Original F1 Research Articles

Quantitative Trait Locus Mapping for Fire Blight Resistance in an F2 Population of Malus fusca MAL0045 Uncovers Novel Resistance Loci.

Several fire blight resistance loci in Malus genotypes map on different linkage groups (LGs) representing chromosomes of the domesticated apple. Prior genetics studies primarily focused on F1 populations. A strong resistance quantitative trait locus (QTL) explained up to 66% of phenotypic variance in an F1 progeny derived from crossing the highly resistant wild apple genotype Malus fusca MAL0045 and the highly susceptible apple cultivar 'Idared', which was previously mapped on LG10 (Mfu10) of MAL0045. Strains of the causative bacterial pathogen Erwinia amylovora, notably those that show a single nucleotide polymorphism in the avrRpt2EA effector protein sequence at position 156 (e.g., Ea3049), are more virulent and overcome some known fire blight resistance donors and their QTLs. However, MAL0045 is resistant to Ea3049 and Mfu10 is not overcome, but most of the F1 progeny were highly susceptible to this strain. This phenomenon led to the assumption that other putative resistance factors not segregating in the F1 progeny might be present in the genome of MAL0045. Here, we crossed F1 progeny together to obtain 135 F2 individuals. Facilitated by genotyping-by-sequencing and phenotypic assessments, we identified and mapped two novel resistance QTLs in these F2 individuals on LGs 4 and 15, which were not identified in the F1. To our knowledge, these are the first resistance QTLs mapped in F2 progeny in Malus. In addition, we report that neither MAL0045 nor Mfu10 is broken down by a highly aggressive U.S. strain, LA635, after analyses in the original F1 individuals. [Formula: see text] Copyright © 2023 The Author(s). This is an open access article distributed under the CC BY-NC-ND 4.0 International license.

The dominance model for heterosis explains culm length genetics in a hybrid sorghum variety

Heterosis helps increase the biomass of many crops; however, while models for its mechanisms have been proposed, it is not yet fully understood. Here, we use a QTL analysis of the progeny of a high-biomass sorghum F1 hybrid to examine heterosis. Five QTLs were identified for culm length and were explained using the dominance model. Five resultant homozygous dominant alleles were used to develop pyramided lines, which produced biomasses like the original F1 line. Cloning of one of the uncharacterised genes (Dw7a) revealed that it encoded a MYB transcription factor, that was not yet proactively used in modern breeding, suggesting that combining classic dw1or dw3, and new (dw7a) genes is an important breeding strategy. In conclusion, heterosis is explained in this situation by the dominance model and a combination of genes that balance the shortness and early flowering of the parents, to produce F1 seed yields.

Selección de plantas de papaya sobresalientes en ambientes comerciales con fines de mejoramiento

Por la demanda del mercado, en las regiones productoras de México la variedad dominante es la Maradol, en diferentes niveles de calidad de fruta, ya que el tipo de semilla que se utiliza varía desde las originales F1, hasta selecciones descendientes identificadas como F2 a F5. Lo anterior, conlleva a una degeneración y segregación genotípica. De esta manera es necesaria una adecuada selección de progenitores y polinización controlada para la producción de semilla, y que este adaptada a la región de interés. Por ello, el objetivo fue seleccionar plantas de papaya sobresalientes y adaptadas a la zona productora de Michoacán. Mediante exploraciones de campo en parcelas comerciales, se identificaron y caracterizaron plantas de papaya del tipo Maradol hermafroditas y con características sobresalientes. En las plantas se seleccionaron botones florales y fueron cubiertos para provocar la autopolinización. Los frutos se colectaron en etapa de madurez y se obtuvieron las semillas. Se registraron variables morfológicas de planta y frutos, sanidad y germinación. 12 huertas presentaron plantas sobresalientes. La caracterización del progreso en el desarrollo de botón floral a frutos fue variable, pues se redujo la cantidad de frutos formados con relación a los botones cubiertos inicialmente, así como también, redujo la cantidad de frutos colectados en relación con los frutos formados. En cuanto al estado fitosanitario, se registró la presencia de virus, ácaros y otros insectos en la mayoría de las huertas registradas. La germinación fue aceptable. Se concluyó que se identificaron y caracterizaron plantas de papaya seleccionadas con base a características sobresalientes y adaptadas a la zona productora de Michoacán, para posterior estudio de mejoramiento.

