Identification of quantitative trait loci associated with acylsugar accumulation using intraspecific populations of the wild tomato, Lycopersicon pennellii

 Lycopersicon pennellii LA716, a wild relative of tomato, is resistant to a number of insect pests due to the accumulation of acylsugars exuded from type IV trichomes. These acylsugars are a class of compounds including both acylglucoses and acylsucroses. Intraspecific populations between L. pennellii LA716 and L. pennellii LA1912, the latter an accession that assorts for low-level acylsugar accumulation, were created to study the inheritance of type IV trichome density, acylsugar accumulation levels, percentage of acylsugars that are acylglucoses, and leaf area. The F₂ population was subsequently used to determine genomic regions associated with these traits. The relative proportion of acylglucoses and acylsucroses was found to be largely controlled by a single locus near TG549 on chromosome 3. One locus on chromosome 10 showed significant associations with acylsugar levels. In addition, 1 locus on chromosome 4 showed significant associations with leaf area. Ten additional loci showed modest associations with one or more of the traits examined, 5 of which have been previously reported. Received: 13 March 1997 / Accepted: 19 September 1997     

Development of PCR-based markers from the tomato glutamate oxaloacetate transaminase isozyme gene family as a means of revitalising old isozyme markers and recruiting new ones

Sequences annotated as aspartate aminotransferases (synonymous with glutamate oxaloacetate transaminases) in the SOL Genomics Network unigene database were used to design 10 pairs of PCR primers for genetic marker development. These primer pairs generated nine CAPS markers, two SCAR markers and one SSR marker, which were bin-mapped using a set of tomato introgression lines (IL) derived from Lycopersicon esculentum cv. M82 and Lycopersicon pennellii LA716. Based on their bin locations, these markers are largely dispersed throughout the tomato genome and appear to have tagged all four of the glutamate oxaloacetate transaminase (Got) isozyme marker genes placed on the classical genetic map of tomato. Orthologous relationships with Arabidopsis aspartate aminotransferase (Asp) genes suggest the existence of at least two additional functional Got genes in tomato that have also been tagged by these markers and likewise an additional functional Asp gene in Arabidopsis. The Got-2 isozyme marker has often been used for the marker-assisted breeding of the I-3 gene for Fusarium wilt resistance introgressed from L. pennellii LA716. The Got-2 CAPS marker that we have developed offers a facile PCR-based alternative to the isozyme marker for the marker-assisted breeding of I-3. However, all of the PCR-based markers we have developed have the potential to assist the breeding of linked traits introgressed from wild relatives of tomato.     

RFLP mapping of I1, a new locus in tomato conferring resistance against Fusarium oxysporum f. sp. lycopersici race 1

Summary The inheritance and linkage relationships of a gene for resistance to Fusarium oxysporum f. sp. lycopersici race 1 were analyzed. An interspecific hybrid between a resistant Lycopersicon pennellii and a susceptible L. esculentum was backcrossed to L. esculentum. The genotype of each backcross-1 (BC₁) plant with respect to its Fusarium response was determined by means of backcross-2 progeny tests. Resistance was controlled by a single dominant gene, I1, which was not allelic to I, the traditional gene for resistance against the same fungal pathogen that was derived from L. pimpinellifolium. Linkage analysis of 154 molecular markers that segregated in the BC₁ population placed I1 between the RFLP markers TG20 and TG128 on chromosome 7. The flanking markers were used to verify the assignment of the I1 genotype in the segregating population. The results are discussed with reference to the possibility of cloning Fusarium resistance genes in tomato.     

High resolution genetic and physical mapping of the I-3 region of tomato chromosome 7 reveals almost continuous microsynteny with grape chromosome 12 but interspersed microsynteny with duplications on Arabidopsis chromosomes 1, 2 and 3

