Differentiation of Meloidogyne incognita and M. arenaria novel resistance phenotypes in Lycopersicon peruvianum and derived bridge-lines

Lycopersicon peruvianum PI 270435 clone 2R2 and PI 126443 clone 1MH were crossed reciprocally with three L. esculentum-L. peruvianum bridge-lines. The incongruity barrier between the two plant species was overcome; F₁ progeny were obtained from crosses between four parental combinations without embryo-rescue culture. Hybridity was confirmed by leaf and flower morphology and by the production of nematode-resistant F₁ progeny on homozygous susceptible parents. Clones of the five F₁ bridgeline hybrids were highly resistant to Mi-avirulent root-knot nematode (Meloidogyne incognita) at both 25°C and 30°C soil temperatures. However, only clones from PI 270435-3MH and PI 126443-1MH, and hybrids from PI 126443-1MH, were resistant to Mi-virulent M. incognita isolates at high soil temperature. Clones and hybrids from PI 270435-2R2 were not resistant to two Mi-virulent M. incognita isolates at high soil temperature. A source of heat-stable resistance was identified in bridge-line EPP-2, and was found to be derived from L. peruvianum LA 1708. Accessions of the L. peruvianum Maranon races, LA 1708 and LA 2172, and bridge-line EPP-2, segregated for heat-stable resistance to Mi-avirulent M. incognita, but were susceptible to Mi-virulent M. incognita isolates. Clone LA 1708-I conferred heat-stable resistance to M. arenaria isolate W, which is virulent to heat-stable resistance genes in L. peruvianum PI 270435-2R2, PI 270435-3MH, and PI 126443-1MH. Clone LA 1708-I has a distinct heat-stable factor for resistance to Mi-avirulent M. arenaria isolate W, for which the gene symbol Mi-4 is proposed. A Mi-virulent M. arenaria isolate Le Grau du Roi was virulent on all Lycopersicon spp. accessions tested, including those with novel resistance genes.     

Identification of resistance to Meloidogyne javanica in the Lycopersicon peruvianum complex

Clones of Lycopersicon peruvianum PI 2704352R2, PI 270435-3MH and PI 126443-1MH expressed novel resistance to three Mi-avirulent M. javanica isolates in greenhouse experiments. Clones from PI 126443-1MH were resistant to the three M. javanica isolates at 25°C. The three isolates were able to reproduce on one embryorescue hybrid of PI 126443-1MH, but not on three L. peruvianum-L. esculentum bridge-line hybrids of PI 1264431MH when screened at 25°C (Mi-expressed temperature). Clones of PI 270435-2R2 and all its hybrids with susceptible genotypes were resistant to the three M. javanica isolates at 25°C. The bridge-line hybrid EPP-2xPI 2704352R2 was susceptible to M. javanica isolate 811 at 32°C, whereas PI 270435-2R2 and all other hybrids of PI 27043 5-2R2 crossed with susceptible genotypes were resistant at 32°C. At 32°C, one F₂ progeny of PI 126443-IMHxEPP-1, and three test-cross progenies of PI 1264409MHx[PI 270435-3MHxPI 126443-1MH], and reciprocal test-cross progenies of [PI 270435-3MHxPI 2704352R2]xPI 126440-9MH, each segregated into resistant: susceptible (RS) ratios close to 31. The results from the F₂ progeny indicated that heat-stable resistance to Mi-avirulent M. javanica in PI 126443 -1MH is conferred by a single dominant gene. The results from the test-crosses indicated that this gene in PI 126443-1MH is different from the resistance gene in PI 270435-3MH. The resistance gene in PI 270435-3MH was also shown to differ from the resistance factor in PI 270435-2R2. The expression of differential susceptibility and resistance to M. javanica and M. incognita in individual plants of the bridge-line hybrid, embryo-rescue hybrid, F₂, and test-crosses indicated that at least some genes governing resistance to M. javanica differ from the genes conferring resistance to M. incognita. A new source of heat-stable resistance to M. javanica was identified in Lycopersicon chilense.     