Cornus×elwinortonii and Cornus×rutgersensis (Cornaceae), new names for two artificially produced hybrids of big-bracted dogwoods.

Big-bracted dogwoods (Cornus sp.) are well-known plants in North America and eastern Asia where they occur as wild, generally spring-flowering understory trees. They are also popular ornamental landscape plants, and many economically important cultivars are propagated and sold across North America, Europe, and Asia. Starting in the late 1960s, Elwin Orton of Rutgers University in New Jersey (USA) utilized three geographically disjunct species of dogwoods, Cornus florida (eastern North America), Cornus nuttallii (western North America), and Cornus kousa (East Asia), in an extensive interspecific hybridization program. He was successful in developing the first-ever interspecific F1 hybrids of these species, several of which have become staple items in the ornamental nursery trade due to their enhanced ornamental qualities and resistance to diseases. The original F1 plants are still alive at Rutgers University. While they have been available for decades in horticultural commerce, the interspecific hybrid crosses were never formally described and their scientific hybrid names were never published. For the Cornus kousa × Cornus florida hybrids, the name Cornus ‘rutgersensis’ has been used on occasion in the horticultural trade, but without proper citation and description. Here, it is formally named Cornus × rutgersensis Mattera, T. Molnar, & Struwe, hybr. nov. For the Cornus kousa × Cornus nuttallii hybrids, no previous name has been used, and it is hereby named Cornus × elwinortonii Mattera, T. Molnar, & Struwe, hybr. nov. The need for providing scientific names for commonly used horticultural hybrids is discussed. Holotype material for both hybrid names was collected from the original F1 hybrids for full documentation, typification, and description. The comparative intermediate development of leaves, inflorescence structures, and fruit types of the hybrids and their parents is discussed and illustrated. Etymology, phenology, and cultivation aspects of these hybrids and their cultivars including backcrosses to Cornus kousa are also presented.

Flooding tolerance in maize (Zea mays subsp. mays) F1 hybrids containing a QTL introgressed from teosinte (Zea nicaraguensis)

Worldwide climate change has increased the risk of crop yield losses caused by flooding. Here, we report the development of maize F1 hybrids containing a teosinte QTL for flooding tolerance on chromosome 4 and the evaluation of flooding tolerance under reducing soil conditions. We used F1 hybrid Mi29 × Mi47 (cultivar Yumechikara) and two versions of Yumechikara into which the QTL has been introgressed, Mi47 × IL#18 (heterozygous for the QTL) and IL#18 × BC3#90 (homozygous for the QTL). Flooding tolerance was evaluated at three treatment levels (i.e., soluble starch at 1, 0.5 or 0 g L−1) and the non-flooded control at an early growth stage (2.6- to 3.0-leaf stage) and at the 5.5-leaf stage. Tolerance was assessed by measuring leaf injury, leaf SPAD values, and shoot and root dry weight ratios (treatment/control). The F1 hybrids containing the QTL were significantly more tolerant (as evaluated by leaf injury and SPAD values) than the original F1 cultivar (Mi29 × Mi47) at both growth stages. When flooding tolerance was evaluated by shoot dry weight ratio at an early growth stage, IL#18 × BC3#90 was significantly more tolerant than Mi29 × Mi47 at the 1 and 0 g L−1 treatment levels. At the 5.5-leaf stage, there were no significant differences among the three F1 hybrids under the three treatment levels. Although plants at an early growth stage are generally more sensitive to flooding than at later stages, IL#18 × BC3#90 should overcome flooding stress at the sensitive early stage of growth.

