F2-derived Families Research Articles

Mapping Quantitative Trait Loci (QTL) for Resistance to Late Blight in Tomato.

Late blight caused by Phytophthora infestans (Montagne, Bary) is a devastating disease of tomato worldwide. There are three known major genes, Ph-1, Ph-2, and Ph-3, conferring resistance to late blight. In addition to these three genes, it is also believed that there are additional factors or quantitative trait loci (QTL) conferring resistance to late blight. Precise molecular mapping of all those major genes and potential QTL is important in the development of suitable molecular markers and hence, marker-assisted selection (MAS). The objective of the present study was to map the genes and QTL associated with late blight resistance in a tomato population derived from intra-specific crosses. To achieve this objective, a population, derived from the crossings of NC 1CELBR × Fla. 7775, consisting of 250 individuals at F2 and F2-derived families, were evaluated in replicated trials. These were conducted at Mountain Horticultural Crops Reseach & Extension Center (MHCREC) at Mills River, NC, and Mountain Research Staion (MRS) at Waynesville, NC in 2011, 2014, and 2015. There were two major QTL associated with late blight resistance located on chromosomes 9 and 10 with likelihood of odd (LOD) scores of more than 42 and 6, explaining 67% and 14% of the total phenotypic variation, respectively. The major QTLs are probably caused by the Ph-2 and Ph-3 genes. Furthermore, there was a minor QTL on chromosomes 12, which has not been reported before. This minor QTL may be novel and may be worth investigating further. Source of resistance to Ph-2, Ph-3, and this minor QTL traces back to line L3707, or Richter’s Wild Tomato. The combination of major genes and minor QTL may provide a durable resistance to late blight in tomato.

Participatory tomato breeding for organic conditions in Italy

Participatory plant breeding (PPB) has been proposed as a breeding methodology to increase/optimize the development and adoption of several new varieties each one different and each adapted to individual microclimates. Tomato is an important horticultural crop in Europe, and in particular in Italy, and there is an increasing demand for organic tomatoes. Therefore, a PPB program was considered as the ideal approach to rapidly develop and deliver in each organic microclimate, a number of different tomato lines directly selected under organic conditions. Visual selection was applied by farmers and researchers in four F2 populations and in the F3 families obtained from selected F2 plants grown in unreplicated row and column designs in five locations. The agronomic performance of the F2-derived F4 families at the end of two cycles of selection was evaluated by comparing the selected materials with commercial hybrids. Production traits and qualitative plant and fruit traits (plant vigor and hardiness, disease resistance, fruit quality) were analysed. A total of 15 F4 families was identified, of which three out-yielded significantly the respective commercial F1 hybrid, and twelve did not differ significantly from it. The putative superior varieties that are being developed from the F4 families represent important genetic material for organic agriculture, since they would assure good productive capacity and a seed availability at low cost.

Mapping Quantitative Trait Loci for Powdery Mildew Resistance in Flax (Linum usitatissimum L.)

ABSTRACTPowdery mildew (Oidium lini Skoric, [PM]), an obligate biotrophic ascomycete, is a common and widespread foliar disease of flax in most flax‐growing areas of the world. In this study, we constructed a linkage map using 143 simple sequence repeat (SSR) markers and an F2 population of 300 individuals generated from a cross between the susceptible cultivar NorMan and the resistant cultivar Linda. The F2–derived F3 families were phenotyped in the field and the F3–derived F4 families were phenotyped in a controlled environment growth chamber. The 15 linkage group map covered 1241 cM and was largely collinear with the previously published consensus map. Quantitative trait loci (QTL) analysis was performed and three PM resistance QTL located on LG1, 7, and 9 were identified consistently using phenotyping data from both field and growth chamber studies. These QTL explained 97% of the phenotypic variation exhibiting a mainly dominant gene action. This work represents a first step toward understanding the genetics of PM resistance in flax and map‐based cloning of candidate genes underlying the QTL.