The tomato I-3 gene introgressed from the Lycopersicon pennellii accession LA716 confers resistance to race 3 of the fusarium wilt pathogen Fusarium oxysporum f. sp. lycopersici. We have improved the high-resolution map of the I-3 region of tomato chromosome 7 with the development and mapping of 31 new PCR-based markers. Recombinants recovered from L. esculentum cv. M82 × IL7-2 F2 and (IL7-2 × IL7-4) × M82 TC1F2 mapping populations, together with recombinants recovered from a previous M82 × IL7-3 F2 mapping population, were used to position these markers. A significantly higher recombination frequency was observed in the (IL7-2 × IL7-4) × M82 TC1F2 mapping population based on a reconstituted L. pennellii chromosome 7 compared to the other two mapping populations based on smaller segments of L. pennellii chromosome 7. A BAC contig consisting of L. esculentum cv. Heinz 1706 BACs covering the I-3 region has also been established. The new high-resolution map places the I-3 gene within a 0.38 cM interval between the molecular markers RGA332 and bP23/gPT with an estimated physical size of 50–60 kb. The I-3 region was found to display almost continuous microsynteny with grape chromosome 12 but interspersed microsynteny with Arabidopsis thaliana chromosomes 1, 2 and 3. An S-receptor-like kinase gene family present in the I-3 region of tomato chromosome 7 was found to be present in the microsyntenous region of grape chromosome 12 but was absent altogether from the A. thaliana genome. Electronic supplementary material  The online version of this article (doi:) contains supplementary material, which is available to authorized users.     

QTL analysis of pest resistance in the wild tomato Lycopersicon pennellii: QTLs controlling acylsugar level and composition

Some accessions of Lycopersicon pennellii, a wild relative of the tomato Lycopersicon esculentum, are resistant to a number of important pests of cultivated tomato due to the accumulation of acylsugars, which constitute 90% of the exudate of type-IV trichomes in L. pennellii LA716. An interspecific F₂ population, created by the cross L. esculentum x L. pennellii LA 716, was surveyed for acylsugar accumulation and subjected to RFLP/QTL analysis to determine the genomic regions associated with the accumulation of acylglucoses, acylsucroses, and total acylsugars, as well as with acylglucoses as a percentage of total acylsugars (mole percent acylglucoses). Data were analyzed using MAPMAKER/QTL with and without a log₁₀ transformation. A threshold value of 2.4 (default value for MAPMAKER/QTL) was used, as well as 95% empirically derived threshold values. Five genomic regions, two on chromosome 2 and one each on chromosomes 3, 4 and 11, were detected as being associated with one or more aspects of acylsugar production. The L. esculentum allele is partially dominant to the L. pennellii allele in the regions on chromosomes 2 and 11, but the L. pennellii allele is dominant in the region on chromosome 3. Throughout this study, we report the comparative effects of analytical methodology on the identification of acylsugar QTLs. Similarities between our results and published results for the genus Solanum are also discussed.R. W. Doerge · S.-C. Liu · J. P. Kuai contributed equally to the paper, and we ordered randomly     

fw 2.2:a major QTL controlling fruit weight is common to both red- and green-fruited tomato species

We have shown that a major QTL for fruit weight (fw2.2) maps to the same position on chromosome 2 in the green-fruited wild tomato species, Lycopersicon pennellii and in the red-fruited wild tomato species, L. pimpinellifolium. An introgression line F₂ derived from L. esculentum (tomato) x L. pennellii and a backcross 1 (BC₁) population derived from L. esculentum x L. pimpinellifolium both place fw2.2 near TG91 and TG167 on chromosome 2 of the tomato highdensity linkage map. fw2.2 accounts for 30% and 47% of the total phenotypic variance in the L. pimpinellifolium and L. pennellii populations, respectively, indicating that this is a major QTL controlling fruit weight in both species. Partial dominance (d/a of 0.44) was observed for the L. pennellii allele of fw 2.2 as compared with the L. esculentum allele. A QTL with very similar phenotypic affects and gene action has also been identified and mapped to the same chromosomal region in other wild tomato accessions: L. cheesmanii and L. pimpinellifolium. Together, these data suggest that fw2.2 represents an orthologous QTL (i.e., derived by speciation as opposed to duplication) common to most, if not all, wild tomato species. High-resolution mapping may ultimately lead to the cloning of this key locus controlling fruit development in tomato.     

Genetics of resistance to Fusarium oxysporum f.sp. cucumerinum races 1 and 2 in cucumber line Wisconsin-2757

The mode of inheritance of resistance to Fusarium oxysporum f.sp. cucumerinum races 1 and 2 in Wisconsin-2757 (WI-2757), a gynoecious cucumber (Cucumis sativus L.), was determined by analysing segregation of F₁, F₂ and BC₁ populations of crosses with susceptible cultivar Straight-8. Resistance to either race 1 or race 2 in WI-2757 was conferred by a single dominant gene. In allelism tests, resistance to either race in WI-2757 was determined by the gene Fcu-1, which also confers resistance in line SMR-18.     

