Formation of a new original material based on genetic heterogeneity of disomal wheat plants from offspring of aneuploids

A series of disomic lines of spring wheat Opal selected on the basis of monosomic lines of this cultivar has been studied. The lines have been tested for combining ability, and the heterosis effect has been studied in disomic lines of F1 hybrids obtained by topcrossing. The line have been demonstrated to differ both from one another and from cultivar Opal in the expression of quantitative traits, combining ability, and the degree of heterosis in F1. These data suggest that recombinations accompanying the formation of the monosomic series have changed their genetic program. To test this suggestion, intramolecular heterogeneity of 42-chromosome plants has been analyzed using polymerase chain reaction (PCR) and isoenzyme analysis. The results confirmed the differences at the DNA and protein levels. According to the results of molecular analyses, A-genome lines are the most polymorphic. Strong heterosis effects have been detected in hybrid combinations contributed by D- and B-genome lines, which are characterized by medium and low degrees of molecular genetic polymorphism. Lines that are promising in terms of breeding programs have been identified.     

Assignment of RFLP linkage groups to chromosomes using monosomic F1 analysis in hexaploid oat

The availability of molecular genetic maps in oat (Avena spp.) and improved identification of chromosomes by C-banding are two recent developments that have made locating linkage groups to chromosomes possible in cultivated hexaploid oat, 2n=6x=42. Monosomic series derived from Avena byzantina C. Koch cv Kanota and from Avena sativa L. cv Sun II were used as maternal plants in crosses with the parents, Kanota-1 and Ogle-C, of the oat RFLP mapping population. Monosomic F₁ plants were identified by root-tip cell chromosome counts. For marker analysis, DNAs of eight F₂ plants from a monosomic F₁ were combined to provide a larger source of DNA that mimicked that of the monosomic F₁ plant. Absence of maternal alleles in monosomic F₁s served to associate linkage groups with individual chromosomes. Twenty two linkage groups were associated with 16 chromosomes. In seven instances, linkage groups that were independent of each other in recombination analyses were associated with the same chromosome. Five linkage groups were shown to be associated with translocation differences among oat lines. Additionally, the results better-characterized the oat monosomic series through the detection of duplicates and translocation differences among the various monosomic lines. The F₁ monosomic series represents a powerful cytogenetic tool with the potential to greatly improve understanding of the oat genome. Received: 24 April 2000 / Accepted: 10 May 2000     

Thinopyrum distichum addition lines: production,morphological and cytological characterisation of 11 disomic addition lines and stable addition-substitution line

Plants of the partial amphiploid Inia 66/Thinopyrum distichum (2n = 70)//Inia 66 (2n = 56) were used as male parents in crosses with the monosomic series in the common wheat cultivar Inia 66. The genome and homoeologous group of the monosomic used in the cross affected the distribution of chromosome number of the progeny plants in the F₂ and F₄. Meiosis in the pollen mother cells of the B₁F₇ partial amphiploids was not stable, and not different from that of the B₁F₁ in which univalents and multivalents were observed. Disomic addition lines were selected on the basis of morphology and meiotic stability in the F₂, F₄ and F₅. Eleven of the fourteen possible wheat-Th. distichum disomic addition lines were identified using chromosome C-band pattern, as well as size and arm ratio, as genetic markers. Addition of T. distichum chromosome J ^dll produced a phenotype indicating homoeology with wheat group-2 chromosomes. Clear indications of homoeology based on morphological characteristics were not obtained in any of the other addition lines, probably due to the mixed homoeology of the Th. distichum chromosomes relative to wheat. The addition lines were all susceptible to leaf rust, unlike the germplasm-line Indis which carries a leaf rust resistance gene on a translocation segment derived from Th. distichum. Instability of meiotic pairing was observed in all addition lines. The stability, or not, of progeny chromosome counts did not reflect the level of chromosome pairing instability in the parental plants. SDS-PAGE for gliadin-type seed proteins revealed two addition lines which expressed seed storage proteins uncommon to Inia 66 but typical of Th. distichum.     

