Contributions of heterosis and epistasis to hybrid fitness

Early-generation hybrid fitness is difficult to interpret because heterosis can obscure the effects of hybrid breakdown. We used controlled reciprocal crosses and common garden experiments to distinguish between effects of heterosis and nuclear and cytonuclear epistasis among morphotypes and advanced-generation hybrid derivative populations in the Piriqueta caroliniana (Turneraceae) plant complex. Seed germination, growth, and sexual reproduction of first-generation hybrids, inbred parental lines, and outbred parental lines were compared under field conditions. Average vegetative performance was greater for hybrids than for inbred lines, and first-season growth was similar for hybrids and outbred parental lines. Hybrid survival surpassed that of inbred lines and was equal to or greater than outbred lines' survival, and more F(1) than parental plants reproduced. Reductions in hybrid fitness due to Dobzhansky-Muller incompatibilities (epistasis among divergent genetic elements) were expressed as differences in vegetative growth, survival, and reproduction between plants from reciprocal crosses for both F(1) and backcross hybrid generations. Comparing performance of hybrids against parental genotypes from intra- and interpopulation crosses allowed a more robust prediction of F(1) hybrids' success and more accurate interpretations of the genetic architecture of F(1) hybrid vigor.     

Inbred line versus Ft hybrid breeding in swedes (Brassica napus L. var. napobrassica Peterm)

In 1989 seven F[hybrid swedes were produced in polythene tunnels. The female parent was an inbred line derived from cv. Ruta 0tofte, and made self-incompatible with an S-allele introduced by backcrossing. Three more hybrids were produced in 1990 in a similar way with an inbred line from cv. Criffel as female parent. In trials in Dundee over the period 1990 to 1992, better parent heterosis for dry matter yield in the hybrids ranged from 1.9% to 19.2%. There was no such heterosis for dry matter percentage nor for mildew resistance. The highest yielding hybrid was the one between the Criffel line and an inbred line from cv. Magres. It averaged 9.9% heterosis over the two years 1991 and 1992, outyielded seven commercial cultivars, but had a lower yield (12.3 t/ha) than an F₆ line SS5 (13.3 t/ha) derived from the cross between the Criffel and Magres lines by pedigree inbreeding with selection. It is concluded that high yielding heterotic F/s should be the starting point rather than the finishing point of swede breeding programmes, and that research into breeding methods should concentrate on improving the efficiency of pedigree inbreeding following hybridisations, rather than on ways of producing Fj hybrids.     

Heterosis Is Prevalent for Multiple Traits in Diverse Maize Germplasm

Background

Heterosis describes the superior phenotypes observed in hybrids relative to their inbred parents. Maize is a model system for studying heterosis due to the high levels of yield heterosis and commercial use of hybrids.

Methods

The inbred lines from an association mapping panel were crossed to a common inbred line, B73, to generate nearly 300 hybrid genotypes. Heterosis was evaluated for seventeen phenotypic traits in multiple environments. The majority of hybrids exhibit better-parent heterosis in most of the hybrids measured. Correlations between the levels of heterosis for different traits were generally weak, suggesting that the genetic basis of heterosis is trait-dependent.

Conclusions

The ability to predict heterosis levels using inbred phenotype or genetic distance between the parents varied for the different traits. For some traits it is possible to explain a significant proportion of the heterosis variation using linear modeling while other traits are more difficult to predict.     

Analysis of nonadditive protein accumulation in young primary roots of a maize (Zea mays L.) F(1)-hybrid compared to its parental inbred lines

Heterosis describes the superior performance of heterozygous F(1)-hybrids compared to their homozygous parental inbred lines. Heterosis is already manifested during early maize (Zea mays L.) primary root development. In this study, the most abundant soluble proteins have been investigated before the phenotypic manifestation of heterosis in 3.5-day-old primary roots in the flint inbred line UH002, the dent inbred line UH301 and the corresponding hybrid UH301 x UH002. In CBB-stained 2-DE gels, 150 of 304 detected proteins (49%) were accumulated in a nonadditive fashion in the hybrid compared to the average of their parental inbred lines (Student's t-test: p < 0.05). Remarkably, expression of 51% (76/150) of the nonadditively accumulated proteins exceeded the high parent or was below the low parent. ESI-MS/MS identified 75 of the 76 proteins that belonged to these expression classes. The most abundant functional classes among the 75 proteins that were encoded by 60 different genes were metabolism (58%) and disease and defense (19%). Nonadditive protein accumulation in primary roots of maize hybrids might be associated with heterosis manifestation. Identification of these proteins could therefore contribute to the better understanding of the molecular basis of heterosis.     