Identification of QTLs controlling acylsugar fatty acid composition in an intraspecific population of Lycopersicon pennellii (Corr.) D’Arcy

 Acylsugars exuded by type IV glandular trichomes are responsible for insect resistances found in many Lycopersicon pennellii accessions. Acylsugars are complex mixtures composed of polyacylated sugars (glucose or sucrose) esterified to branched and straight-chain 4 : 0 to 12 : 0 fatty acids. The biogeneses of these unusual fatty acid constituents have their origins in branched-chain amino acid pathways. However, the mechanism of fatty acid elongation in these systems and the genetic control of carbon flux from amino acid to fatty acid pathways remain unclear. In this study, we used an intraspecific F₂ population derived from the cross between L. pennellii LA716 and L. pennellii LA1912 to examine the genetic basis of acylsugar fatty acid composition. Six QTLs were detected which, combined, explain 23–60% of the variance observed for each of the nine segregating fatty acid constituents. Both correlation data and QTL analysis data indicate that branched medium-chain fatty acids are synthesized through elongation of short-chain precursors in two-carbon increments. The proportion of iso-branched acylsugar fatty acids that have an even-carbon chain length was found to be primarily determined by a single locus that maps to a location 5.5 cM above TG117 on chromosome 8. QTL function in several cases can be inferred from discrete patterns of fatty acid composition; in other cases, control of acylsugar fatty acid composition appears to be complex. Received: 7 April 1998 / Accepted: 28 December 1998     

Inheritance of potato aphid resistance in hybrids between Lycopersicon esculentum and L. pennellii

Summary The potato aphid, Macrosiphum euphorbiae Thomas, is an important pest of tomato, Lycopersicon esculentum Mill., because it transmits tomato viruses and directly reduces crop yields by its feeding. This study was conducted to determine whether the wild tomato species, Lycopersicon pennellii (Corr.) D'Arcy, would be useful as a source of potato aphid resistance for tomato. Type IV trichome density and aphid resistance were assessed in six generations (P₁, P₂, F₁, F₂, BC₁P₁, and BC₁P₂) from crosses between L. pennellii (LA 716) and two tomato cultivars, New Yorker and VF Vendor. Weighted leastsquares were used in joint scaling tests to estimate the relative importance of gene effects on type IV trichome density and potato aphid resistance of the hybrids. A simple additive-dominance model adequately explained the variation in type IV trichome density. Models which included digenic epistatic effects were required to explain the variation in aphid resistance. Standard unit heritability estimates of aphid resistance in the backcross to L. esculentum were obtained by regression of BC₁F₂ off-spring families on BC₁F₁ parents. Regression coefficients and heritability estimates varied between years with the level and uniformity of the aphid infestation. In the 1985–1986 growing seasons, when aphid infestations were uniform, aphid resistance exhibited a moderate level of heritability (29.8% ± 14.1% and 47.1% ± 11.5% in New Yorker and VF Vendor backcross populations, respectively). The non-uniform aphid infestation of 1984 resulted in lower heritability estimates in the 1984–1985 growing seasons (16.1% ± 15.7% and 21.9% ± 14.8% in the New Yorker and VF Vendor backcross populations, respectively). Selection for potato aphid resistance would probably be most efficient if it were delayed until gene combinations are fixed in later generations, because of the large epistatic effects and the low heritability of this trait in seasons with variable aphid infestations.     

High-resolution linkage analysis and physical characterization of the EIX-responding locus in tomato

An ethylene-inducing xylanase (EIX) from Tricohoderma viride is a potent elicitor of ethylene biosynthesis, localized cell death and other defense responses in specific cultivars of tobacco (Nicotiana tabacum) and tomato (Lycopersicon esculentum). Wild species of tomato, such as Lycopersicon cheesmanii and Lycopersicon pennellii, do not respond to EIX treatment. The F₁ progeny of a L. esculentum×L. cheesmanii and a L. esculentum×L. pennellii cross responded to EIX treatment with an increase in ethylene biosynthesis and the induction of localized cell death. The F₂ progeny of the above mentioned crosses segregated 3:1 (responding:non-responding). We mapped the EIX-responding locus (Eix) to the short arm of chromosome 7 using a population of introgression lines (ILs), containing small RFLP-defined chromosome segments of L. pennellii introgressed into L. esculentum. RFLP analysis of 990 F₂ plants that segregated for the introgressed segment mapped the Eix locus 0.1 cM and 0.9 cM from the flanking markers TG61 and TG131, respectively. Using the marker TG61 we isolated a yeast artificial chromosome (YAC) clone that carries 300-kb DNA segments derived from the Eix region. By mapping the ends of this YAC clone we show that it spans the Eix locus. Thus, positional cloning of the Eix locus appears feasible. Received: 20 March 1999 / Accepted: 30 April 1999     