Inheritance of heat-stable resistance to Meloidogyne incognita in Lycopersicon peruvianum and its relationship to the Mi gene

Summary The inheritance of heat-stable resistance to the root-knot nematode, Meloidogyne incognita (Kofoid and White) Chitwood, was studied in crosses between different accessions and clones of Lycopersicon peruvianum L. F₁, F₂ and BC₁ generations were evaluated for their index of resistance based on numbers of eggs and infective second-stage juveniles (J₂) per gram of root, and the segregation ratios were determined in experiments carried out at constant soil temperatures of 25 °C and 30 °C. L. peruvianum P.I. 270435 clones 3 MH and 2R2 and P.I. 126443 clone 1 MH, all heatstable resistant, were crossed with L. peruvianum P.I. 126440 clone 9 MH, which is susceptible at both 25 °C and 30 °C. All F₁ progeny were resistant at 25 °C and 30 °C; F₂ and BC₁ generations at 25 °C gave resistant: susceptible (RS) ratios of 151 and 31, respectively, which suggests that resistance is conditioned by two independently assorting genes. However, at 30 °C, RS ratios of 31 and 11 were observed for the F₂ and BC₁ generations, respectively. These results indicate that heat-stable resistance is conferred by a single dominant gene expressed at 30 °C, while the second resistance gene is heat unstable and not expressed at 30 °C. P.I. 270435 clones 2R2 and 3 MH and P.I. 126443 clone 1 MH were crossed with P.I. 128657 clone 3 R4 (source of gene Mi), which is resistant at 25 °C but susceptible at 30 °C. All of the F₁ progeny were resistant at 25 °C and 30 °C.TC₁ progeny of 270435-2 R2 x 128657-3 R4, 270435-3 MH x 128657-3 R4 and 126443-1 MH x 128657-3 R4 crossed with susceptible 126440-9 MH were all resistant at 25 °C and segregated in a 11 ratio at 30 °C. These results also suggest that the heat-stable resistance is monogenic and that it is non-allelic to gene Mi. The non-segregation of TC₁ progenies at 25 °C, suggests that the heat-unstable resistance factor in L. peruvianum P.I. 270435 clones 2 R2 and 3 MH and in P.I. 126443 clone 1 MH is allelic to or the same as gene Mi. We propose the symbol Mi-2 for the gene in P.I. 270435 that confers heat-stable resistance to M. incognita.     

Mapping a novel heat-stable resistance to Meloidogyne in Lycopersicon peruvianum

 The root-knot nematode heat-stable resistance locus from L. peruvianum LA2157 was mapped on chromosome 6. All wild tomato LA2157 entries and the LA2157 S₁ progeny tested were resistant to Mi-avirulent Meloidogyne spp. isolates at 32°C, indicating that the self-compatible accession is homozygous for heat-stable nematode resistance. The novel resistance locus was mapped on a RFLP linkage map; this map was based on a segregating F₂ population obtained from the interspecific F₁ between L. esculentum cv ‘Solentos’ and L. peruvianum LA2157. The inheritance of the heat-stable resistance was evaluated in 100 F₃ lines derived from one F₁ interspecific hybrid. The genotype of the resistance locus of the individual F₂ plants was based on the phenotypic classification of their F₃ lines, and the data were used to map the resistance locus on the arm of chromosome 6 with the closest linkage to TG178. The position of the novel heat-stable resistance of LA2157 was localized in the resistance genes’ cluster close to the location of gene Mi-1. Cuttings of the F₃ lines expressed resistance to Mi-1-avirulent M. incognita and M. javanica biotypes at 25°C and at 32°C (a temperature at which Mi-1 resistance is not expressed). There was no difference in the segregating population for expression of heat-unstable resistance and heat-stable resistance to Mi-1-avirulent Meloidogyne spp. However, LA2157 and cuttings of the above F₃ lines were susceptible to a Mi-1-virulent M. incognita isolate at 30°C and to a M. hapla isolate at 25°C. Received: 6 July 1998 / Accepted: 28 July 1998     

Reproduction of Mi-Virulent Meloidogyne incognita Isolates on Lycopersicon spp.