Powdery mildew resistance in soybean PI 243540 is controlled by a single dominant gene

Powdery mildew (Microsphaera diffusa Cooke & Peck) is a common disease of soybean in many countries of the world, including the northern United States of America and parts of Canada. The genetic resistance of soybean to M. diffusa is known to be controlled by a single locus with three alleles designated as Rmd, Rmd-c and rmd. Identification and characterization of sources of resistance is a prerequisite for the development of resistant cultivars. The objective of this study was to determine the inheritance of powdery mildew resistance in a plant introduction (PI) from Japan, PI 243540. The inheritance of powdery mildew was determined in a segregating population from a cross between powdery mildew susceptible Ohio cultivar Wyandot and PI 243540. The parents and the progeny showed a consistent response to powdery mildew for all growth stages of plants. The two parents, the F1, F2, and F2:3 families from the cross were screened in a greenhouse and field following inoculation with M. diffusa. All F1 plants were resistant to M. diffusa and χ2 analysis for segregation in the population of 343 F2 plants indicated a tight fit for a 3:1 (P = 0.78) ratio, indicative of a single dominant gene. In the next generation, the 334 F2-derived families fit an expected 1 resistant:2 segregating:1 susceptible segregation ratio (P =0.88), which confirmed the results obtained in the F2 generation. Our results indicate that the powdery mildew resistance derived from PI 243540 is controlled by a single dominant gene linked to the Rmd/ Rmd-c/rmd locus. The simple inheritance of this gene should make it relatively easy to find linked DNA markers and transfer the gene to susceptible elite cultivars using the backcross breeding approach.

Identification and genetics of resistance to cercospora leaf spot (Cercospora zonata) in faba bean (Vicia faba)

The fungal disease cercospora leaf spot CLS (Cercospora zonata) has affected major faba bean (Vicia faba) production regions in southern Australian in the last several years. This study offers the first report of sources of resistance to CLS in faba bean and describes techniques to evaluate resistance to C. zonata in faba bean genotypes within a controlled environment. The method was rapid (43 days), repeatable (R 2 > 0.74) and demonstrated positive correlations (R 2 > 0.45–0.80) to data collected from field disease nurseries under naturally established CLS epiphytotics. All faba bean cultivars currently adopted by the Australian industry were found to be susceptible to CLS and defoliation was found to be an important component of disease expression. Genetic analysis of segregation patterns in F 2 derived F 3 families of 1322/2*Farah (resistant*susceptible) showed the mode of inheritance of resistance to C. zonata was monogenic dominant. F 3 families were shown to segregate in the ratio of 1:2:1 for homozygous resistant: heterozygous: homozygous susceptible (χ 2 2 = 2.78; P > 0.05) and individual plants within heterozygous F 3 families segregated in the ratio of 3:1 for resistant: susceptible responses (χ 1 2 = 2.93; P > 0.05). Monogenic dominant inheritance also explained the change in frequency of resistant and susceptible plants within a population of cv. Cairo following one generation of self-pollination (χ2 = 0.88, 0.3 < P < 0.5). The sources of resistance identified in this study are being used to transfer CLS resistance to adapted faba bean genotypes for future cultivar releases to the southern Australian industry.

QTLs associated with resistance in soybean PI567516C to synthetic nematode population infecting cv. Hartwig

Worldwide, soybean cyst nematode (SCN, Heterodera glycines Ichinohe) is the most destructive pathogen of soybean [Glycine max (L.) Merr.]. Crop losses are primarily mitigated by the use of resistant cultivars. Nematode populations are variable and have adapted to reproduce on resistant cultivars over time because resistance primarily traces to two soybean accessions, Plant Introduction (PI) 88788 and Peking. Soybean cultivar Hartwig, derived primarily from PI437654, was released for its comprehensive resistance to most SCN populations. A synthetic nematode population (LY1) was recently selected for its reproduction on Hartwig. The LY1 nematode population currently infects known sources of resistance except soybean PI567516C; however, the resistance to LY1 has not been characterized. The objective of this study was to identify quantitative trait loci (QTLs) underlying resistance to the LY1 SCN population in PI567516C, identify diagnostic DNA markers for the LY1 resistance, and confirm their utility for marker-assisted selection (MAS). Resistant soybean line PI567516C was crossed to susceptible cultivar Hartwig to generate 105 recombinant inbred lines (F2-derived F5 families). QTLs were mapped using simple sequence repeats (SSRs) covering 20 Linkage Groups (LGs) and three diagnostic markers, Satt592, Satt331, and Sat_274, were identified on LG O. These markers have a combined efficacy of 90% in identifying resistant lines in a second cross that has been generated by crossing a susceptible cultivar 5601T with resistant PI567516C. F2-derived F4 segregating population was used in MAS to identify resistant lines.