QTL mapping for combining ability in different population-based NCII designs: a simulation study

The NCII design (North Carolina mating design II) has been widely applied in studies of combining ability and heterosis. The objective of our research was to estimate how different base populations, sample sizes, testcross numbers and heritability influence QTL analyses of combining ability and heterosis. A series of Monte Carlo simulation experiments with QTL mapping were then conducted for the base population performance, testcross population phenotypic values and the general combining ability (GCA), specific combining ability (SCA) and Hmp (midparental heterosis) datasets. The results indicated that: (i) increasing the number of testers did not necessarily enhance the QTL detection power for GCA, but it was significantly related to the QTL effect. (ii) The QTLs identified in the base population may be different from those from GCA dataset. Similar phenomena can be seen from QTL detected in SCA and Hmp datasets. (iii) The QTL detection power for GCA ranked in the order of DH(RIL) based > F₂ based > BC based NCII design, when the heritability was low. The recombinant inbred lines (RILs) (or DHs) allows more recombination and offers higher mapping resolution than other populations. Further, their testcross progeny can be repeatedly generated and phenotyped. Thus, RIL based (or DH based) NCII design was highly recommend for combining ability QTL analysis. Our results expect to facilitate selecting elite parental lines with high combining ability and for geneticists to research the genetic basis of combining ability.     

Genetic distance and heterosis in Indian mustard: developmental isozymes as indicators of genetic relationships

The use of isozymes as indicators of genetic diversity and as markers for the selection of agronomic traits has been proposed in different crop species. The present investigation was conducted to study the use of isozyme-derived genetic distance between parents in predicting the F₁ heterosis in Indian mustard. In addition, the interaction of isozyme-based diversity with quantitative trait and pedigree-based diversity measures, and its role in predicting hybrid heterosis has also been examined. Sixteen Indian mustard lines and their 48 crosses (12 × 4, line x tester crossing) were evaluated over two environments for isozyme and quantitative morphological characters. The results from this study suggest that the heterotic response to isozymic changes is more responsive in crosses derived from morphologically and pedigree-wise related parents in comparison to crosses derived from unrelated parents. It was possible to improve heterosis predictions by partitioning the isozyme-based genetic distance into general genetic distance and specific genetic distance and correlating the latter with the specific combining ability of morphological traits. The possible reasons for these observations are discussed.     

Breeding Potential of Introgression Lines Developed from Interspecific Crossing between Upland Cotton (Gossypium hirsutum) and Gossypium barbadense: Heterosis,Combining Ability and Genetic Effects

Upland cotton (Gossypium hirstum L.), which produces more than 95% of the world natural cotton fibers, has a narrow genetic base which hinders progress in cotton breeding. Introducing germplasm from exotic sources especially from another cultivated tetraploid G. barbadense L. can broaden the genetic base of Upland cotton. However, the breeding potential of introgression lines (ILs) in Upland cotton with G. barbadense germplasm integration has not been well addressed. This study involved six ILs developed from an interspecific crossing and backcrossing between Upland cotton and G. barbadense and represented one of the first studies to investigate breeding potentials of a set of ILs using a full diallel analysis. High mid-parent heterosis was detected in several hybrids between ILs and a commercial cultivar, which also out-yielded the high-yielding cultivar parent in F₁, F₂ and F₃ generations. A further analysis indicated that general ability (GCA) variance was predominant for all the traits, while specific combining ability (SCA) variance was either non-existent or much lower than GCA. The estimated GCA effects and predicted additive effects for parents in each trait were positively correlated (at P<0.01). Furthermore, GCA and additive effects for each trait were also positively correlated among generations (at P<0.05), suggesting that F₂ and F₃ generations can be used as a proxy to F₁ in analyzing combining abilities and estimating genetic parameters. In addition, differences between reciprocal crosses in F₁ and F₂ were not significant for yield, yield components and fiber quality traits. But maternal effects appeared to be present for seed oil and protein contents in F₃. This study identified introgression lines as good general combiners for yield and fiber quality improvement and hybrids with high heterotic vigor in yield, and therefore provided useful information for further utilization of introgression lines in cotton breeding.     

Chromosomal location of a gene suppressing powdery mildew resistance genes Pm8 and Pm17 in common wheat (Triticum aestivum L. em. Thell.)