Effects of the C57BL/6 strain background on tauopathy progression in the rTg4510 mouse model

BackgroundCross-breeding of transgenic mice is commonly used to assess gene-gene interactions, particularly in the context of disease. Strain background changes can influence the phenotype of mouse models and can confound crossbreeding studies. We sought to determine if changing the strain background of a commonly used mouse model of tauopathy (rTg4510) would significantly impact the originally reported phenotype. On the original F1 FVB/N x 129S6 background, rTg4510 mice present with progressive cognitive decline, increased insoluble tau, robust tau pathology and age-dependent neurodegeneration. One of the most common strains in mouse modeling is C57BL/6. We and others have previously reported that this strain background alters the phenotypes of various models, including the JNPL3 model of tauopathy. To determine if the phenotype of rTg4510 mice was similarly affected by the introduction of the C57BL/6 background, we compared rTg4510 mice on the original F1 FVB/N x 129S6 background to rTg4510 mice on an F1 FVB/N x C57BL/6NTac (B6/NTac) background, herein termed rTg4510B6.ResultsDespite a small, but significant increase in soluble human tau levels, young rTg4510B6 mice had equivalent levels of tau phosphorylation, aggregation and cognitive impairments as age-matched rTg4510 mice. At 6.5 months of age, rTg4510B6 mice displayed hyperphosphorylated insoluble tau and robust cortical tau neuropathology that was equivalent to age-matched rTg4510 mice; however, 10.5-month-old rTg4510B6 mice had greater amounts of phospho-tau in the cortex and hippocampus when compared to age-matched rTg4510 mice. Non-transgenic (NT) littermates of rTg4510B6 (NTB6) mice also had greater amounts of cortical and hippocampal phospho-tau at 10.5 months of age when compared to NT littermates of rTg4510 mice. Additionally, older rTg4510B6 mice had gross forebrain neurodegeneration that was equivalent to age-matched rTg4510 mice.ConclusionsOverall, our data shows that introduction of the C57BL/6 strain into the rTg4510 mouse background modestly alters the tau pathology that was originally reported in rTg4510 on the F1 FVB/129 background. In contrast, behavioral and neurodegenerative outcomes were not altered. These studies support the use of the rTg4510 mouse model on a partial C57BL/6 strain background without losing fidelity of the phenotype and suggest that the C57BL/6 background does not inherently protect against tauopathy.

Development of EST-SSR markers and construction of a linkage map in faba bean (Vicia faba).

To develop a high density linkage map in faba bean, a total of 1,363 FBES (Faba bean expressed sequence tag [EST]-derived simple sequence repeat [SSR]) markers were designed based on 5,090 non-redundant ESTs developed in this study. A total of 109 plants of a ‘Nubaria 2’ × ‘Misr 3’ F2 mapping population were used for map construction. Because the parents were not pure homozygous lines, the 109 F2 plants were divided into three subpopulations according to the original F1 plants. Linkage groups (LGs) generated in each subpopulation were integrated by commonly mapped markers. The integrated ‘Nubaria 2’ × ‘Misr 3’ map consisted of six LGs, representing a total length of 684.7 cM, with 552 loci. Of the mapped loci, 47% were generated from multi-loci diagnostic (MLD) markers. Alignment of homologous sequence pairs along each linkage group revealed obvious syntenic relationships between LGs in faba bean and the genomes of two model legumes, Lotus japonicus and Medicago truncatula. In a polymorphic analysis with ten Egyptian faba bean varieties, 78.9% (384/487) of the FBES markers showed polymorphisms. Along with the EST-SSR markers, the dense map developed in this study is expected to accelerate marker assisted breeding in faba bean.

AFLP-based bulk segregant analysis for tagging the slow-ripening trait in peach [Prunus persica (L.) Batsch

SummaryFlesh texture is an important quality trait in peach, affecting both fruit firmness and the ripening process. An interesting variation, “slow ripening” (SR), described in some progeny from ‘Fantasia’ nectarine, interferes with the completion of ripening. In these plants, fruit development appears to cease before the end of the cell expansion phase. The SR process was studied at the genetic level using amplified fragment length polymorphisms (AFLPs) combined with bulked segregant analysis (BSA).A marker linked to the SR trait was found at 5.6 cM in a segregating ‘BO 95021’ selfed progeny related to the cultivars ‘Fantasia’ and ‘Fairlane’.The original F1 parent tree was not available at the time of this trial which prevented traditional gene mapping and required the use of BSA. The F1 plant was derived by crossing ‘Fantasia’, a normal melting-flesh (M) nectarine, and ‘BO 7803302’, an SR selection generated by open-pollination of ‘Fairlane’, another M-flesh nectarine, putatively heterozygous for the SR trait. The generally high level of inbreeding in peach could explain the low level of genetic variability observed in our AFLP patterns.