Inheritance and genetic linkage of fusarium wilt {Fusarium oxysporum f.sp. cucumerinum race 1) and scab {Cladosporium cucumerinum) resistance genes in cucumber (Cucumis sativus)

The inheritance of resistance of the cucumber cv. SMR 18 to the race 1 of Fusarium oxysporum f.sp. cucumerinum, the linkage relationship between resistance to race 1 of F. oxysporum f.sp. cucumerinum, resistance to Cladosporium cucumerinum and fruit spine colour, and the reactions of several cucumber cultivars to inoculations with race 1 of F. oxysporum f.sp. cucumerinum and C. cucumerinum were examined. The inbred line Straight 8 (P,), which has white fruit spines and is susceptible to both fusarium wilt and scab was crossed with the inbred line SMR 18 (P₂), which has black fruit spines and resistance to both diseases. When F, F₂, F₃, BC₁P₁ BC₁P₂ and BC₁P₁ selfed progenies were inoculated at the cotyledon stage with a suspension of spores of race 1 of F. oxysporum f.sp. cucumerinum, the ratios of resistant to susceptible plants indicated that resistance was conferred by a single dominant gene, designated Fcu-1. When 171 BC^! plants were selfed and from each resulting F₂ family different groups of 15–25 seedlings each were tested for resistance to either disease, segregation data indicated that the Fcu-1 locus and the Ccu locus for C. cucumerinum resistance were completely linked. No evidence for linkage was found between the Fcu-1 (Ccu) locus and the B locus for fruit spine colour. Among the 59 cultivars tested at the seedling stage, 15 were susceptible, while the remainder were highly resistant to inoculations with both pathogens.     

Chromosomal locations of ten isozyme loci in rice (Oryza sativa L.) through trisomic analysis

Chromosomal locations of 10 isozyme loci in rice (Oryza sativa L.) were determined through trisomic analysis. All 10 genes produced altered allozyme banding patterns in specific F₁ trisomics. This served as the primary source of evidence for chromosome locations ofEst-5, Icd-1, Acp-1, andPgd-1. The locations ofAmp-1, Amp-2, Amp-4, Pox-5, Got-1, andCat-1 were further confirmed from segregation data in BC₁ generations, as the ratios deviated significantly from 1:1 in the critical trisomics but agreed with the expected trisomic ratios. Triallelic heterozygotes were recovered forAmp-1 andAmp-2. On the basis of these dataGot-1, Est-5, andIcd-1 were located to chromosome 1,Amp-1 to chromosome 2,Cat-1 andPox-5 to chromosome 3,Acp-1 to chromosome 6,Amp-2 andAmp-4 to chromosome 8, andPgd-1 to chromosome 11. BecauseAcp-2 andPox-2 are known to be linked withAcp-1, they must also be on chromosome 6. The gene order and recombination values between isozyme loci on chromosomes 3, 6, 8, and 11 are presented.The senior author wishes to acknowledge the financial support from the Chinese government.     

Mapping salt-tolerance genes in tomato (Lycopersicon esculentum) using trait-based marker analysis

The germination responsiveness of an F₂ population derived from the cross Lycopersicon esculentum (UCT5) x L. pennellii (LA716) was evaluated for salt tolerance at two stress levels, 150 mM NaCl + 15 mM CaCl₂ and 200 mM NaCl + 20 mM CaCl₂. Individuals were selected at both tails of the response distribution. The salt-tolerant and salt-sensitive individuals were genotyped at 16 isozyme loci located on 9 of the 12 tomato chromosomes. In addition, an unselected (control) F₂ population was genotyped at the same marker loci, and gene frequencies were estimated in both selected and unselected populations. Trait-based marker analysis was effective in identifying genomic locations (quantitative trait loci, QTLs) affecting salt tolerance in the tomato. Three genomic locations marked by Est-3 on chromosome 1, Prx-7 on chromosome 3, and 6Pgdh-2 and Pgi-1 on chromosome 12 showed significant positive effects, while 2 locations associated with Got-2 on chromosome 7 and Aps-2 on chromosome 8 showed significant negative effects. The identification of genomic locations with both positive and negative effects on this trait suggests the likelihood of recovering transgressive segregants in progeny derived from these parental lines. Similar genomic locations were identified when selection was made either for salt tolerance or salt sensitivity and at both salt-stress treatments. Comparable results were obtained in uni- and bidirectional selection experiments. However, when marker allele gene frequencies in a control population are unknown, bidirectional selection may be more efficient than unidirectional selection in identifying marker-QTL associations. Results from this study are discussed in relationship to the use of molecular markers in developing salt-tolerant tomatoes.     