The chromosomal location of a suppressor for the powdery mildew resistance genes Pm8 and Pm17 was determined by a monosomic set of the wheat cultivar Caribo. This cultivar carries a suppressor gene inhibiting the expression of Pm8 in cv Disponent and of Pm17 in line Helami-105. In disease resistance assessments, monosomic F₁ hybrids (2n=41) of Caribo x Disponent and Caribo x Helami-105 lacking chromosome 7D were resistant, whereas monosomic F₁ hybrids involving the other 20 chromosomes, as well as disomic F₁ hybrids (2n=42) of all cross combinations, were susceptible revealing that the suppressor gene for Pm8 and Pm17 is localized on chromosome 7D. It is suggested that genotypes without the suppressor gene be used for the exploitation of genes Pm8 and Pm17 in enhancing powdery mildew resistance in common wheat.     

Effects of genetic distance on heterosis in a <Emphasis Type="Italic">Drosophila melanogaster</Emphasis> model system

Habitat fragmentation and small population sizes can lead to inbreeding and loss of genetic variation, which can potentially cause inbreeding depression and decrease the ability of populations to adapt to altered environmental conditions. One solution to these genetic problems is the implementation of genetic rescue, which re-establishes gene flow between separated populations. Similar techniques are being used in animal and plant breeding to produce superior production animals and plants. To optimize fitness benefits in genetic rescue programs and to secure high yielding domestic varieties in animal and plant breeding, knowledge on the genetic relatedness of populations being crossed is imperative. In this study, we conducted replicated crosses between isogenic Drosophila melanogaster lines from the Drosophila Genetic Reference Panel. We grouped lines in two genetic distance groups to study the effect of genetic divergence between populations on the expression of heterosis in two fitness components; starvation resistance and reproductive output. We further investigated the transgenerational effects of outcrossing by investigating the fitness consequences in both the F₁- and the F₃-generations. High fitness enhancements were observed in hybrid offspring compared to parental lines, especially for reproductive output. However, the level of heterosis declined from the F₁- to the F₃-generation. Generally, genetic distance did not have strong impact on the level of heterosis detected, although there were exceptions to this pattern. The best predictor of heterosis was performance of parental lines with poorly performing parental lines showing higher hybrid vigour when crossed, i.e. the potential for heterosis was proportional to the level of inbreeding depression. Overall, our results show that outcrossing can have very strong positive fitness consequences for genetically depauperate populations.     

A diallel analysis of heterosis in elite hybrid rice based on RFLPs and microsatellites

Hybrid rice has contributed significantly to the dramatic increase of rice production in the world. Despite this, little attention has been given to studying the genetic basis of heterosis in rice. In this paper, we report a diallel analysis of heterosis using two classes of molecular markers: restriction fragment length polymorphisms, (RFLPs) and microsatellites. Eight lines, which represent a significant portion of hybrid rice germ plasm, were crossed in all possible pairs, and the F₁s were evaluated for yield and yield component traits in a replicated field trial. The parental lines were surveyed for polymorphisms with 117 RFLP probes and ten microsatellites, resulting in a total of 76 polymorphic markers well-spaced in the rice RFLP map. The results indicated that high level heterosis is common among these crosses: more than 100% midparent and 40% better-parent heterosis were observed in many F₁s, including some crosses between maintainer lines. Heterosis was found to be much higher for yield than for yield component traits, which fits a multiplicative model almost perfectly. Between 16 and 30 marker loci (positive markers) detected highly significant effects on yield or its component traits. Heterozygosity was significantly correlated with several attributes of performance and heterosis. Correlations based on positive markers (specific heterozygosity) were large for midparent heterosis of yield and seeds/panicle and also for F₁ kernel weight. These large correlations may have practical utility for predicting heterosis.     

Understanding the genetic and molecular constitutions of heterosis for developing hybrid rice

The wide adoption of hybrid rice has greatly increased rice yield in the last several decades. The utilization of heterosis facilitated by male sterility has been a common strategy for hybrid rice development. Here, we summarize our efforts in the genetic and molecular understanding of heterosis and male sterility together with the related progress from other research groups. Analyses of F1 diallel crosses show that strong heterosis widely exists in hybrids of diverse germplasms, and inter-subsp...     

