Cause Of Heterosis Research Articles

Comprehensive molecular evaluation of the histone methyltransferase gene family and their important roles in two-line hybrid wheat

BackgroundHistone methylation usually plays important roles in plant development through post-translational regulation and may provide a new visual field for heterosis. The histone methyltransferase gene family has been identified in various plants, but its members and functions in hybrid wheat related in heterosis is poorly studied.ResultsIn this study, 175 histone methyltransferase (HMT) genes were identified in wheat, including 152 histone lysine methyltransferase (HKMT) genes and 23 protein arginine methyltransferase (PRMT) genes. Gene structure analysis, physicochemical properties and subcellular localization predictions of the proteins, exhibited the adequate complexity of this gene family. As an allohexaploid species, the number of the genes (seven HKMTs orthologous groups and four PRMTs orthologous groups) in wheat were about three times than those in diploids and showed certain degrees of conservation, while only a small number of subfamilies such as ASH-like and Su-(var) subfamilies have expanded their members. Transcriptome analysis showed that HMT genes were mainly expressed in the reproductive organs. Expression analysis showed that some TaHMT genes with different trends in various hybrid combinations may be regulated by lncRNAs with similar expression trends. Pearson correlation analysis of the expression of TaHMT genes and two yield traits indicated that four DEGs may participate in the yield heterosis of two-line hybrid wheat. ChIP-qPCR results showed that the histone modifications (H3K4me3, H3K36me3 and H3K9ac) enriched in promoter regions of three TaCCA1 genes which are homologous to Arabidopsis heterosis-related CCA1/LHY genes. The higher expression levels of TaCCA1 in F1 than its parents are positive with these histone modifications. These results showed that histone modifications may play important roles in wheat heterosis.ConclusionsOur study identified characteristics of the histone methyltransferase gene family and enhances the understanding of the evolution and function of these members in allohexaploid wheat. The causes of heterosis of two-line hybrid wheat were partially explained from the perspective of histone modifications.

Construction of a high-density genetic linkage map and QTL mapping for growth traits in hybrid Epinephelus fuscoguttatus (♀) and Epinephelus tukula (♂) progeny

QTL mapping is an effective method for marker assisted selection (MAS) in animal breeding, and high-density linkage genetic maps are necessary for fine mapping of QTLs. In this study, high-density genetic maps of hybrid Epinephelus fuscoguttatus (♀) and Epinephelus tukula (♂) progeny (which demonstrated improved growth rates compared with the female parent) were constructed using RAD technology. Sex-average, female-specific, and male-specific linkage maps were composed of 24 linkage groups (LGs). The genetic map developed in this study had the highest number of SNPs among reported genetic maps for groupers. The sex-average linkage map contained 2759 bin makers (150,468 SNPs) and spanned 1627.28 cM, with an average marker interval of 0.59 cM. The female-specific (E. fuscoguttatus) linkage map contained 648 bin makers (38,269 SNPs) and spanned 1414.68 cM, with an average marker interval of 2.22 cM. The male-specific (E. tukula) linkage map contained 2126 bin makers (112,015 SNPs) and spanned 1839.88 cM, with an average marker interval of 0.86 cM. In total, 45 QTLs comprising 318 SNPs for growth-related traits were identified, including 10 QTLs for body weight, 6 QTLs for full length, 7 QTLs for body length, 14 QTLs for head length, 5 QTLs for body height, and 3 QTLs for body thickness. In addition, 56 candidate genes related to growth, metabolism, bone formation, muscle development, and neural regulation were identified, including eight genes associated with growth in other fish. Among these, gpr126, fam20a, fam20c, sox9b, foxm1, foxl1, myf6, zbtb38, and sl should be a focus of further research. These results provide a foundation for fine QTL mapping in grouper, and theoretical guidance for exploring the causes of heterosis, which is of great significance for MAS in the breeding of hybrids and male and female parents.

