brassica napus to wild weed relatives could potentially have an environmental effect. To evaluate the introgression of transgenic B. napus into wild Brassica juncea, the hybrid F₁ and backcross progenies derived from B. juncea (genome constitution AABB) and transgenic B. napus (AACC) crosses were investigated. C-genome-specific simple sequence repeat (SSR) markers corresponding to linkage groups N11–N19 in B. napus were screened and used to estimate the marker frequency in hybrid F₁ and backcross progenies. C-genome-specific markers could be stably detected in hybrid F₁ and backcross BC₁ plants, but were only rarely found in the BC₂–BC₅ generations. For example, a specific SSR marker for linkage group N12 segregated in BC₂ generation but were completely lost in BC₃–BC₅, while a specific SSR marker of linkage group N15 segregated in BC₁, BC₂ and BC₃ generations and was absent in more advanced backcrossed generations (BC₄ and BC₅). The results indicate that a certain gene regions in Brassica napus plants are transmitted at a relatively lower frequency to wild relatives, and more rapidly disappeared in subsequent backcross generations. We propose that a foreign gene or transgene that is integrated in the C-chromosome of Brassica napus could reduce the risk of introgression in nature.     

Combining transgenic and natural resistance to obtain broad resistance to tospovirus infection in tomato (Lycopersicon esculentum mill)

This study was undertaken to develop tomato plants with broad resistanceto tospoviruses which are a major limiting factor to tomato productionworldwide. A nontransgenic tomato line Stevens-Rodale (S-R), six transgenictomato lines expressing the nucleocapsid (N) protein gene of the lettuceisolate of tomato spotted wilt virus (TSWV-BL), and progeny of the crosses between S-Rand three of the transgenic lines homozygous for the N gene were evaluated fortheir resistance to tospovirus infection in greenhouse inoculation tests. S-Rhas the Sw-5 gene that confers resistance to several TSWVisolates. The six transgenic lines showed high levels of resistance wheninoculated with either TSWV-BL or a tomato isolate from Hawaii (TSWV-H).However, these same plants were highly susceptible to the Brazilian isolate ofgroundnut ringspot virus (GRSV-BR). Plants with the Sw-5gene were resistant to TSWV-BL and GRSV-BR, but were susceptible to TSWV-H.When inoculated with any of the three viruses, the F₁ progeny of thecrosses exhibited a susceptible, tolerant, or resistant phenotype with a higherproportion of the plants being either tolerant or resistant. When F₂progeny from F₁ resistant plants of each cross were inoculated withany of the three viruses, a higher proportion of tolerant and resistant plantswas observed compared to the F₁ progeny. Our results show thepotential to obtain broad resistance to tospoviruses by combining transgenicand natural resistance in a single plant.     

Genetic analysis of interspecific incompatibility in Brassica rapa

In interspecific pollination of Brassica rapa stigmas with Brassica oleracea pollen grains, pollen tubes cannot penetrate stigma tissues. This trait, called interspecific incompatibility, is similar to self-incompatibility in pollen tube behaviors of rejected pollen grains. Since some B. rapa lines have no interspecific incompatibility, genetic analysis of interspecific incompatibility was performed using two F₂ populations. Analysis with an F₂ population between an interspecific-incompatible line and a self-compatible cultivar ‘Yellow sarson’ having non-functional alleles of S-locus genes and MLPK, the stigmas of which are compatible with B. oleracea pollen grains, revealed no involvement of the S locus and MLPK in the difference of their interspecific incompatibility phenotypes. In QTL analysis of the strength of interspecific incompatibility, three peaks of LOD scores were found, but their LOD scores were as high as the threshold value, and the variance explained by each QTL was small. QTL analysis using another F₂ population derived from selected parents having the highest and lowest levels of interspecific incompatibility revealed five QTLs with high LOD scores, which did not correspond to those found in the former population. The QTL having the highest LOD score was found in linkage group A02. The effect of this QTL on interspecific incompatibility was confirmed by analyzing backcrossed progeny. Based on synteny of this QTL region with Arabidopsis thaliana chromosome 5, a possible candidate gene, which might be involved in interspecific incompatibility, is discussed.     