Studying the Immunogenic Properties of Plague Microbe Capsule Antigen F1 Conjugated with Nanoparticles of Colloid Gold and Silver

Influence of nanoparticles of colloid gold (CG) and silver (CS) on immunobiological properties of antigens and possibility of their application as adjuvants for antibody formation stimulation were studied. It was shown that F1 capsule antigen of plague microbe conjugated with nanoparticles of CG and CS possesses more strongly expressed immunogenic properties than original F1 antigen independently of the way of injection. Gold and silver sols with antigens are stable, capable to stimulate specific antibody generation subject to less antigen burden, comfortable for use, do not cause general and local reactions. All these facts give the opportunity to use them in immunologic practice as adjuvants when engineering immunobiological preparations.

Experimental infection of domestic cats with passaged genotype I Bartonella henselae

The influence of in vitro passage on Bartonella henselae pathogenesis in cats has not been thoroughly evaluated. Our objective was to examine the bacterial kinetics and humoral immune responses in cats experimentally infected with three different in vitro passages of B. henselae F1, a genotype I strain of feline origin. The F1 strain was in vitro passaged 20 and 40 times, and each was inoculated into a group of 5 cats. The kinetics of bacteremia and the feline humoral immune response to bacterial antigens were compared to a previous study involving a group of six cats inoculated with the original F1 strain. Among the three groups of cats, the kinetics of bacteremia profiles and the humoral immune responses to B. henselae lysates were similar. The influence of passage on bacterial membrane proteins was examined. In vitro passage altered the expression of 4/17 (23.5%) bacterial membrane proteins and 6/15 (40%) bacterial membrane antigens. An association between poor seroreactivity to three lysate antigens (15-, 18- and 45 kDa), prolonged bacteremia and decreased serum bactericidal activity was noted. Our data show that in vitro passage of B. henselae did not alter the kinetics of bacteremia, including the occurrence of relapsing bacteremia, in experimentally infected cats. This suggests that highly passaged strains may not be suitable for future vaccination studies. Furthermore, in vitro passage results in phenotypic and antigenic changes in the bacterial membrane protein profile, which warrants caution in the interpretation of studies involving passaged B. henselae strains.

Synthesis and preliminary characterization of a new species (amphidiploid) in Cucumis

A successful interspecific hybridization between cucumber (Cucumis sativus L., 2n = 14) and a wild it Cucumis species, C. hystrix Chakr. (2n = 24) was made via embryo rescue. Hybrid plants (2n = 19; 7 from cucumber and 12 from C. hystrix) were sterile, but morphologically uniform. Self-pollination and backcrossing of F1 hybrid plants to either parent confirmed presence of both male- and female-sterility that were likely caused by lack of homology and improper pairing during meiosis. While the multiple-branching habit, densely brown hairs (on corolla and pistil), orange-yellow corolla, and ovate fruit of F1 hybrid plants were similar to that of the C. hystrix parent, the appearance of the first pistillate flower was more similar to that of C. sativus parent. The diameter and internode length of the stem, and the shape and size of leaves and flowers were intermediate when compared to the parents. The chromosome number in the hybrid was doubled through somaclonal variation during embryo culture and regeneration process to restore the fertility. Pollen grains were released and fruits with viable seeds matured on fertile, synthetic amphidiploid plants. The results from flow cytometry indicated that, on average, 7.3% of the morphologically unique regenerants had the 4C DNA content of 2.35 pg relative to the 2C DNA content of the original F1 hybrid at 1.17 pg and, therefore, were likely chromosome-doubled F1 hybrids (2n = 38). Nutrition alanalysis indicated that the synthetic species had higher protein (0.78%)and mineral (0.35%) content compared to the normal pickling cucumber(0.62% and 0.27%, respectively), and could be considered a new Cucumis crop having a special place in the future agriculture. Preliminarily evaluation indicates that C. hystrix possesses a high level of root-knot nematode resistance, and that this resistance is partially expressed in the interspecific F1 and chromosome-doubled F1. This and the fact that the fruit morphology of the fertile amphidiploid differs during the growing season (e.g., short and long fruit) suggest that it could be useful in broadening the germplasm base of cucumber.