Quantitative analysis and QTL mapping for agronomic and fiber traits in an RI population of upland cotton

Genetic mapping is an essential tool for cotton (Gossypium hirsutum L.) molecular breeding and application of DNA markers for cotton improvement. In this present study, we evaluated an RI population including 188 RI lines developed from 94 F2-derived families and their two parental lines, ‘HS 46’ and ‘MARCABUCAG8US-1-88’, at Mississippi State, MS, for two years. Fourteen agronomic and fiber traits were measured. One hundred forty one (141) polymorphic SSR markers were screened for this population and 125 markers were used to construct a linkage map. Twenty six linkage groups were constructed, covering 125 SSR loci and 965 cM of overall map distance. Twenty four linkage groups (115 SSR loci) were assigned to specific chromosomes. Quantitative genetic analysis showed that the genotypic effects accounted for more than 20% of the phenotypic variation for all traits except fiber perimeter (18%). Fifty six QTLs (LOD > 3.0) associated with 14 agronomic and fiber traits were located on 17 chromosomes. One QTL associated with fiber elongation was located on linkage group LGU01. Nine chromosomes in sub-A genome harbored 27 QTLs with 10 associated with agronomic traits and 17 with fiber traits. Eight chromosomes in D sub-genome harbored 29 QTLs with 13 associated with agronomic traits and 16 with fiber traits. Chromosomes 3, 5, 12, 13, 14, 16, 20, and 26 harbor important QTLs for both yield and fiber quality compared to other chromosomes. Since this RI population was developed from an intraspecific cross within upland cotton, these QTLs should be useful for marker assisted selection for improving breeding efficiency in cotton line development.

Quantitative Trait Loci for High-Temperature Adult-Plant and Slow-Rusting Resistance to Puccinia striiformis f. sp. tritici in Wheat Cultivars

Stripe rust, caused by Puccinia striiformis f. sp. tritici, is one of the most damaging diseases of wheat (Triticum aestivum) globally. High-temperature adult-plant resistance (HTAPR) and slow-rusting have great potential for sustainable management of the disease. The wheat cultivars Luke and Aquileja have been previously reported to possess HTAPR and slow-rusting to stripe rust, respectively. Aquileja displayed less number of stripes per unit leaf area than Luke, while Luke showed lower infection type than Aquileja at adult-plant stages of growth under high-temperature conditions. The objectives of this study were to confirm the resistances and to map the resistance genes in Luke and Aquileja. Luke was crossed with Aquileja, and 326 of the F(2) plants were genotyped using 282 microsatellite primer pairs. These F(2) plants and their derived F(3) families were evaluated for resistance to stripe rust by inoculation in the fields and greenhouses of high- and low-temperatures. Infection type was recorded for both seedlings and adult plants, and stripe number was recorded for adult plants only. Two quantitative trait loci (QTL) were identified, on the short arm of chromosome 2B, to be significantly associated with infection type at adult-plant stages in the fields and in the high-temperature greenhouse. The locus distal to centromere, referred to as QYrlu.cau-2BS1, and the locus proximal to centromere, referred to as QYrlu.cau-2BS2, were separated by a genetic distance of about 23 cM. QYrlu.cau-2BS1 was flanked by the microsatellite markers Xwmc154 and Xgwm148, and QYrlu.cau-2BS2 was flanked by Xgwm148 and Xabrc167. QYrlu.cau-2BS1 and QYrlu.cau-2BS2 explained up to 36.6 and 41.5% of the phenotypic variation of infection type, respectively, and up to 78.1% collectively. No significant interaction between the two loci was detected. Another QTL, referred to as QYraq.cau-2BL, was detected on the long arm of chromosome 2B to be significantly associated with stripe number. QYraq.cau-2BL was flanked by the microsatellite markers Xwmc175 and Xwmc332, and it explained up to 61.5% of the phenotypic variation of stripe number. It is possible that these three QTL are previously unmapped loci for resistance to stripe rust.