Manifestation of heterosis during early maize (Zea mays L.) root development

Heterosis is typically detected in adult hybrid plants as increased yield or vigor compared to their parental inbred lines. Only little is known about the manifestation of heterosis during early postembryonic development. Objective of this study was to identify heterotic traits during early maize root development. Four German inbred lines of the flint (UH002 and UH005) and dent (UH250 and UH301) pool and the 12 reciprocal hybrids generated from these inbred lines were subjected to a morphological and histological analysis during early root development. Primary root length and width were measured daily in a time course between 3 and 7 days after germination (DAG) and displayed average midparent heterosis (MPH) of 17–25% and 1–7%, respectively. Longitudinal size of cortical cells in primary roots was determined 5 DAG and displayed on average 24% MPH thus demonstrating that enlarged primary roots of hybrids can mainly be attributed to elongated cortical cells. The number of seminal roots determined 14 DAG showed on average 18% MPH. Lateral root density of all tested hybrids was determined 5 DAG. This root trait showed the highest degree of heterosis with an average MPH value of 51%. This study demonstrated that heterosis is already manifesting during the very early stages of root development a few days after germination. The young root system is therefore a suitable model for subsequent molecular studies of the early stages of heterosis manifestation during seedling development.     

Growth rate rather than growth duration drives growth heterosis in maize B104 hybrids

Research in maize is often performed using inbred lines that can be readily transformed, such as B104. However, because the B104 line flowers late, the kernels do not always mature before the end of the growing season, hampering routine seed yield evaluations of biotech traits introduced in B104 at many geographical locations. Therefore, we generated five hybrids by crossing B104 with the early‐flowering inbred lines CML91, F7, H99, Mo17, and W153R and showed in three consecutive years that the hybrid lines proved to be suitable to evaluate seed yield under field conditions in a temperate climate. By assessing the two main processes driving maize leaf growth, being rate of growth (leaf elongation rate or LER) and the duration of growth (leaf elongation duration or LED) in this panel of hybrids, we showed that leaf growth heterosis was mainly the result of increased LER and not or to a lesser extent of LED. Ectopic expression of the transgenes GA20‐oxidase (GA20‐OX) and PLASTOCHRON1 (PLA1), known to stimulate the LER and LED, respectively, in the hybrids showed that leaf length heterosis can be stimulated by increased LER, but not by LED, indicating that LER rather than LED is the target for enhancing leaf growth heterosis.     

Morphometric studies in inbred and hybrid house mice. Heterosis, homeostasis and heritability of size and shape

One size (SIZE) and four shape measures (SHAPE 1-SHAPE 4) were derived from a multiple group principal components analysis of 15 osteometric variables in inbred and hybrid house mice. In both sexes, SIZE and two of the four SHAPE variables showed positive heterosis, the other two SHAPE variables exhibiting negative heterosis. SIZE showed a greater magnitude of heterosis (average of about 2.3 standard deviations) than all SHAPE characters except SHAPE 2, a skull length/width contrast. Inbreds were more variable than hybrids (positive homeostasis) for all characters, and there was a significant, positive correlation between heterosis and homeostasis in these characters. The reciprocals category in hybrids was more important for SIZE than for the SHAPE variables, presumably because maternal effects have a greater influence on growth characters. Broad-sense heritabilities for SIZE were 0.8 in inbreds and 0.6 in hybrids whereas they averaged only 0.4 for the SHAPE variables. It was postulated that there is a greater number of loci governing SIZE compared to SHAPE, and that this explains both the heritability and heterosis differences between these characters.     