The combining ability of recombinant S-lines developed from an F2 maize population

Starting with the F2 generation of the single-cross commercial hybrid Lorena (PR3183), recombinant lines were developed combining half-sib/S1 evaluation on widely spaced plants in the direction of high yielding per se. Combining ability tests consisted of crosses between: (a) recombinant lines of common pedigree and (b) recombinant lines and freely available inbred lines. The highest-yielding crosses between recombinant lines reached 100% of the original F1 hybrid in a percentage of 14·2. Low heterosis was estimated owing to additive gene action of recombinant lines. Crosses between recombinant lines and freely available inbred lines outyielded significantly the commercial F1 hybrid in a percentage of 33·3. Heterosis was greater and the original F1 hybrid was outyielded significantly because of non-additive gene action. When the applied breeding procedure on a source population with high yield adaptability is adopted and where effects of intergenotypic competition masking the inherent genotypic value are controlled, population improvement may be substituted by combined half-sib/S1 selection for productivity of lines per se in low stress conditions during the very early stages.

Evaluation of Rapid Cycle Recurrent Selection for Early Flowering in Oat

More efficient methods of recurrent selection are important to oat (Avena sativa L.) breeding. Although recurrent selection methods have been utilized successfully for the improvement of a number of traits in oat, rapid‐cycle population improvement methods for moderately to highly heritable traits have not been evaluated. Our objectives were (i) to evaluate effectiveness of rapid‐cycle recurrent selection for early flowering, and (ii) to determine the effect of selection for early flowering on grain yield, maturity, and plant height. Phenotypic recurrent selection for early flowering was practiced for four cycles. The scheme requires one generation per cycle and up to three cycles can be completed in 1 yr. From each of the 30 selected plants in all four cycles, a single F2:4 line was derived for replicated testing. All derived lines and the original parents were evaluated at Rosemount and St. Paul, MN, in 1992 and 1993. Analysis of F2:4 lines from the earliest 12 plants per cycle indicated that selection reduced days‐to‐flowering by 1.25 days per cycle. Grain yield and plant height were reduced by 25 and 8%, respectively. Although the selection was effective, this population resulted in an unacceptably high frequency of aneuploid and phenotypically off‐type progeny. Cytological analysis of the original F1 hybrids indicated the presence of chromosomal interchanges between some of the C0 parents. We hypothesize that the absence of a selfing generation in a closed recurrent selection system enhanced chromosomal interchange differences, resulting in a high frequency of off‐types and aneuploids in later cycles. Therefore, if interchanges are present between breeding lines, recurrent selection without selling in oat may not be a useful method for long‐term selection programs.

Identification of Juvenile Hybrids ofMoroneBased on Meristics and Morphometrics

We made meristic and morphometric measurements on juveniles (25–100 mm total length) of hatchery-produced striped bass Morone saxatilis, white bass M. chrysops, original and reciprocal F1 hybrids, original F2 hybrids, and backcrosses with the original F1 female. We compared both sets of measurements with published data and among themselves to delineate progeny differences. Meristic and morphometric values for all hybrids were generally intermediate between those of their parents, but exhibited considerable variation. We examined 11 meristic characters for detection of statistically significant differences (P < 0.05) between crosses that might be used for identification; of these, the numbers of lateral line scales, soft dorsal rays, and gill rakers differed among crosses. We examined 31 morphometric ratios with linear discriminant function tests for statistical differences between crosses. With the six best of these ratios, the overall discriminant analysis resulted in an 83% acceptable classification, and several crosses (16 out of 84 comparisons) were correctly segregated 100% of the time. After meristic differences were used to initially group samples of fish in a blind test, the discriminant function derived from the morphometric ratios allowed individual fish to be properly classified with a high degree of accuracy.