Identification of two novel genes for blackleg resistance in Brassica napus

Blackleg, caused by Leptosphaeria maculans, is a major disease of Brassica napus. Two populations of B. napus DH lines, DHP95 and DHP96, with resistance introgressed from B. rapa subsp. sylvestris, were genetically mapped for resistance to blackleg disease with restriction fragment length polymorphism markers. Examination of the DHP95 population indicated that a locus on linkage group N2, named LepR1, was associated with blackleg resistance. In the DHP96 population, a second locus on linkage group N10, designated LepR2, was associated with resistance. We developed BC₁ and F₂ populations, to study the inheritance of resistance controlled by the genes. Genetic analysis indicated that LepR1 was a dominant nuclear allele, while LepR2 was an incompletely dominant nuclear resistance allele. LepR1 and LepR2 cotyledon resistance was further evaluated by testing 30 isolates from Canada, Australia, Europe, and Mexico. The isolates were from B. napus, B. juncea, and B. oleracea and represented different pathogenicity groups of L. maculans. Results indicated that LepR1 generally conferred a higher level of cotyledon resistance than LepR2. Both genes exhibited race-specific interactions with pathogen isolates; virulence on LepR1 was observed with one isolate, pl87-41, and two isolates, Lifolle 5, and Lifolle 6, were virulent on LepR2. LepR1 prevented hyphal penetration, while LepR2 reduced hyphal growth and inhibited sporulation. Callose deposition was associated with resistance for both loci.     

Molecular markers linked to white rust resistance in mustard Brassica juncea

 White rust, caused by Albugo candida (Pers.) Kuntze, is an economically important disease of Brassica juncea (L.) Czern. and Coss mustard, particularly in India. The most efficient and cost-effective way of protecting mustard plants from white rust disease is through genetic resistance. The objective of this study was to identify RAPD markers for white rust resistance in an F₁-derived doubled-haploid (DH) population originating from a cross between white rust-susceptible and white rust-resistant breeding lines of B. juncea from the canola-quality B. juncea breeding project of the Agriculture and Agri-Food Canada-Saskatoon Research Centre. The DH population was used to screen for RAPD markers associated with white rust resistance/susceptibility using bulked segregant analysis. Two markers, WR2 and WR3, linked to white rust resistance, flanked the resistance locus Ac2 ₁ and were highly effective in identifying the presence or absence of the resistance gene in the DH population. These two markers were shown to be specific to the Russian source of white rust resistance utilized in this project. It is concluded that the availability of these RAPD markers will enhance the breeding for white rust resistance in B. juncea. Received: 17 December 1997 / Accepted: 7 April 1998     

Interspecific hybrids between Brassica napus L. and B. oleracea L. developed by embryo culture

Summary Interspecific hybrids between Brassica napus and B. oleracea are difficult to produce, and previous attempts to transfer economic characters from one species to the other have largely been unsuccessful. In these studies, oilseed rape cv. Tower (2n38) (B. napus) was crossed with broccoli and kale (2n18) (B. oleracea), and hybrid plants were developed from embryos in culture by either organogenesis or somatic embryogenesis. In rape × broccoli, F₁ plants were regenerated from hybrid embryos and the plants produced viable selfed seeds. F₅ plants (2n38) homozygous for white flower colour were selected for high oil content (47%) and Line 15; a selection from these plants produced fertile hybrids with rape, broccoli and kale without embryo culture. In reciprocal crosses between oilseed rape cv. Tower and an aphid resistant diploid kale, 28 and 56 chromosome F₁ hybrid plants were regenerated from somatic embryos. The 56 chromosome plants were self-fertile and it was concluded from F₂ segregation ratios that a single dominant gene controls resistance to cabbage aphid in kale. The 28 chromosome F₁'s were self-sterile, but these and the 56 chromosome F₁'s could be backcrossed to rape and kale. A cross between the F₁ (2n56) and a forage rape resulted in the selection of a cabbage aphid (Brevicoryne brassicae L.) resistant line (Line 3). Both Line 15 and Line 3 can serve as bridges for gene interchange between B. campestris, B. napus and B. oleracea, which has not been possible hitherto. Hybridisations between rape and tetraploid kale produced F₁ plants with 37 chromosomes. One F₂ plant possessed coronal scales and the inheritance was shown to be controlled by a single recessive gene unlinked to petal colour.This paper is dedicated to Mr. T. P. Palmer, a colleague and close friend who retired from the DSIR as Assistant Director of the Crop Research Division in September 1984     