DNA divergence as a criterion of a sweet pepper (Capsicum annuum L.) selection for heterosis

The identification of perspective parental lines for the creation of high-yield hybrids is the most labor-consuming stage of selection, because it needs extensive trials of combining ability. Based on evaluation of the genetic divergence of the parental lines, the prediction accuracy of F₁ hybrids performance was investigated. The value of the divergence was calculated using biometric and molecular methods, such as inter simple sequence repeats (ISSR) and random amplified polymorphic DNA (RAPD). Based on estimates of divergence, 10 lines were chosen for cyclic cross (scheme I) and testcross (scheme II). In most crosses, the F₁ hybrids were significantly superior to the parents in the main economically valuable traits. The level of heterosis was higher among hybrids of scheme I. Analysis of the relationship between parental divergence and F₁ performance showed that the hybrid productivity of scheme I was predetermined by ISSR divergence in 86%, and productivity was caused by RAPD divergence in 69%, whereas the F₁ yield of scheme II was not related to the value of genetic distances. Since the values of DNA divergence were closely associated both with midparent level and F₁ performance, we assumed that part of the polymorphic DNA fragments of the parental lines of scheme I is related to heterotic loci (HTL), which may be considered potential key markers for the heterotic selection of the sweet pepper.     

Chromosomal location of genes for stem rust resistance derived from ‘Waldron’ wheat

The chromosomal locations of genes for resistance to stem rust (Puccinia graminis Pers.: Pers. f. sp. tritici Eriks. & E. Henn.) in the wheat (Triticum aestivum L.) cultivar ‘Waldron’ (WDR) were determined by monosomic analyses. Wheat lines WDR-B1, -C2, -E4, and -F1,which have single genes for resistance to stem rust derived previously from WDR sel. ‘Little Club’, were crossed onto a complete set of 21 ‘Chinese Spring’ monosomics. The F₂ and backcross-F₁ (BC₁F₁) seedlings from each of the 84 crosses were tested for reaction to culture 111-SS2 (CRL-LCBB) of stem rust, and a few selected segregants were analyzed cytologically for chromosome number. The F₂ from 2 crosses of WDR-C2, -E4 and -F1 and the BC₁F₁ from 2 crosses of WDR-F1 were tested also with culture Or11c (CRL-QBCN). Significant deviations from disomic ratios towards monosomic ratios in the F₂ and BC₁F₁ were used to determine which chromosomes carried the genes for resistance. Cytological analyses of certain BC₁F₁ and susceptible F₂ plants were used to help identify the location of the genes for rust resistance. WDR-B1 has a gene, herein designated Sr41, for resistance on chromosome 4D. WDR-C2 has a gene on chromosome 7 A that may be the same as one previously designated SrWld2. WDR-E4 has a gene on chromosome 2A, possibly SrWld1, which is effective against most or all North American stem rust cultures. WDR-F1 has a gene on chromosome 6B that is the same as or similar to Sr11.     

Establishment of a complete series of a monosomic tomato chromosome addition lines in the cultivated potato using RFLP and GISH analyses

With the aim of establishing a complete monosomic alien tomato chromosome addition series in a potato background, the backcross progenies derived from repeated crossing of potato (+) tomato fusion hybrids to potato were screened through RFLP and GISH analyses. Because of the availability from our previous work of seven of the possible 12 tomato monosomic additions, selected from BC₂ populations, attention was paid to those alien additions that were missing. Thus, since the alien additions were already available for tomato chromosomes 1, 2, 4, 6, 8, 10 and 12, efforts were made to select for chromosomes 3, 5, 7, 9 and 11 by screening specific BC₃ populations. In all, 105 plants from four BC₃ populations were screened through a combination of RFLP and GISH analyses in order to complete the series. Among the newly selected alien addition lines, five were monosomic additions for all the remaining chromosomes and one was a disomic addition for chromosome 11. When using conventional cytogenetics the selection of monosomic alien additions is highly laborious. All the tomato chromosomes showed a variable rate of transmission. Chromosome 6 was transmitted at 29.6% and 81.5% frequency in populations 2705 and 2701 respectively. The present study showed that molecular markers and molecular cytogenetics applied in this study were most efficient and rapid because a pre-selection for the desired genotypes was possible by screening a population with chromosome-specific markers for the presence of one tomato chromosome at a time. Received: 9 January 2001 / Accepted: 26 January 2001     