Locations of Quantitative Trait Loci Conferring Russian Wheat Aphid Resistance in Barley Germplasm STARS‐9301B

Russian wheat aphid (RWA, Diuraphis noxia Kurdjumov) infestations reduce grain yield and quality and have caused more than $1 billion in losses for barley (Hordeum vulgare L.) and wheat (Triticum aestivum L.) in the western United States since 1986. Our objective was to map quantitative trait loci (QTLs) conferring resistance to RWA feeding damage in the germplasm line STARS‐9301B via polymerase chain reaction–based marker assays of 191 F2–derived F3 families from the cross ‘Morex’/STARS‐9301B. Morex is a susceptible six‐rowed malting barley. STARS‐9301B is a selection from RWA‐resistant Afghanistan introduction PI366450. Replicated seedling reactions to RWA infestations were used to phenotype each family based on a 1 to 9 visual rating of chlorosis. A total of 107 molecular markers was used to construct a linkage map. Quantitative trait loci analysis identified two major QTLs for resistance. The QTL on the short arm of chromosome 1H was associated with B‐hordein and explained 26% of the variation for RWA reaction. A QTL on chromosome 3H associated with EBmac0541 explained 38% of the variation. A minor QTL on chromosome 2H was associated with marker GBM1523 and explained 6% of the variation. A combined analysis indicated that the marker–QTL associations explained 59% of the phenotypic variation for RWA resistance. These markers linked with QTLs will be valuable in breeding for RWA resistance. Pyramiding the genes from STARS‐9301B with genes from other sources will be helpful for long‐term protection against RWA in barley.

Identification and mapping of pm2026: a recessive powdery mildew resistance gene in an einkorn (Triticum monococcum L.) accession

Triticum monococcum accession TA2026 showed resistance to wheat powdery mildew. To identify the resistance gene and transfer it to common wheat, genetic analysis and molecular mapping were conducted using an F2 population and derived F3 families from the cross of TA2026xM389. The results indicated that TA2026 possessed a recessive powdery mildew resistance gene. This gene was mapped to the terminal portion of chromosome 5AmL and flanked by SSR marker loci Xcfd39 and Xgwm126. Eight RFLP markers previously mapped to the terminal chromosome 5AmL were converted into STS markers. Three loci, detected by MAG1491, MAG1493 and MAG1494, the STS markers derived from RFLP probes CDO1312, PSR164 and PSR1201, respectively, were linked to this resistance gene with Xmag1493 only 0.9 cM apart from it. In addition, the STS marker MAG2170 developed from the tentative consensus wheat cDNA encoding the Mlo-like protein identified a locus co-segregating with Xmag1493. This is the first recessive powdery mildew resistance gene identified on chromosome 5Am, and is temporarily designated pm2026. We have successfully transferred it to a tetraploid background, and this resistance stock will now be used as the bridge parent for its transfer to common wheat.

TheMi-9Gene fromSolanum arcanumConferring Heat-Stable Resistance to Root-Knot Nematodes Is a Homolog ofMi-1