Selection of detectable numbers of Mi-virulent root-knot nematodes has necessitated a greater understanding of nematode responses to new sources of resistance. During the course of this research, we compared the reproduction of four geographically distinct Mi-virulent root-knot nematode isolates on three resistant accessions of Lycopersicon peruvianum. Each accession carried a different resistant gene, Mi-3, Mi-7, or Mi-8. All nematode isolates were verified as Meloidogyne incognita using diagnostic markers in the mitochondrial genome of the nematode. Reproduction of Mi-virulent isolates W1, 133 and HM, measured as eggs per g of root, was greatest on the Mi-7 carrying accession and least on the Mi-8 carrying accession. In general, Mi-3 behaved similar to the Mi-8 carrying accession. Reproduction of the four nematode isolates was also compared on both Mi and non-Mi-carrying L. esculentum cultivars and a susceptible L. peruvianum accession. Resistance mediated by Mi in L. esculentum still impacted the Mi-virulent nematodes with fewer eggs per g of root on the resistant cultivar (P ≤ 0.05). Preliminary histological studies suggests that Mi-8 resistance is mediated by a hypersensitive response, similar to Mi.     

Resistance in Lycopersicon peruvianum to Isolates of Mi Gene-Compatible Meloidogyne Populations

Root-knot nematode resistance of F₁ progeny of an intraspecific hybrid (Lycopersicon peruvianum var. glandulosum Acc. No. 126443 x L. peruvianum Acc. No. 270435), L. esculentum cv. Piersol (possessing resistance gene Mi), and L. esculentum cv. St. Pierre (susceptible) was compared. Resistance to 1) isolates of two Meloidogyne incognita populations artificially selected for parasitism on tomato plants possessing the Mi gene, 2) the wild type parent populations, 3) four naturally occurring resistance (Mi gene)-breaking populations of M. incognita, M. arenaria, and two undesignated Meloidogyne spp., and 4) a population of M. hapla was indexed by numbers of egg masses produced on root systems in a greenhouse experiment. Artificially selected M. incognita isolates reproduced abundantly on Piersol, but not (P = 0.01) on resistant F₁ hybrids. Thus, the gene(s) for resistance in the F₁ hybrid differs from the Mi gene in Piersol. Four naturally occurring resistance-breaking populations reproduced extensively on Piersol and on the F₁ hybrid, demonstrating ability to circumvent both types of resistance. Meloidogyne hapla reproduced on F₁ hybrid plants, but at significantly (P = 0.01) lower levels than on Piersol.     

Histological response of resistant tomato cultivars to infection of virulent Tunisian root-knot nematode (Meloidogyne incognita) populations

The response of a susceptible tomato cultivar (Solanum lycopersicum cv. Rio Grande) to infection by three populations of root-knot nematode (Meloidogyne incognita) was compared histologically with that of Lycopersicon esculentum cv. Monita, L. esculentum cv. VFN8 and Solanum lycopersicum cv. Nemador possessing the Mi-1 resistance gene and accession PI126443 of L. peruvianum possessing the Mi-3 gene. The resistant cultivars showed susceptibility to the Tunisian Meloidogyne populations. Feeding sites were characterised by the development of giant cells that contained granular cytoplasm and several hypertrophied nuclei. The cytoplasm of giant cells was aggregated along their thickened cell walls and consequently the vascular tissues within galls appeared disrupted and disorganised. Feeding site formed on resistant L. esculentum lines and susceptible cultivar Rio Grande are similar according to cell and nucleus number, and the nurse superficies. Resistant accession L. peruvianum PI126443, known to possess heat-stable nematode resistance, also showed susceptible reaction to Tunisian Meloidogyne incognita populations; however, nematode development was reduced in comparison with susceptible plants and less developed feeding cells were observed.     

Effects of high grafting on tomato plants infected by Meloidogyne incognita and Ralstonia solanacearum

The root‐knot nematode Meloidogyne incognita is known to increase the severity of bacterial wilt in many solanaceous crops. In Japan, several bacterial wilt‐resistant rootstocks that have the M. incognita resistance (Mi) gene in their genome have been developed for tomatoes. In this study, we aimed to examine whether the presence of Mi gene‐breaking M. incognita population affects the development of bacterial wilt in bacterial‐wilt‐resistant tomato rootstocks with Mi in their genetic background. We also aimed to examine the possibility of using high‐grafted tomatoes to control bacterial wilt in plants infected by M. incognita. Our results indicate that the resistance to bacterial wilt was easy to break in usual‐grafted tomato plants infected with M. incognita and that M. incognita enhanced the vertical movement of Ralstonia solanacearum in the bacterial‐wilt‐resistant tomato rootstocks. In addition, our results suggest that high grafting led to significantly less wilting in the plants infected by M. incognita than did usual grafting.     