Progressive heterosis in genetically defined tetraploid maize

Progressive heterosis, i.e., the additional hybrid vigor in double-cross tetraploid hybrids not found in their single-cross tetraploid parents, has been documented in a number of species including alfalfa,potato, and maize. In this study, four artificially induced maize tetraploids, directly derived from standard inbred lines, were crossed in pairs to create two single-cross hybrids. These hybrids were then crossed to create double-cross hybrids containing genetic material from all four original lines. Replicated fieldbased phenotyping of the materials over four years indicated a strong progressive heterosis phenotype in tetraploids but not in their diploid counterparts. In particular, the above ground dry weight phenotype of double-cross tetraploid hybrids was on average 34% and 56% heavier than that of the single-cross tetraploid hybrids and the double-cross diploid counterparts, respectively. Additionally,whole-genome resequencing of the original inbred lines and further analysis of these data did not show the expected spectrum of alleles to explain tetraploid progressive heterosis under the complementation of complete recessive model. These results underscore the reality of the progressive heterosis phenotype,its potential utility for increasing crop biomass production, and the need for exploring alternative hypothesis to explain it at a molecular level.     

Heterosis in hybrids within and between yeast species

The performance of hybrids relative to their parents is an important factor in speciation research. We measured the growth of 46 Saccharomyces yeast F1 interspecific and intraspecific hybrids, relative to the growth of each of their parents, in pairwise competition assays. We found that the growth of a hybrid relative to the average of its parents, a measure of mid‐parent heterosis, correlated with the difference in parental growth relative to their hybrid, a measure of phenotypic divergence, which is consistent with simple complementation of low fitness alleles in one parent by high fitness alleles in the other. Interspecific hybrids showed stronger heterosis than intraspecific hybrids. To manipulate parental phenotypic divergence independently of genotype, we also measured the competitive growth of a single interspecific hybrid relative to its parents in 12 different environments. In these assays, we not only identified a strong relationship between parental phenotypic divergence and mid‐parent heterosis as before, but, more tentatively, a weak relationship between phenotypic divergence and best‐parent heterosis, suggesting that complementation of deleterious mutations was not the sole cause of interspecific heterosis. Our results show that mating between different species can be beneficial, and demonstrate that competition assays between parents and offspring are a useful way to study the evolutionary consequences of hybridization.     

Nitrate Reductase Activity: A Biochemical Criterion of Hybrid Vigour in Sorghum bicolor (L.) Moench

F₁ hybrids of Sorghum bicolor (L.) Moench and their inbred parentswere analysed for NADH-nitrate reductase activity during theearly stages of seedling growth. In all the hybrids both mid-parentaland better parental heterosis were discernible in shoots whereasin roots two hybrids out of the three tested, showed heteroticlevels. It is suggested that in sorghum nitrate reductase activityduring seedling stages can be used as a biochemical criterionfor evaluating hybrid vigour. Sorghum bicolor (L.) Moench, sorghum, hybrid vigour, nitrate reductase     

A unifying theory for general multigenic heterosis: energy efficiency, protein metabolism, and implications for molecular breeding

Hybrids between genetically diverse varieties display enhanced growth, and increased total biomass, stress resistance and grain yield. Gene expression and metabolic studies in maize, rice and other species suggest that protein metabolism plays a role in the growth differences between hybrids and inbreds. Single trait heterosis can be explained by the existing theories of dominance, overdominance and epistasis. General multigenic heterosis is observed in a wide variety of different species and is likely to share a common underlying biological mechanism. This review presents a model to explain differences in growth and yield caused by general multigenic heterosis. The model describes multigenic heterosis in terms of energy-use efficiency and faster cell cycle progression where hybrids have more efficient growth than inbreds because of differences in protein metabolism. The proposed model is consistent with the observed variation of gene expression in different pairs of inbred lines and hybrid offspring as well as growth differences in polyploids and aneuploids. It also suggests an approach to enhance yield gains in both hybrid and inbred crops via the creation of an appropriate computational analysis pipeline coupled to an efficient molecular breeding program.     