Resistance conferred by the Mi-1 gene from Solanum peruvianum is effective and widely used for limiting root-knot nematode (Meloidogyne spp.) yield loss in tomato (Solanum lycopersicum), but the resistance is ineffective at soil temperatures above 28 degrees C. Previously, we mapped the heat-stable resistance gene Mi-9 in Solanum arcanum accession LA2157 to the short arm of chromosome 6, in a genetic interval as Mi-1 and the Cladosporium fulvum resistance gene Cf2. We developed a fine map of the Mi-9 region by resistance and marker screening of an F2 population and derived F3 families from resistant LA2157 x susceptible LA392. Mi-1 intron 1 flanking primers were designed to amplify intron 1 and fingerprint Mi-1 homologs. Using these primers, we identified seven Mi-1 homologs in the mapping parents. Cf-2 and Mi-1 homologs were mapped on chromosome 6 using a subset of the F2. Cf-2 homologs did not segregate with Mi-9 resistance, but three Mi-1 homologs (RH1, RH2, and RH4) from LA2157 and one (SH1) from LA392 colocalized to the Mi-9 region. Reverse transcriptase-polymerase chain reaction analysis indicated that six Mi-1 homologs are expressed in LA2157 roots. We targeted transcripts of Mi-1 homologs for degradation with tobacco (Nicotiana tabacum) rattle virus (TRV)-based virus-induced gene silencing using Agrobacterium infiltration with a TRV-Mi construct. In most LA2157 plants infiltrated with the TRV-Mi construct, Mi-9-mediated heat-stable root-knot nematode resistance was compromised at 32 degrees C, indicating that the heat-stable resistance is mediated by a homolog of Mi-1.

Tracing B-genome chromatin in Brassica napus × B. juncea interspecific progeny

We used polymerase chain reaction (PCR) and fluorescence in situ hybridization (FISH) techniques to demonstrate the presence of Brassica B-genome chromosomes and putative B-genome introgressions in B. napus x B. juncea interspecific progeny. The B-genome--specific repeat sequence pBNBH35 was used to generate PCR products and FISH probes. The highest frequencies of viable progeny were obtained when B. napus was the maternal parent of the interspecific hybrid and the first backcross. B-genome--positive PCR assays were found in 34/51 fertile F2 progeny (67%), which was more than double the proportion found in fertile BC(1) progeny. Four B-genome--positive F(2)-derived families and 1 BC(1)-derived family were fixed or segregating for B. juncea morphology in the F(4) and BC(1)S(2), respectively, but in only 2 of these families did B. juncea-type plants exhibit B. juncea chromosome count (2n = 36) and typical B-genome FISH signals on 16 chromosomes. The remaining B. juncea-type plants had B. napus chromosome count (2n = 38) and no B-genome FISH signals, except for 1 exceptional F(4)-derived line that exhibited isolated and weak B-genome FISH signals on 11 chromosomes and typical A-genome FISH signals. B. juncea morphology was associated with B-genome--positive PCR signals but not necessarily with 16 intact B-genome chromosomes as detected by FISH. B-genome chromosomes tend to be eliminated during selfing or backcrossing after crossing B. juncea with B. napus, and selection of lines containing B-genome chromatin during early generations would be promoted by use of this B-genome repetitive marker.

Mapping QTLs and QTL × environment interaction for CIMMYT maize drought stress program using factorial regression and partial least squares methods

The study of QTL x environment interaction (QEI) is important for understanding genotype x environment interaction (GEI) in many quantitative traits. For modeling GEI and QEI, factorial regression (FR) models form a powerful class of models. In FR models, covariables (contrasts) defined on the levels of the genotypic and/or environmental factor(s) are used to describe main effects and interactions. In FR models for QTL expression, considerable numbers of genotypic covariables can occur as for each putative QTL an additional covariable needs to be introduced. For large numbers of genotypic and/or environmental covariables, least square estimation breaks down and partial least squares (PLS) estimation procedures become an attractive alternative. In this paper we develop methodology for analyzing QEI by FR for estimating effects and locations of QTLs and QEI and interpreting QEI in terms of environmental variables. A randomization test for the main effects of QTLs and QEI is presented. A population of F2 derived F3 families was evaluated in eight environments differing in drought stress and soil nitrogen content and the traits yield and anthesis silking interval (ASI) were measured. For grain yield, chromosomes 1 and 10 showed significant QEI, whereas in chromosomes 3 and 8 only main effect QTLs were observed. For ASI, QTL main effects were observed on chromosomes 1, 2, 6, 8, and 10, whereas QEI was observed only on chromosome 8. The assessment of the QEI at chromosome 1 for grain yield showed that the QTL main effect explained 35.8% of the QTL + QEI variability, while QEI explained 64.2%. Minimum temperature during flowering time explained 77.6% of the QEI. The QEI analysis at chromosome 10 showed that the QTL main effect explained 59.8% of the QTL + QEI variability, while QEI explained 40.2%. Maximum temperature during flowering time explained 23.8% of the QEI. Results of this study show the possibilities of using FR for mapping QTL and for dissecting QEI in terms of environmental variables. PLS regression is efficient in accounting for background noise produced by other QTLs.