Mapping a new nematode resistance locus in Lycopersicon peruvianum

Accessions of the wild tomato species L. peruvianum were screened with a root-knot nematode population (557R) which infects tomato plants carrying the nematode resistance gene Mi. Several accessions were found to carry resistance to 557R. A L. peruvianum backcross population segregating for resistance to 557R was produced. The segregation ratio of resistant to susceptible plants suggested that a single, dominant gene was a major factor in the new resistance. This gene, which we have designated Mi-3, confers resistance against nematode strains that can infect plants carrying Mi. Mi-3, or a closely linked gene, also confers resistance to nematodes at 32°C, a temperature at which Mi is not effective. Bulked-segregant analysis with resistant and susceptible DNA pools was employed to identify RAPD markers linked to this gene. Five-hundred-and-twenty oligonucleotide primers were screened and two markers linked to the new resistance gene were identified. One of the linked markers (NR14) was mapped to chromosome 12 of tomato in an L. esculentum/L. pennellii mapping population. Linkage of NR14 and Mi-3 with RFLP markers known to map on the short arm of chromosome 12 was confirmed by Southern analysis in the population segregating for Mi-3. We have positioned Mi-3 near RFLP marker TG180 which maps to the telomeric region of the short arm of chromosome 12 in tomato.     

Retention of Resistance to Meloidogyne incognita in Lycopersicon Genotypes at High Soil Temperature

Lycopersicon glandulosum and L. peruvianum clones and L. esculentum cultivars ''VFN8'' (resistant) and ''Rutgers'' (susceptible) were tested for their resistance to Meloidogyne incognita (race l) at soil temperatures of 25 and 32 C. L. esculentum cv. VFN8 and L. peruvianum Acc. No. 128657, both of which possess the Mi gene, were resistant at 25 C but were susceptible at 32 C. L. glandulosum Acc. No. 126443 and L. peruvianum Acc. No. 270435, with combined resistance to M. hapla and M. incognita, and L. peruvianum Acc. Nos. 129152 and LA2157, with resistance to M. incognita, were highly resistant at both temperatures. In a second experiment three of these accessions under heat stress simulated by 32 C ambient and soil temperature retained a high level of resistance. Two clones of L. glandulosum Acc. No. 126440, with resistance to M. hapla, were moderately susceptible to M. incognita at 25 and highly susceptible at 32 C. M. incognita produced significantly (P = 0.01) more eggs on L. esculentum cv. Rutgers at 32 than at 25 C. This study supports the existence of genes other than the Mi gene that confer resistance to M. incognita and are functional at high soil temperatures.     

Mapping of a Heat-Stable Gene for Resistance to Southern Root-Knot Nematode in Solanum lycopersicum

Root-knot nematodes (Meloidogyne spp.) can cause severe problems in tomato production in warm climates. To date, Mi-1 is the only gene that has been used widely to develop cultivars for controlling disease caused by nematodes around the world. However, Mi-1 does not provide resistance to the pest when the soil temperature is above 28 °C. Tomato breeding line ZN17 has been reported to possess resistance to Meloidogyne incognita at high temperatures (32 °C). To identify markers linked tightly to resistance, an F₂ population was developed by crossing the resistance line ZN17 to susceptible line 09C84. Analysis of F₂ individuals by inoculating M. incognita suggested that resistance in ZN17 is conditioned by a single dominant gene temporarily designated as Mi-HT. Linkage analysis suggested that Mi-HT is located on the short arm of chromosome 6. One marker, W737, co-segregated with Mi-HT. Comparisons of map positions of Mi-HT, Mi-1, and Mi-9, as well as marker genotypes in LA2157, Motelle, and ZN17 suggested that Mi-HT might be a homologue of Mi-1 and Mi-9 or a new gene. The results obtained in this study will facilitate fine-mapping and cloning of the gene as well as marker-assisted breeding for heat-stable resistance to southern root-knot nematodes in tomato.     