Effect of the over-dominant expression of proteins on nicotine heterosis via proteomic analysis

Heterosis is a common biological phenomenon that can be used to optimize yield and quality of crops. Using heterosis breeding, hybrids with suitable nicotine content have been applied to tobacco leaf production. However, the molecular mechanism of the formation of nicotine heterosis has never been explained from the perspective of protein. The DIA proteomics technique was used to compare the differential proteomics of the hybrid Va116 × Basma, showing strong heterosis in nicotine content from its parent lines Va116 and Basma. Proteomics analysis indicated that 65.2% of DEPs showed over-dominant expression patterns, and these DEPs included QS, BBL, GS, ARAF and RFC1 which related to nicotine synthesis. In addition, some DEPs (including GST, ABCE2 and ABCF1 and SLY1) that may be associated with nicotinic transport exhibited significant heterosis over the parental lines. These findings demonstrated that the efficiency of the synthesis and transport of nicotine in hybrids was significantly higher than that in the parent lines, and the accumulation of over-dominant expression proteins may be the cause of heterosis of nicotinic content in hybrids.

Transcriptomic and proteomic analyses of a new cytoplasmic male sterile line with a wild Gossypium bickii genetic background

BackgroundCotton is an important fiber crop but has serious heterosis effects, and cytoplasmic male sterility (CMS) is the major cause of heterosis in plants. However, to the best of our knowledge,no studies have investigated CMS Yamian A in cotton with the genetic background of Australian wild Gossypium bickii. Conjoint transcriptomic and proteomic analysis was first performed between Yamian A and its maintainer Yamian B.ResultsWe detected 550 differentially expressed transcript-derived fragments (TDFs) and at least 1013 proteins in anthers at various developmental stages. Forty-two TDFs and 11 differentially expressed proteins (DEPs) were annotated by analysis in the genomic databases of G. austral, G. arboreum and G. hirsutum. Gene Ontology and Kyoto Encyclopedia of Genes and Genomes pathway analyses were performed to better understand the functions of these TDFs and DEPs. Transcriptomic and proteomic results showed that UDP-glucuronosyl/UDP-glucosyltransferase, 60S ribosomal protein L13a-4-like, and glutathione S-transferase were upregulated; while heat shock protein Hsp20, ATPase, F0 complex, and subunit D were downregulated at the microspore abortion stage of Yamian A. In addition, several TDFs from the transcriptome and several DEPs from the proteome were detected and confirmed by quantitative real-time PCR as being expressed in the buds of seven different periods of development. We established the databases of differentially expressed genes and proteins between Yamian A and its maintainer Yamian B in the anthers at various developmental stages and constructed an interaction network based on the databases for a comprehensive understanding of the mechanism underlying CMS with a wild cotton genetic background.ConclusionWe first analyzed the molecular mechanism of CMS Yamian A from the perspective of omics, thereby providing an experimental basis and theoretical foundation for future research attempting to analyze the abortion mechanism of new CMS with a wild Gossypium bickii background and to realize three-line matching.

Exploring the molecular basis of heterosis for plant breeding.

Since approximate a century ago, many hybrid crops have been continually developed by crossing two inbred varieties. Owing to heterosis (hybrid vigor) in plants, these hybrids often have superior agricultural performances in yield or disease resistance succeeding their inbred parental lines. Several classical hypotheses have been proposed to explain the genetic causes of heterosis. During recent years, many new genetics and genomics strategies have been developed and used for the identifications of heterotic genes in plants. Heterotic effects of the heterotic loci and molecular functions of the heterotic genes are being investigated in many plants such as rice, maize, sorghum, Arabidopsis and tomato. More and more data and knowledge coming from the molecular studies of heterotic loci and genes will serve as a valuable resource for hybrid breeding by molecular design in future. This review aims to address recent advances in our understanding of the genetic and molecular mechanisms of heterosis in plants. The remaining scientific questions on the molecular basis of heterosis and the potential applications in breeding are also proposed and discussed.