SNP-based codominant markers for a recessive gene conferring resistance to corky root rot (Rhizomonas suberifaciens) in lettuce (Lactuca sativa).

The analysis of F2 progeny and derived F3 families of Lactuca sativa segregating for resistance to corky root rot caused by Rhizomonas suberifaciens permitted the identification of restriction fragment length polymorphism (RFLP) and single nucleotide polymorphism (SNP) markers linked to the recessive resistance gene cor. PCR-based markers were identified by bulked segregant analysis (BSA). Allele-specific primers were generally designed with the 3 terminal base coinciding with an SNP, matching one of the alleles and mismatching the other, and with an additional subterminal 3 base mismatching both alleles. Codominant, robust, and inexpensive molecular markers were obtained that used standardized PCR conditions. Some of the markers could be analyzed in multiple Lactuca mapping populations that did not segregate for disease resistance allowing the cor locus to be located on several maps. The consistent low density of markers around cor in these maps suggests that cor may be in an area with an elevated rate of recombination. Evaluation of these markers in a large sample of cultivars and landraces identified pairs of flanking polymorphic markers that can be used for marker-assisted selection of corky root resistance.

Narrow-Sense Heritability of Selected Sensory Descriptors in Virginia-Type Peanut (Arachis hypogaea L.)1

Abstract The sweet, bitter and roasted peanut attributes of roasted peanut (Arachis hypogaea L.) flavor have been shown to be heritable traits. Previous research has estimated broad-sense heritability (H) and breeding values of numerous peanut cultivars and breeding lines for these attributes, but no study has estimated narrow-sense heritability (h2) in a specific population derived through hybridization and inbreeding. A population of 120 F3-derived families was developed without selection from the cross of NC 7/NC Ac 18431, a virginia-type line identified in 1990 as having a good flavor profile. The parents and F3:5 families were grown at two locations in North Carolina in 1995. SMK samples from each plot were roasted, ground to paste, tasted by a sensory panel, and scored for roasted peanut, sweet, bitter and astringent attributes. Additive and nonadditive genetic variances were estimated by equating variances among F2-derived families and among F3-derived families within F2-derived families to genetic covariances among inbred relatives. Regardless of whether the genetic model included dominance or additive-by-additive epistasis, the estimates of additive genetic variance for flavor attributes were small compared to those for nonadditive genetic variance. Narrow-sense heritability in the F2 generation was estimated at 0 for roasted peanut and astringent, 0.02 to 0.04 for sweet, and 0.01 to 0.03 for bitter, depending on the model used. Because of the low values of h2, which are specific to this population, gain from selection in early generations is expected to be limited within this population. Selection in this population should be practiced in late generations. Other parents have been identified whose crosses should produce greater improvement in sensory quality than can be expected from the NC 7 / NC Ac 18431 population.