Pi35(t), a new gene conferring partial resistance to leaf blast in the rice cultivar Hokkai 188

The japonica rice cultivar Hokkai 188 shows a high level of partial resistance to leaf blast. For mapping genes conferring the resistance, a set of 190 F₂ progeny/F₃ families was developed from the cross between the indica rice cultivar Danghang-Shali, with a low level of partial resistance, and Hokkai 188. Partial resistance to leaf blast in the F₃ families was assessed in upland nurseries. From a primary microsatellite (SSR) linkage map and QTL analysis using a subset of 126 F₂ progeny/F₃ families randomly selected from the above set, one major QTL located on chromosome 1 was detected in the vicinity of SSR marker RM1216. This QTL was responsible for 69.4% of the phenotypic variation, and Hokkai 188 contributed the resistance allele. Segregation analysis in the F₃ families for partial resistance to leaf blast was in agreement with the existence of a major gene, and the gene was designated as Pi35(t). Another QTL detected on chromosome 8 was minor, explained 13.4% of the phenotypic variation, and an allele of Danghang-Shali increased the level of resistance in this QTL. Additional SSR markers of the targeted Pi35(t) region were further surveyed in the 190 F₂ plants, and Pi35(t) was placed in a 3.5-cM interval flanked by markers RM1216 and RM1003.     

Inheritance of the photoperiodically induced cold acclimation response in Cornus sericea L., red-osier dogwood

Summary Cold acclimation responses of latitudinal ecotypes of Cornus sericea L. (C. stolonifera Michx.) and F₁, F₂ and BC₁ hybrid progenies were measured under natural photoperiod conditions in St. Paul, MN and artificially shortened photoperiods in the glass-house. The 65 °N and 62 °N ecotypes (Alaska and Northwest Territories, respectively) were characterized by a short night length for hardiness induction, the 42 °N ecotype (Utah A and B) by a long night length for hardiness induction, while the F₁ was intermediate to the parents. Results from reciprocal crosses indicated there was no significant unilateral maternal influence on cold acclimation. Acclimation responses of the F₂ were highly variable but generally ranged between the parental extremes. However, three individuals from the 42 ° × 62 °N crosses exhibited greater cold resistance than the northern parent on two successive freezing test dates. F₂ plants were also found with less freezing resistance than the southern parent. Backcrosses to the southern parent produced progeny with acclimation patterns resembling that of the southern parent and were significantly less hardy than the F₂ in early freezing tests.Scientific Journal Series Paper No. 12,075 of the Minnesota Agricultural Experiment Station     

Development of synthetic Brassica amphidiploids by reciprocal hybridization and comparison to natural amphidiploids

In a previous study we proposed that cytoplasmic genomes have played an important role in the evolution of Brassica amphidiploid species. Based on this and other studies, we hypothesized that interactions between the maternal cytoplasmic genomes and the paternal nuclear genome may cause alterations in genome structure and/or gene expression of a newly synthesized amphidiploid, which may play an important role in the evolution of natural amphidiploid species. To test this hypothesis, a series of synthetic amphidiploids, including all three analogs of the natural amphidiploids B. napus, B. juncea, and B. Carinata and their reciprocal forms, were developed. These synthetic amphidiploids were characterized for morphological traits, chromosome number, and RFLPs revealed by chloroplast, mitochondrial, and nuclear DNA clones. The maternal transmission of chloroplast and mitochondrial genomes was observed in all of the F₁ hybrids examined except one hybrid plant derived from the B. rapa x B. oleracea combination, which showed a biparental transmission of organelles. However, the paternal chloroplast and mitochondrial genomes were not observed in the F₂ progeny. Nuclear genomes of synthetic amphidiploids had combined RFLP patterns of their parental species for all of the nuclear DNA clones examined. A variation in fertility was observed among self-pollinated progenies of single amphidiploids that had completely homozygous genome constitutions. Comparisons between natural and synthetic amphidiploids based on restriction fragment length polymorphism (RFLP) patterns indicated that natural amphidiploids are considerably more distant from the progenitor diploid species than the synthetic amphidiploids. The utility of these synthetic amphidiploids for investigating the evolution of amphidiploidy is discussed.     