Inheritance of Arabidopsis DNA in offspring from Brassica napus and A. thaliana somatic hybrids

 Chromosome counts and RFLP markers mapped to Arabidopsis thaliana were used to determine the proportion of eliminated chromosomes and retained A. thaliana DNA in the back-crossed (BC) progeny derived from symmetric and asymmetric somatic hybrids between Brassica napus and A. thaliana. All plants were analysed for the presence of two RFLP markers per chromosome, preferably with one located on each chromosome arm. A reduction in both A. thaliana RFLP markers and chromosome numbers was found in the BC₁ and BC₂ generations of the symmetric hybrids as well as in the BC₁ generation of the asymmetric hybrids. In the symmetric hybrids, two back-crosses to B. napus were required to reduce the frequency of retained A. thaliana loci to 42.4% and mean chromosome number to 39.4. In comparison, the BC₁ progeny of the asymmetric hybrids had 16% of the analysed A. thaliana loci present and an average of 38.4 chromosomes maintained. When the frequency of A. thaliana chromosomes with both analysed loci maintained was compared with the frequency of chromosomes with one locus lost and one kept, a reduction in the number of complete chromosomes between BC₁ and BC₂ derived from the symmetric hybrids was observed. Among the BC₁ plants in the asymmetric group the situation was different, with higher amounts of incomplete donor chromosomes compared to whole chromosomes. The results indicate that A. thaliana chromosome fragments are more often found in the progeny of irradiated hybrids, while back-crossed symmetric hybrids have more complete chromosomes. Received: 2 April 1998 / Accepted: 14 July 1998     

Production and molecular and cytogenetic analyses of euploid (2n = 42) and telocentric addition (2n = 42 + 2t) alloplasmic lines (Hordeum marinum subsp. gussoneanum)-Triticum aestivum

Individual plants from the BC₁F₆ and BC₁F₈ backcross progenies of barley-wheat [H. marinum subsp. gussoneanum Hudson (=H. geniculatum All.) (2n = 28) × T. aestivum L. (2n = 42)] and the BC₁F₆ progeny of their amphiploids were used to obtain alloplasmic euploid (2n = 42) lines L-28, L-29, and L-49 and alloplasmic telocentric addition (2n = 42 + 2t) lines L-37, L-38, and L-50. The lines were examined by genomic in situ hybridization (GISH), microsatellite analysis, chromosome C-banding, and PCR analysis of the mitochondrial 18S/5S repeat. Lines L-29 and L-49 were characterized by substitution of wild barley chromosome 7H¹ for common wheat chromosome 7D. In line L-49, common wheat chromosomes 1B, 5D, and 7D were substituted with homeologous barley chromosomes. Lines L-37, L-38, and L-50 each contained a pair of telocentric chromosomes, which corresponded to barley chromosome arm 7H¹L. All lines displayed heteroplasmy for the mitochondrial 18S/5S locus; i.e., both barley and wheat sequences were found. Original Russian Text ? N.V. Trubacheeva, E.D. Badaeva, I.G. Adonina, L.I. Belova, E.P. Devyatkina, L.A. Pershina, 2008, published in Genetika, 2008, Vol. 44, No. 1, pp. 81–89.     

Breeding tomatoes for salt tolerance: inheritance of salt tolerance and related traits in interspecific populations

Summary Interspecific segregating populations derived from a cross between tomato (Lycopersicon esculentum) cv M82-1 -8 (M82) and the wild species L. pennellii accession LA-716 (Lpen716) were used to study the genetic basis of salt tolerance and its implications for breeding. BC₁ (M82 x (M82 x Lpen716)) and BC₁ S₁ (progenies of selfed BC₁ plants) populations were grown under arid field conditions and irrigated with water having electrical conductivities of 1.5 (control), 10 and 20 dSm^-1. The evaluation of salt tolerance was based on total fruit yield (TY), total dry matter (TD) and TD under salinity relative to the control (RD). Sodium, potassium and chloride concentrations were measured in the leaves and stems. The methods for estimating heritability were adapted to BC₁ plants and BC₁S₁ families. TY, TD and RD had heritability estimates of 0.3–0.45, indicating that salt tolerance can be improved by selection. Genetic correlations between traits indicated that high yield may be combined with salt tolerance and that ion contents are not likely to provide an efficient selection criteria for salt tolerance. Genetic correlations between performances under various salinity levels suggested that similar mechanisms affect the responses to salinity treatments of 10 and 20 dSm^-1. Responses to paper selection confirmed that salt tolerance of the tomato may be improved by selection, and that this selection should be based on dry matter and yield parameters under salinity.Passed away May 1986     