Heterosis for horticultural traits in Broccoli

Over the last three decades, broccoli (Brassica oleracea L., Italica Group) hybrids made by crossing two inbred lines replaced open-pollinated populations to become the predominant type of cultivar. The change to hybrids evolved with little or no understanding of heterosis or hybrid vigor in this crop. Therefore, the purpose of the present study was to determine levels of heterosis expressed by a set of hybrids derived by crossing relatively elite, modern inbreds (n = 9). An additional objective was to determine if PCR-based marker derived genetic similarities among the parents can be useful to predict heterosis in this crop. Thirty-six hybrids derived from a diallel mating design involving nine parents were evaluated for five horticultural characters including the head characteristics of head weight, head stem diameter, and maturity (e.g., days from transplant to harvest), and the plant vigor characteristics of plant height, and plant width in four environments. A total of 409 polymorphic markers were generated by 24 AFLP, 23 SRAP and 17 SSR primer combinations. Euclidean distances between parents were determined based on phenotypic traits. About half of the hybrids exhibited highparent heterosis for head weight (1–30 g) and stem diameter (0.2–3.5 cm) when averaged across environments. Almost all hybrids showed highparent heterosis for plant height (1–10 cm) and width (2–13 cm). Unlike other traits, there was negative heterosis for maturity, indicating that heterosis for this character in hybrids is expressed as earliness. Genetic similarity estimates among the nine parental lines ranged from 0.43 to 0.71 and were significantly and negatively correlated with highparent heterosis for all traits except for stem diameter and days from transplant to harvest. Euclidean distances were not correlated with heterosis. With modern broccoli inbreds, less heterosis was observed for head characteristics than for traits that measured plant vigor. In addition, genetic similarity based on molecular markers was more highly correlated with plant vigor characteristics than head traits. Unlike with molecular marker-based estimates of genetic similarity, euclidean distance determined using phenotypic trait data was not predictive of heterosis. In conclusion, this study has documented heterosis in Brassica oleracea L., and the ability to predict heterosis in this crop using molecular marker-based estimates of genetic similarity among parents used in producing the hybrid. The contents of this publication do not necessarily reflect the views or policies of the USDA, nor does the mention of trade names, commercial products, or organizations imply endorsement by the US Government. The cost of publishing this paper was defrayed in part by the payment of page charges. Under postal regulations, this paper therefore must be hereby marked as advertisement solely to indicate this fact.     

Genetic and non-genetic factors affecting anther culture of Brussels sprouts (Brassica oleracea var. gemmifera)

Summary Eight inbred lines of Brussels sprouts and ten F₁ hybrids derived from them were tested for their response to anther culture. From 5–19 plants per genotype were tested, and each plant was tested on 3–6 separate occasions. Results from the inbred lines were broadly similar to those from the F₁ hybrids, despite the inbreds producing fewer buds and having a higher frequency of anther deformities. The maximum embryo yield from an inbred line was 215 embryos per 100 anthers, and from a hybrid was 275. From estimation of the variance components it was calculated that, for both inbreds and hybrids, about half the total variation was genetic whereas variation due to plants within genotypes and to occasions within plants were each about 13% of the total. The narrow sense heritability of responsiveness to anther culture (estimated by the proportion of variation between inbred lines which was genetic) was 0.48, and there was partial dominance for this character. In three cases the hybrid outyielded the better inbred, and this heterosis may well be due to dispersed dominant genes.     

盐度对长牡蛎和近江牡蛎及其杂交稚贝生长和存活的影响

2011年8月以长牡蛎自繁组GG(Crassostrea gigas♀×C.gigas♂),近江牡蛎自繁组AA(C.ariakensis♀×C.ariakensis♂)、正交组GA(Crassostrea gigas♀×C.ariakensis♂)、反交组AG(C.ariakensis♀×C.gigas♂)为实验材料,开展了稚贝对盐度的适应性研究。结果发现长牡蛎的最适生存盐度为15—35,最适生长盐度为25—35;近江牡蛎的最适生存盐度为10—25,最适生长盐度为20—25;GA的最适生存盐度为15—30,最适生长盐度为15—30,AG的最适生存盐度为20—30,最适生长盐度为20—25。GG对低盐度敏感,AA对高盐度敏感,AG具有高盐度存活的杂种优势,在盐度30时,中亲杂种优势HG×A为13.32,单亲杂种优势HGA和HAG分别为1.89和27.88,在盐度40时,HAG上升到400,GA和AG都不具有生长优势。杂种稚贝对盐度适应介于双亲之间,且表现出一定程度的父系遗传特点。     

A comparison of shoot regeneration from protoplasts and leaf discs of different genotypes of the cultivated tomato

Summary Shoot regeneration from leaf discs and leaf mesophyll protoplasts of 11 genotypes of Lycopersicon esculentum (the cultivated tomato), were compared. In both regeneration procedures genotypic differences were observed between inbred lines, and also between F1 hybrids and their parental lines. In the tested hybrid genotypes no heterosis effect with respect to shoot regeneration capacity was observed. A correlation between shoot regeneration from leaf discs and from leaf mesophyll protoplasts was apparent in the tested genotypes. This suggests that using the described procedure, shoot regeneration from leaf discs can be usef for rapid pre-screening for regeneration capacity from protoplasts of tomato genotypes.     