INHERITANCE OF NUCLEAR 2C DNA CONTENT VARIATION IN INTRASPECIFIC AND INTERSPECIFIC HYBRIDS OF MICROSERIS (ASTERACEAE)

Nuclear DNA content varies over 20% within the diploid (2n = 18) species M. douglasii and M. bigelovii. Two different intraspecific crosses were made between M. douglasii biotypes which differed by about 10% in 2C nuclear DNA content. The F₂ progeny of one intraspecific cross showed no striking evidence of segregation for DNA content. The mean DNA contents of F₂ progeny from two sister hybrids from the second intraspecific cross were significantly different at the 1% level. An interspecific cross was made between biotypes of M. douglasii and M. bigelovii that differed by approximately 10% in DNA amount. The 12 F₁ progeny did not cluster around the parental midpoint, but instead encompassed nearly the entire range between the parental means. The five families of F₂ progeny studied each had a mean DNA content corresponding to that of the particular F₁ from which they were derived, indicating that the F₁ plants were not of identical DNA content. The results of this study suggest that DNA sequences which account for the DNA content differences among the plants are unstable and can undergo deletion or amplification in a hybrid. The altered DNA content may be heritably stable and show little or no segregation in the F₂ progeny.     

Intergeneric hybridization between Brassica napus and Sinapis pubescens,and the cytology and crossability of their progenies

The cytological possibility of gene transfer from Sinapis pubescens to Brassica napus was investigated. Intergeneric hybrids between Brassica napus (2n = 38) and Sinapis pubescens (2n = 18) were produced through ovary culture. The F₁ hybrids were dihaploid and the chromosome configurations were (0–1) _III + (2–11) _II + (5–24) _I . One F₂ plant with 38 chromosomes was obtained from open pollination of the F₁ hybrid. Thirty-one seeds were obtained from the backcross of the F₂ plant with B. napus. Five out of seven plants had 38 chromosomes, and the pollen stainability ranged from 0% to 81.4%. In the B₂ plants obtained from the backcross of B₁ plants with B. napus, 66.7% of the plants examined had 38 chromosomes. S. pubescens may become a gene source for the improvement of B. napus.     

Inheritance of bipyridyl herbicide resistance in Arctotheca calendula and Hordeum leporinum

The mode of inheritance of resistance to bipyridyl herbicides in bipyridyl-resistant biotypes of Arctotheca calendula and of Hordeum leporinum was investigated. F₁ plants from reciprocal crosses between diquat-resistant and -susceptible plants of A. calendula showed an intermediate response to diquat application that was nuclearly inherited. Treatment of F₂ plants with 100 g ai ha^-1 of diquat or 800 g ai ha^-1 of paraquat killed all homozygous-susceptible plants, caused severe injury to heterozygous plants but only slight or no injury to homozygous-resistant plants. Back crosses of F₁ to susceptible plants exhibited intermediate and susceptible phenotypes. The observed segregation ratios in F₂ and test-cross populations fitted predicted segregation ratios, 1:2:1 (R:I:S) and 1:1 (I:S) respectively, showing that bipyridyl resistance is conferred by a single incompletely-dominant gene. Biotypes of paraquat-resistant and -susceptible H. leporinum were crossed reciprocally. F₁ plants from reciprocal crosses showed an intermediate response to paraquat application. The F₂ progeny showed segregation ratios that fitted the predicted segregation ratio of 1:2:1 (R:I:S) forinheritance of resistance being governed by a single partially-dominant gene.