GISH analysis of disomic Brassica napus-Crambe abyssinica chromosome addition lines produced by microspore culture from monosomic addition lines

Two Brassica napus--Crambe abyssinica monosomic addition lines (2n=39, AACC plus a single chromosome from C. abyssinca) were obtained from the F₂ progeny of the asymmetric somatic hybrid. The alien chromosome from C. abyssinca in the addition line was clearly distinguished by genomic in situ hybridization (GISH). Twenty-seven microspore-derived plants from the addition lines were obtained. Fourteen seedlings were determined to be diploid plants (2n=38) arising from spontaneous chromosome doubling, while 13 seedlings were confirmed as haploid plants. Doubled haploid plants produced after treatment with colchicine and two disomic chromosome addition lines (2n=40, AACC plus a single pair of homologous chromosomes from C. abyssinca) could again be identified by GISH analysis. The lines are potentially useful for molecular genetic analysis of novel C. abyssinica genes or alleles contributing to traits relevant for oilseed rape (B. napus) breeding.     

Molecular cytogenetic evaluation of chromosome instability inTriticum aestivum—Secale cereale disomic addition lines

The genetic stability of wheat/rye (‘Chinese Spring’/‘Imperial’) disomic addition lines was checked using the Feulgen method and fluorescent in situ hybridization (FISH). Feulgen staining detected varying proportions of disomic, monosomic, and telosomic plants among the progenies of the disomic addition lines. The greatest stability was observed for the 7R addition line, while the most unstable lines were those with 2R and 4R additions. Chromosome rearrangements were also detected using FISH. Based on the specific hybridization patterns of repetitive DNA probes pSc119.2 and (AAC)5, as well as ribosomal DNA probes (5S and 45S), isochromosomes were identified in the progenies of 1R and 4R addition lines. The results draw attention to the importance of continuous cytological checks on basic genetic materials by using FISH, because this method reveals chromosome rearrangements that could not be detected either with the conventional Feulgen staining technique or with molecular markers.     

Genetic diversity for restriction fragment length polymorphisms and heterosis for two diallel sets of maize inbreds

Summary Changes that may have occurred over the past 50 years of hybrid breeding in maize (Zea maize L.) with respect to heterosis for yield and heterozygosity at the molecular level are of interest to both maize breeders and quantitative geneticists. The objectives of this study were twofold: The first, to compare two diallels produced from six older maize inbreds released in the 1950's and earlier and six newer inbreds released during the 1970's with respect to (a) genetic variation for restriction fragment length polymorphisms (RFLPs) and (b) the size of heterosis and epistatic effects, and the second, to evaluate the usefulness of RFLP-based genetic distance measures in predicting heterosis and performance of single-cross hybrids. Five generations (parents, F₁; F₂, and backcrosses) from the 15 crosses in each diallel were evaluated for grain yield and yield components in four Iowa environments. Genetic effects were estimated from generation means by ordinary diallel analyses and by the Eberhart-Gardner model. Newer lines showed significantly greater yield for inbred generations than did older lines but smaller heterosis estimates. In most cases, estimates of additive x additive epistatic effects for yield and yield components were significantly positive for both groups of lines. RFLP analyses of inbred lines included two restriction enzymes and 82 genomic DNA clones distributed over the maize genome. Eighty-one clones revealed polymorphisms with at least one enzyme. In each set, about three different RFLP variants were typically found per RFLP locus. Genetic distances between inbred lines were estimated from RFLP data as Rogers' distance (RD), which was subdivided into general (GRD) and specific (SRD) Rogers' distances within each diallel. The mean and range of RDs were similar for the older and newer lines, suggesting that the level of heterozygosity at the molecular level had not changed. GRD explained about 50% of the variation among RD values in both sets. Cluster analyses, based on modified Rogers' distances, revealed associations among lines that were generally consistent with expectations based on known pedigree and on previous research. Correlations of RD and SRD with f₁ performance, specific combining ability, and heterosis for yield and yield components, were generally positive, but too small to be of predictive value. In agreement with previous studies, our results suggest that RFLPs can be used to investigate relationships among maize inbreds, but that they are of limited usefulness for predicting the heterotic performance of single crosses between unrelated lines.Joint contribution from Cereal and Soybean Research Unit, USDA, Agricultural Research Service and Journal Paper no. J-13929 of the Iowa Agric and Home Economics Exp Stn, Ames, IA 50011. Projects no. 2818 and 2778A.E.M. is presently at the Iowa State University on leave from University of Hohenheim, D-7000 Stuttgart 70, Federal Republic of Germany     