Do rice hybrids have heterosis for insect resistance? A study with Nilaparvata lugens (Hemiptera: Delphacidae) and Marasmia patnalis (Lepidoptera: Pyralidae)

Antibiosis-based resistance to two insect pests of rice, Nilaparvata lugens (St?l) (Hemiptera: Delphacidae) and Marasmia patnalis Bradley (Lepidoptera: Pyralidae), was compared in 11 F1 hybrids and their parental lines. Our objective was to determine whether hybrids show heterosis (hybrid vigor) for insect resistance or susceptibility. Heterosis is defined as the amount by which a hybrid exceeds its midparent value or its better parent. Overall, we did not find evidence of heterosis or heterobeltiosis (a type of heterosis in which a hybrid exceeds its better parent) for antibiosis-based resistance or susceptibility to either of the insects. One hybrid, IR64616H, seemed more resistant to M. patnalis than its better parent but none of the other hybrids showed heterobeltiosis for resistance or susceptibility to either insect. Three hybrids had resistance to N. lugens that exceeded their midparent value, possibly due to dominant resistance in one of the parents. The increased frequency and severity of insect outbreaks on hybrid rice that have been reported in China may be attributable to factors other than diminished antibiosis in hybrids, such as greater attractiveness of hybrids to migrating or dispersing insects or differences in agronomic practices applied to hybrids and inbred rice cultivars.     

Hybrid wheat: quantitative genetic parameters and consequences for the design of breeding programs

Key message

Commercial heterosis for grain yield is present in hybrid wheat but long-term competiveness of hybrid versus line breeding depends on the development of heterotic groups to improve hybrid prediction.

Abstract

Detailed knowledge of the amount of heterosis and quantitative genetic parameters are of paramount importance to assess the potential of hybrid breeding. Our objectives were to (1) examine the extent of midparent, better-parent and commercial heterosis in a vast population of 1,604 wheat (Triticum aestivum L.) hybrids and their parental elite inbred lines and (2) discuss the consequences of relevant quantitative parameters for the design of hybrid wheat breeding programs. Fifteen male lines were crossed in a factorial mating design with 120 female lines, resulting in 1,604 of the 1,800 potential single-cross hybrid combinations. The hybrids, their parents, and ten commercial wheat varieties were evaluated in multi-location field experiments for grain yield, plant height, heading time and susceptibility to frost, lodging, septoria tritici blotch, yellow rust, leaf rust, and powdery mildew at up to five locations. We observed that hybrids were superior to the mean of their parents for grain yield (10.7 %) and susceptibility to frost (?7.2 %), leaf rust (?8.4 %) and septoria tritici blotch (?9.3 %). Moreover, 69 hybrids significantly (P < 0.05) outyielded the best commercial inbred line variety underlining the potential of hybrid wheat breeding. The estimated quantitative genetic parameters suggest that the establishment of reciprocal recurrent selection programs is pivotal for a successful long-term hybrid wheat breeding.     

Heterosis in the flour moth,Ephestia kühniella

Four lines of Ephestia kühniella each homozygous for one of three different allozyme alleles at the Est-2 locus and two alleles at the Adh locus were crossed in order to study the extent of somatic, reproductive and adaptive heterosis in F₁ hybrids in comparison with the mean performance of the simultaneously reared inbred parent lines. With regard to adult weight and wing length (somatic heterosis) hybrids exhibit maximally 20% (males 10%) and 9% heterosis, respectively. As concerns the production of eggs and hatched larvae (reproductive heterosis) hybrids exceed the parental mean by 90% and 200%. Adaptive heterosis is realized by a shorter development period of the hybrids (maximally by 30%) as well as by significant lower variance of all metrical characters studied. In the F₂ the degree of heterosis diminishes. There is neither an excess of heterozygotes among the segregating allozyme genotypes nor superior performance of the heterozygotes concerning any one of the traits studied. Therefore, it is concluded that the pronounced heterosis in F₁ is not a single-gene-heterosis operating at the Est-2 and the Adh-locus.