Photosystem II function and herbicide binding sites during photoinhibition of spinach chloroplasts in-vivo and in-vitro

The time courses of some Photosystem II (PS II) parameters have been monitored during in-vivo and in-vitro photoinhibition of spinach chloroplasts, at room temperature and at 10 °C or 0 °C. Exposing leaf discs of low-light grown spinach at 25 °C to high light led to photoinhibition of chloroplasts in-vivo as manifested by a parallel decrease in the number of functional PS II centres, the variable chlorophyll fluorescence at 77K (F _v /F _m ), and the number of atrazine-binding sites. When the photoinhibitory treatment was given at 10 °C, the former two parameters declined in parallel but the loss of atrazine-binding sites occurred more slowly and to a lesser extent. During in-vitro photoinhibition of chloroplast thylakoids at 25 °C, the loss of functional PS II centres proceeded slightly more rapidly than the loss of atrazine-binding sites, and this difference in rate was further increased when the thylakoids were photoinhibited at 0 °C. During the recovery phase of leaf discs (up to 9 h) the increases in F _v /F _m preceded that of the number of functional PS II centres, while only a further decline in the number of atrazine-binding sites was observed. The recovery of variable chlorophyll fluorescence and the concentration of functional PS II centres occurred more rapidly at 25 °C than at 10 °C. These results suggest that the photoinhibition of PS II function is a relatively temperature-independent early photochemical event, whereas the changes in the concentration of herbicide-binding sites appear to be a more complex biochemical process which can occur with a delayed time course.Abbreviations BSA bovine serum albumin - Chl chlorophyll - D1 32kDa herbicide-binding polypeptide in photosystem II and product of the psbA gene - D2 34kDa polypeptide in photosystem II which is the product of the psbD gene - DCMU 3-(3,4-dichlorophenyl)-1,1-dimethylurea - DCPIP 2,6-dichlorophenolin-dophenol - F ₀, F _v , F _m chlorophyll fluorescence with reaction centres open, variable and maximum fluorescence, respectively - LDS lithium dodecyl sulfate - MES 2-(N-morpholino) ethanesulfonic acid - PSII photosystem II - Q_A, Q_B first and second quinone-type PS II acceptor, respectively     

Effect of Temperature on Expression of Resistance to Meloidogyne spp. In Common Bean (Phaseolus vulgaris)

The effect of soil temperature on the expression of resistance in several common bean lines carrying resistance to root-knot nematodes (Meloidogyne spp.) was studied under controlled temperatures in temperature tank and growth chamber conditions. Resistance to M. javanica and M. incognita race 1 in bean lines A315, A328, A445, G1805, and G2618 was stable at 24-30 C. However, there was a significant increase in reproduction of M. javanica on A315, A328, and A445 when temperature was increased from 26 to 30 C. This increase did not reflect a change from a resistant to a susceptible reaction or classification. Resistance in A315 is derived from G1805, whereas resistance in A328 and A445 is derived from G2618. Alabama No. 1, PI 165426, and PI 165435, with resistance to M. incognita race 2, were heat stressed at temperatures above 27 C. Resistance to M. incognita race 2 in Alabama No. 1 and PI 165435 was lost at 30 C, but PI 165426 supported low reproduction of M. incognita race 2 at all temperatures. Poor root development at 30 C may have been responsible, in part, for the poor development of M. incognita race 2 on PI 165426.     