Predicting the properties of second cycle hybrids produced by intercrossing random samples of recombinant inbred lines.

Distributive properties of the second cycle hybrids that are produced by inter-crossing the recombinant inbreds extractable from the F2 of a cross between two pure breeding lines can be predicted from the early generations of the original cross. Hence the frequency of such hybrids that will outperform the extreme recombinant inbreds or the original F1 can be predicted. Basic generations and triple test cross families provide the most reliable estimates of the predictors and therefore should be used whenever possible although, in the presence of linkage, randomly mated F2's may give improved predictions. Simpler experiments consisting of the parental varieties and their F1 and F2 generations, however, provide all the information that is likely to be necessary for most practical purposes. The predictive power of the new approach is demonstrated on material extracted from the cross of varieties 1 and 5 of Nicotiana rustica. The predictors were estimated from the means and variances of V1, V5, F1 and F2 raised in six environments between 1973 and 1983. The predicted frequencies of second cycle F1's which outperform the extreme recombinant inbred lines derived from this cross are compared with those observed among 190 second cycle hybrids in a diallel between 20 recombinant inbreds derived from the same cross.

POLYPLOIDY AND DIPLOIDY: NEW PERSPECTIVES ON CHROMOSOME PAIRING AND ITS EVOLUTIONARY IMPLICATIONS

The last widely accepted classification of polyploidy recognized three major categories: autoploidy, segmental alloploidy, and true alloploidy. Criteria for recognizing these types were based largely on chromosome pairing in F1 hybrids, but until very recently there were no quantitative criteria to distinguish autoploids from segmental alloploids. This is theoretically possible, and Jackson and Casey (1982) and Jackson and Hauber (1982) have presented models and methods to quantitatively predict the frequencies of the different meiotic configurations in autotriploids through autooctoploids. The expected numbers of configurations then can be statistically tested for goodness of fit with observed data. Synthetic and natural autotetraploids have been shown to fit the models well. However, the exceptions that do not fit are of particular interest because they indicate the presence of Ph‐like (Pairing homoeologous) genes that cause fewer multivalents and more bivalents than expected in the autoploid models. The effect of Ph‐like genes at the nuclear level apparently involves placement of chromosome attachment sites on the nuclear membrane such that different genomes occupy slightly different sites. This would be far enough apart in true alloploids so that no pairing occurred in the original F1 hybrid. In true autoploids and normal diploids, homologues are equally close so that pairing obeys probability laws that make it possible to describe expected pairing events with appropriate models and equations. From the foregoing statements, one can deduce that the terms autoploid, segmental alloploid, and true alloploid need not and indeed should not be equated with results of crosses between any taxonomic levels. Rather, they represent conditions that can be attained by one or very few genes acting on nuclear membrane attachment sites. It is possible to deduce from hybrids at the diploid level the type of polyploid that can be obtained, and such data can have great agronomic value. A generally held dogma is that the inability of chromosomes to pair in F1 hybrids is due to considerable genetic divergence and perhaps numerous structural changes at the light microscope and/or ultrastructural level. Data from various sources show that this need not be true, and there is no reason to believe this is a factor in what has been called genome divergence in the classical sense. Genome divergence in terms of pairing ability can occur at the population level due to one or a few genes.

CYTOGENETIC STUDIES ON THE ARTIFICIAL POLYPLOIDS IN THE GENUS &lt;i&gt;ORYZA&lt;/i&gt;.