Comparisons of quantitative trait locus mapping properties between two methods of recombinant inbred line development

Summary Theoretical comparisons for quantitative trait loci (QTL) mapping properties were conducted among simulated recombinant inbred (RI) populations developed by single-hill (SH), complete bulk, and single seed descent (SSD) procedures by Monte Carlo simulations based on various population sizes, heritabilities, and QTL effects. Our simulations included estimation of QTL effects, QTL positions, and statistical testing power in the RI populations by comparing the estimates with preset values. The simulation results showed that the single hill (SH) bulk and single seed descent RI populations were generally not significantly different with respect to quality of estimated QTL effects and positions. Furthermore, when each RI population had 150 lines, each could provide desirable properties for QTL mapping. The results implied that a SH RI population consisting of 75 or more F2-derived families with two lines per family (corresponding population size of 150 or above) was appropriate for QTL mapping and was not significantly different than a SSD RI population of 150. Thus, the SH method could be used to develop large numbers of RI lines for achieving better results in QTL mapping. Simulations also showed that there was no significant difference between means using SH methods with 10 and 100 fruits per family. However, RI populations developed by the complete bulk method where F2 identities are lost were not suitable for QTL mapping.

Comparison of Three Procedures for Early Generation Testing of Soybean

Early generation testing as a breeding procedure for autogamous crops consists of testing heterogeneous families, followed by selection of homozygous lines from superior families. Although early generation testing has been compared with other breeding methods, there have been few comparisons of different early generation testing procedures. Our objective was to assess the effectiveness of testing F1-, F2-, and F3-derived families in soybean [Glycine max (L.) Merr.]. We applied the three procedures to two soybean populations from 1986 to 1993. Selected lines (F3-, F4-, and F5-derived, respectively) were obtained by applying each testing method to each population. Divided into maturity classes, these selected lines were compared with unselected lines of the same generation, population, and maturity to determine genetic gain. The three early generation treatments produced similar genetic gains in seed yield, averaging approximately 4%. Selection of F1-derived families for yield increased plant height and lodging, but the other two selection procedures were satisfactory in this respect. To maximize genetic gain for yield while avoiding undesirable changes in lodging in an early-generation testing program, we recommend testing F2-derived families, unless off-season nurseries permit development of F3-derived families without further loss of time.

Variation within and between F2-derived families for grain yield and barley malting quality

Variation within and between F2-derived families for grain yield and malting quality was investigated using F4 breeding lines derived from F2 families of 4 barley crosses. The variation between F2-derived families was greater than within F2-derived families for grain yield and all malting quality attributes. Superior segregates almost exclusively came from the best performing families. The greater similarity of lines eventually drawn from an F2-derived family has significant implications for selection strategies in barley breeding programs as it facilitates the early discard of F2-derived families. To maximise the exploitation of genetic variation as early as possible, selection for malting quality could start in the F2 generation using near infrared transmittance (NIT) spectroscopy and for grain yield in the F3 generation.

Inheritance and linkage analysis of resistance to zucchini yellow mosaic virus, watermelon mosaic virus, papaya ringspot virus and powdery mildew in melon

A melon (Cucumis melo L.) breeding line derived from PI 414723 is resistant to three potyviruses,watermelon mosaic virus (WMV), zucchini yellow mosaic virus (ZYMV), papaya ringspot virus (PRSV), and to powdery mildew (PM). The inheritance and linkage relationships of these four resistances were studied in a segregating F2 population and derived F3 families from a cross between cultivar Top Mark and the resistant breeding line. Dominant monogenic inheritance of all four resistances was observed. We report that line 414723-4S3, which was initially selected as a source of ZYMV and WMV resistance, is also a source of dominant monogenic resistances to PRSV and PM race 1. We also report on genetic linkage (significant departure from independent segregation, χ2 = 58.1, p≪ 0.0001) between resistance to WMV and ZYMV. The map distance between these loci was estimated to be 7.5 cm. The genes for resistance to PM and PRSV segregated independently from each other, and from ZYMV and WMV resistance.

Inheritance of resistance to yellow mosaic virus in interspecific crosses of soybean

The inheritance of resistance to yellow mosaic virus spread by Bemisia tabaci Gen. in Glycine soja (Linn.) Seib. & Zucc. was studied following natural infection in the field condition. The resistant wild accession, Glycine soja was crossed with susceptible cultivars ‘Ankur’, ‘Bragg’, ‘PK 472’ and ‘Kalitur’ of Glycine max (Linn.) Merr. Resistance reactions of F1 and F2 plants, and individual F2 plant derived F3 families indicated that resistance was controlled by a single dominant gene.