Mathematical modeling of mixing in a horizontal rotating tubular bioreactor: “Spiral flow” model

A horizontal rotating tubular bioreactor (HRTB) was designed as a combination of a thin-layer bioreactor and a biodisc reactor whose interior was divided by O-ring shaped partition walls. For the investigation of mixing in HRTB the temperature step method was applied. Temperature changes in the bioreactor were monitored by six Pt-100 sensors (t ₉₀ response time 0.08 s and resolution 0.002 °C) which were connected with an interface unit and a personal computer. In this work a modified tank in series concept was used to establish a mathematical model. The heat balance of the model compartments was established according to the physical model and the spiral flow pattern. Numerical integration was done by the Runge-Kutta-Fehlberg method. The mathematical mixing model called spiral flow model contained four adjustable parameters (N1, Ni, F _cr and F _p) and five parameters which characterized the plant and experimental conditions. The spiral flow model was capable to describe the mixing in HRTB properly, and its applicability was much better than with the simple flow model, presented earlier.List of Symbols A _ui m² ithpart of inner surface of bioreactor's wall - A _vi m² ith part of outlet surface of bioreactor's wall - C _p kJ kg^–1 K^–1 heat capacity of liquid - c _pr kJ kg^–1 K^–1 heat capacity of bioreactor's wall - D h^–1 dilution rate - E °C°C^–1h^–1 error of mathematical model - F _cr dm³ s^–1 circulation flow in the model - F _p dm³s^–1 back flow in the model - F _t dm³ s^–1 inlet flow in bioreactor - I °C intensity of temperature step, the difference in temperature between the temperature of the inlet liquid flow and the temperature of liquid in bioreactor before temperature step - K1 Wm^–2 K ^–1 heat transfer coefficient between the liquid and bioreactor's wall - K2 Wm^–2 K ^–1 heat transfer coefficient between the bioreactor's wall and air - L m length of bioreactor - m _s kg mass of bioreactor's wall - n min^–1 rotational speed of bioreactor - n _s number of temperature sensors - N1 number of cascades - Ni number of compartments inside cascade - r _u m inner diameter of bioreactor - r _v m outside diameter of bioreactor - s(t) step function - t s time - T °C temperature - T _c °C calculated temperature - T _m °C measured temperature - T _N 1,Ni°C temperature of liquid in defined compartments inside the cascade - T _N 1,S°C temperature of defined part of bioreactor's wall - T _N i,z°C temperature of surrounding air - V _t dm³ volume of liquid in the bioreactor     

Exchange of HCO 3 − for monovalent anions across the human erythrocyte membrane

Summary A stopped-flow rapid reaction apparatus was used for measuring changes in extracellular pH (pH _o ) of red cell suspensions under conditions wheredpH _o /dt was determined by the rate of HCO ₃ ^– /X ^– exchange across the membrane (X ^–=Cl^–, Br^–, F^–, I^–, NO ₃ ^– or SCN^–). The rate of the exchange at 37°C decreased forX ^– in the order: Cl^–>Br^–>F^–>I^–>NO ₃ ^– >SCN^–, with rate constants in the ratios 10.860.770.550.520.31. When HCO ₃ ^– is exchanged for Cl^–, Br^–, F^–, NO ₃ ^– or SCN^–, a change in the rate-limiting step of the process takes place at a transition temperature (T _T ) between 16 and 26°C. In I^– medium, however, no transition temperature is detected between 3 and 42°C. AlthoughT _T varies withX ^–, the activation energies both above and belowT _T are similar for Cl^–, Br^–, NO ₃ ^– and F^–. The values of activation energy are considerably higher whenX ^–=I^– or SCN^–. The apparent turnover numbers calculated for HCO ₃ ^– /X ^– exchange (except forX ^–=I^–) at the correspondingT _T ranged from 140 to 460 ions/site ·sec for our experimental conditions. These findings suggest that: (i) HCO ₃ ^– /X ^– exchange for allX ^– studied takes place via the rapid anion exchange pathway; (ii) the rate of HCO ₃ ^– /X ^– exchange is influenced by the specific anions involved in the 11 obligatory exchange; and (iii) the different transition temperatures in the Arrhenius diagrams of the HCO ₃ ^– /X ^– exchange do not seem to be directly related to a critical turnover number, but may be dependent upon the influence ofX ^– on protein-lipid interactions in the red blood cell membrane.     