1. Allohexaploid, minuta-australiensis (2n=72, BBCCEE), was synthesized successfully, treating with 0.05% colchicine the young shoot vegetatively propagated from buds on the node of the F1 hybrid.2. In MI of PMCs, bivalent chromosomes ranging from 29 to 36, and univalents from 0 to 14 were observed. The average chromosome configuration was 35.12II+1.76I, the mode of bivalents and univalents being 36 and 0 respectively. These bivalents paired tightly. More than a half of univalents observed were assumed to be the result of precocious division of bivalents.3. In spite of the homogenetic association occurring exclusively in respective three sets of different genomes of the amphiploid, several meiotic irregularities were observed. Accordingly, the fertilities of pollen and seed were unexpectedly low, being 22.8% and 14.4% respectively. Some unknown cause or causes other than the chromosomal homology must have been responsible for such situation.4. The present amphiploid minuta-australiensis differs from the male-sterile amphiploid sativa-minuta only in its containing E genome instead of A. Therefore, one of the causes of male-sterility of amphiploid sativa-minuta, and perhaps of sativa-latifolia, seems to be found in A genome itself or to be due to the interaction between A and BC or CD genomes. Accordingly, E genome of O. australiensis expressed clearly different effect from A, on the development of anthers in amphiploid level, when brought together with BC genomes.5. Amphiploid, minuta-australiensis, showed the gigas nature in most characters, and it had fewer tillers and set seeds more sparsely. Under a controlled short-day condition it shot the panicle about 10 days later than the original F1.

Studies on histocompatibility mutations in mouse tumour cells using isogenic strains of mice

The feasibility of using heterozygous tumours induced in hybrids between Snell's isogenic resistant (IR) strains of mice for mutation studies was examined. The system has been described by Mitchison (1956) and Klein, Klein &amp; Révész (1957).Fourteen hybrid sarcomas (A × A.SW F1; H-2 genotype a/s) were tested in mice of the parental strains. Seven were specific for the original F1 hybrid genotype and failed to give any variants. Four tumours gave variants that were compatible with one of the parental strains. The variants grew progressively in all mice of the parental strain, even in pre-immunized mice, and failed to grow in mice of foreign genotypes. Of these, one tumour, as. 14 gave true variants towards both the parental strains. One tumour, as.7, gave variants regularly in 35·5% of the A.SW mice but only in 7·7% of pre-immunized A.SW mice. It failed to give variants towards the other parental strain.The effect of triethylenemelamine (TEM) and X-ray treatment on the rate of variant production in hybrid tumours was studied. The effect of the treatment on the viability of tumour cells was first examined by titrating tumour cell suspensions on groups of mice or on the CAMs of groups of developing chick embryos. The TD50 values (50% end-points) were calculated. Comparable results were obtained by titrating on mice and on the CAM, although titration on mice gave more consistent results. Approximately 102 viable, untreated cells were needed to produce a tumour in mice or on the CAM.Tumour as.7 was tested in the parental strains after treatment with TEM or X-rays. Using comparable cell doses in the untreated controls and treated series, the percentage of variants produced in one of the parental strains (A.SW) was significantly increased after treatment with TEM or X-rays. With the exception of one experiment, no variants were produced in the other parental strain. In one experiment, four variants were produced towards the A strain after treatment with TEM, three of which were tested: only one was found to be specific for the A strain. Three of the A.SW variants, originating after treatment with TEM, were tested: two were specific while one showed incomplete specificity for the A.SW strain. Treatment of specific tumours with TEM or X-rays did not give variants towards either of the parental strains.Two of the hybrid tumours and their derived variants were examined cytologically. Variants originating spontaneously and after treatment with TEM were examined. While most of the original tumours had chromosome numbers with an exactly diploid modal number, the spontaneous and TEM-treated variants had modal numbers which were hyperdiploid. The TEM-treated variants also had a larger proportion of cells which were triploid or hypotetraploid. This would explain the incomplete specificity of the variants isolated after treatment with TEM.While it appears possible to use heterozygous tumours for isolating genetical variants specifically compatible with one of the parental strains, the actual nature of the mechanism of variant production remains dubious.

Comparative Performance of Various Combinations of F1 and F2 Generation Seed of Maize1

SynopsisThe presence of 20% or more of F2 female single cross seed in an original F1 double cross hybrid was necessary to obtain a statistically significant reduction in yield. The vigorous F1 plants partly compensate for the loss effected by the F2 plants.