Expression of unilateral incompatibility in pollen of Lycopersicon pennellii is determined by major loci on chromosomes 1, 6 and 10

Summary We have previously described gene introgression from the wild nightshade Solanum lycopersicoides into tomato (Lycopersicon esculentum) through the use of either diploid or sesquidiploid hybrids (the latter consisting of two genomes of L. esculentum and one genome of S. lycopersicoides). Both types of intergeneric hybrids display pollen sterility, but workable ovule fertility. Unilateral incompatibility prevents their direct hybridization with staminate L. esculentum. Pollen of a self-compattible form of the related wild species L. pennellii is compatible with pistils of L. esculentum x S. lycopersicoides hybrids. This trait was backcrossed from L. pennellii to L. esculentum in order to develop bridging lines that could be used to obtain progeny from the intergeneric hybrids and to study the inheritance of bridging ability. In progeny of L. esculentum x S. lycopersicoides hybrids pollinated with L. pennellii-derived bridging lines, preferential transmission of L. pennellii alleles was observed for certain isozyme and RFLP markers on chromosomes 1, 6 and 10. The skewed segregations suggest linkage to three major pollen-expressed compatibility loci. This was confirmed by observations of pollen tube growth, which indicated that compatibility with pistils of the diploid intergeneric hybrid occurred only in bridging lines at least heterozygous for the L. pennellii markers on chromosomes 1, 6 and 10. Compatibility with the sesquidiploid hybrid required only the chromosome 1 and 6 loci, indicating an apparent effect of gene dosage on expression of incompatibility in the pistil. In an F₂ L. esculentum x L. pennellii population, preferential transmission of L. pennellii alleles was observed for the same markers on chromosomes 1 and 10, as well as other markers on chromosomes 3, 11, and 12, but not 6. The chromosome 1 pollen compatibility locus maps to or near the S-locus, which determines S-allele specificity. The results are discussed in relation to existing genetic models for unilateral incompatibility, including the possible involvement of the S-locus.     

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 inheritance of restriction fragment length polymorphisms in the flax rust Melampsora lini

Summary Random cDNA sequences synthesized from poly A⁺ RNA extracted from germinated urediospores of the flax rust fungus, Melampsora lini, were used as probes to detect restriction fragment length polymorphisms (RFLPs) in three races of M. lini originating from cultivated flax, Linum usitatissimum, and one race originating from Australian native flax, L. marginale. Fourteen out of 22 probes tested detected RFLPs in the three races from cultivated flax while 19 of the probes detected polymorphisms between these three races and the race from L. marginale. The segregation of seven RFLPs was determined in a family of 19 F₂ progeny derived from a cross between two of the rust races. With six of these the inheritance was consistent, in each case, with the segregation of alleles at a single locus. Inheritance of the seventh was unusual and an explanation involving two loci with null alleles at each was proposed. No linkage was detected between any of the RFLP loci and nine unlinked loci specifying avirulence.     

Molecular analysis of the extent of asymmetry in two asymmetric somatic hybrids of tomato

Summary Two somatic hybrid plants generated from a single fusion event between Lycopersicon esculentum and irradiated L. pennellii protoplasts have been analyzed at the molecular level. Over 30 loci have been analyzed using isozymes and RFLPs. All loci tested on chromosomes 2–10 were heterozygous, while those loci on chromosome 12 were homozygous L. pennellii in both somatic hybrids. In one of the somatic hybrids, 2850, loci on chromosome 1 were also homozygous L. pennellii. The other somatic hybrid, 28F5, was heterozygous at all chromosome 1 loci tested, but exhibited altered stoichiometry of parental bands as compared to the sexual hybrid. Loci on chromosome 2 from both somatic hybrids have altered stoichiometry, with L. pennellii alleles being four times more abundant than expected. Both somatic hybrids contain the L. esculentum chloroplast genome, while only L. pennellii polymorphisms have been detected in the mitochondrial genome.