Comparative mapping in F2∶3 and F6∶7 generations of quantitative trait loci for grain yield and yield components in maize

This study was conducted to compare maize quantitative trait loci (QTL) detection for grain yield and yield components in F₂₃ and F₆₇ recombinant inbred (RI) lines from the same population. One hundred and eighty-six F₆₇ RIs from a Mo17×H99 population were grown in a replicated field experiment and analyzed at 101 loci detected by restriction fragment length polymorphisms (RFLPs). Single-factor analysis of variance was conducted for each locus-trait combination to identify QTL. For grain yield, 6 QTL were detected accounting for 22% of the phenotypic variation. A total of 63 QTL were identified for the seven grain yield components with alleles from both parents contributing to increased trait values. Several genetic regions were associated with more than one trait, indicating possible linked and/or pleiotropic effects. In a comparison with 150 F₂₃ lines from the same population, the same genetic regions and parental effects were detected across generations despite being evaluated under diverse environmental conditions. Some of the QTL detected in the F₂₃ seem to be dissected into multiple, linked QTL in the F₆₇ generation, indicating better genetic resolution for QTL detection with RIs. Also, genetic effects at QTL are smaller in the F₆₇ generation for all traits.Abbreviations RFLPs Restriction fragment length polymorphisms - QTL quantitative trait loci - RIs recombinant inbreds Journal Paper no. J-16261 of the Iowa Agric and Home Economics Exp Stn Project no. 3134     

High resolution RFLP map around the root knot nematode resistance gene (Mi) in tomato

Summary In the 1940's the root-knot nematode resistance gene (Mi) was introgressed into the cultivated tomato from the wild species, L. peruvianum, and today it provides the only form of genetic resistance against this pathogen. We report here the construction of a high resolution RFLP map around the Mi gene that may aid in the future cloning of this gene via chromosome walking. The map covers the most distal nine map units of chromosome 6 and contains the Mi gene, nine RFLP markers, and one isozyme marker (Aps-1). Based on the analysis of more than 1,000 F₂ plants from four crosses, we were able to pinpoint the Mi gene to the interval between two of these markers — GP79 and Aps-1. In crosses containing the Mi gene, this interval is suppressed in recombination and is estimated to be 0.4 cM in length. In contrast, for a cross not containing Mi, the estimated map distance is approximately 5 times greater (ca. 2 cM).Using RFLP markers around Mi as probes, it was possible to classify nematode resistant tomato varieties into three types based on the amount of linked peruvianum DNA still present. Two of these types (representing the majority of the varieties tested) were found to still contain more than 5 cM of peruvianum chromosome — a result that may explain some of the negative effects (e.g. fruit cracking) associated with nematode resistance. The third type (represented by a single variety) is predicted to carry a very small segment of peruvianum DNA (<2 cM) and may be useful in the identification of additional markers close to Mi and in the orientation of clones during a chromosome walk to clone the gene.     

Genetic resistance of CBA and a mice to transplanted lymphoid and hemopoietic cells of CBA.M523 mutants and their F1 hybrids

(CBA × M523)F₁, (A × M523)F₁ and M523 lymphocytes grafted into lethally irradiated CBA or A mice temporarily lose their capacity to respond to test antigens (SRBC, Vi-antigenS. typhi). Immunoresponsiveness of F₁ cells is affected to a lesser degree in lethally irradiated M523 mice. Depression of response is absent in the CBA F₁ combination, in the syngeneic combination and in CBA mice which have received transplanted cells from F₁ hybrids which do not share theM523 mutation. The number of hemopoietic (CBA × M523)F₁ colonies was also reduced in CBA mice. Resistance of CBA mice to lymphoid (CBA × M523)F₁ cells develops 18 days after birth. It can be reduced by additional recipient preirradiation or preinoculation with (CBA × M523)F₁ spleen cells. The abrogated resistance can be partially restored by CBA spleen cells. The activity of (CBA × M523)F₁ lymphocytes passaged through CBA spleen is restored in syngeneic F₁ secondary recipients but inhibited again in the CBA secondary recipients. These results are consistent with the suggestion that resistance of lethally irradiated CBA mice to hemopoietic and lymphoid (CBA × M523)F₁ cells is mediated by immunologically competent, radioresistant recipient cells rapidly reacting to transplantation antigens coded by the mutantH-2K ^ka allele. These cells temporarily suppress the functional activity of transplanted cells but do not eliminate them.