Genetic basis of grain yield heterosis in an “immortalized F2” maize population

Key message

Genetic basis of grain yield heterosis relies on the cumulative effects of dominance, overdominance, and epistasis in maize hybrid Yuyu22.

Abstract

Heterosis, i.e., when F₁ hybrid phenotypes are superior to those of the parents, continues to play a critical role in boosting global grain yield. Notwithstanding our limited insight into the genetic and molecular basis of heterosis, it has been exploited extensively using different breeding approaches. In this study, we investigated the genetic underpinnings of grain yield and its components using “immortalized F₂” and recombinant inbred line populations derived from the elite hybrid Yuyu22. A high-density linkage map consisting of 3,184 bins was used to assess (1) the additive and additive-by-additive effects determined using recombinant inbred lines; (2) the dominance and dominance-by-dominance effects from a mid-parent heterosis dataset; and (3) the various genetic effects in the “immortalized F₂” population. Compared with a low-density simple sequence repeat map, the bin map identified more quantitative trait loci, with higher LOD scores and better accuracy of detecting quantitative trait loci. The bin map showed that, among all traits, dominance was more important to heterosis than other genetic effects. The importance of overdominance/pseudo-overdominance was proportional to the amount of heterosis. In addition, epistasis contributed to heterosis as well. Phenotypic variances explained by the QTLs detected were close to the broad-sense heritabilities of the observed traits. Comparison of the analyzed results obtained for the “immortalized F₂” population with those for the mid-parent heterosis dataset indicated identical genetic modes of action for mid-parent heterosis and grain yield performance of the hybrid.     

Molecular divergence and hybrid performance in rice

This study was undertaken to determine the relationship between genetic distance of the parents based on molecular markers and F₁ performance in a set of diallel crosses involving eight commonly used parental lines in hybrid rice production. The F₁s and their parents were measured for five traits including heading date, plant height, straw weight, grain yield and biomass. The parental lines were assayed for DNA polymorphisms using two classes of markers: 140 probes for restriction fragment length polymorphisms (RFLPs) and 12 simple sequence repeats (SSRs), resulting in a total of 105 polymorphic markers well spaced along the 12 rice chromosomes. SSRs detected more polymorphism than RFLPs among the eight lines. A cluster analysis based on marker genotypes separated these eight lines into three groups which agree essentially with the available pedigree information. Correlations were mostly low between general heterozygosity based on all the markers and F₁ performance and heterosis. In contrast, very high correlations were detected between midparent heterosis and specific heterozygosity based on the markers that detected significant effects for all the five traits; these correlations may have practical utility in predicting heterosis. The analyses also suggest the existence of two likely heterotic groups in the rice germplasm represented by these eight lines.     

Allelic compositions at polymorphic gene loci in maize inbred and hybrid lines and their associations with levels of heterosis

We analyzed polymorphism in the parental lines GK26 and Mo17 and testers Od221MV, Od308MV, and Od329 using SSR-analysis. Recombinant inbred lines (RILs) from populations F₄ and F₆ were genotyped at ten polymorphic loci. Allelic compositions and allele frequencies at microsatellite loci were investigated in parental lines and testers, and the best highly heterotic hybrids and their molecular genetic formulae were derived. The allelic composition of microsatellites were investigated in RILs and high-yield hybrids for the best combining ability parameters.