Genetic basis of heterosis for yield and yield components explored by QTL mapping across four genetic populations in upland cotton

BackgroundQuantitative trait loci (QTL) mapping provides a powerful tool to unravel the genetic bases of cotton yield and its components, as well as their heterosis. In the present study, the genetic basis underlying inbreeding depression and heterosis for yield and yield components of upland cotton was investigated in recombinant inbred line (RIL), immortalized F2 (IF2), and two backcross (BCF1) populations based on a high-density SNP linkage map across four environments.ResultsSignificant inbreeding depression of fruit branches per plant (FB), boll numbers per plant (BN), seed cotton yield (SY), and lint yield (LY) in RIL population and high levels of heterosis for SY, LY, and boll weight (BW) in IF2 and two BCF1 populations were observed. A total of 285 QTLs were identified in the four related populations using a composite interval mapping approach. In the IF2 population, 26.60% partially dominant (PD) QTLs and 71.28% over-dominant (OD) QTLs were identified. In two BCF1 populations, 42.41% additive QTLs, 4.19% PD QTLs, and 53.40% OD QTLs were detected. For multi-environment analysis, phenotypic variances (PV) explained by e-QTLs were higher than those by m-QTLs in each of the populations, and the average PV of m-QTLs and e-QTLs explained by QTL × environment interactions occupied a considerable proportion of total PV in all seven datasets.ConclusionsAt the single-locus level, the genetic bases of heterosis varied in different populations. Partial dominance and over-dominance were the main cause of heterosis in the IF2 population, while additive effects and over-dominance were the main genetic bases of heterosis in two BCF1 populations. In addition, the various genetic components to heterosis presented trait specificity. In the multi-environment model analysis, epistasis was a common feature of most loci associated with inbreeding depression and heterosis. Furthermore, the environment was a critical factor in the expression of these m-QTLs and e-QTLs. Altogether, additive effects, over-dominance, epistasis and environmental interactions all contributed to the heterosis of yield and its components in upland cotton, with over-dominance and epistasis more important than the others.

QTL Mapping and Heterosis Analysis for Fiber Quality Traits Across Multiple Genetic Populations and Environments in Upland Cotton.

An “immortalized F2” (IF2) population and two reciprocal backcross (HSBCF1 and MARBCF1) populations were constructed to investigate the genetic bases of fiber quality traits in upland cotton across four different environments. A relatively high level of heterosis for micronaire (MIC) in IF2 population as well as fiber length (FL) and MIC in MARBCF1 population was observed. A total of 167 quantitative trait loci (QTLs) were detected in the three related experimental populations and their corresponding midparental heterosis (MPH) datasets using the composite interval mapping (CIM) approach. An analysis of genetic effects of QTLs detected in different populations and their MPH datasets showed 16 (24.24%) QTLs of partial dominance, and 46 (69.70%) QTLs of overdominance were identified in an IF2 population; 89 (62.68%) additive QTLs, three (2.11%) partial dominant QTLs, and 49 (34.51%) over-dominant QTLs were detected in two BCF1 populations. Multi-environment analysis showed 48 and 56 main-QTLs (m-QTLs) and 132 and 182 epistasis-QTLs (e-QTLs), by inclusive composite interval mapping (ICIM) in IF2 and two BCF1 populations, respectively. Phenotypic variance explained by e-QTLs, except for MARBCF1 population, was higher than that by m-QTLs. Thus, the overdominant, partial dominant, and epistasis effects were the main causes of heterosis in the IF2 population, whereas the additive, overdominant, and epistasis effects were the primary genetic basis of heterosis in the two BCF1 populations. Altogether, additive effect, partial dominance, overdominance, and epistasis contributed to fiber quality heterosis in upland cotton, but overdominance and epistasis were the most important factors.

Another explanation for the cause of heterosis phenomenon.

The explanation for heterosis phenomenon is based on ideas: (i) every trait of an organism depends on many genes. (ii) Inbreeding depression and heterosis are related to individual genetic diversity. To assess individual genetic diversity of an organism, I suggest the term number of genetic properties. Assessing the changes of individual genetic diversity caused by self-pollination and cross-pollination reveals that self-pollinating plants of natural cross-pollinating leads to the decrease in individual genetic diversity of offspring and crossing between pure lines of genetic difference leads to the increase in individual genetic diversity of hybrids. Therefore, I propose that the decrease in individual genetic diversity is the cause the of depression and the increase in individual genetic diversity is the cause of heterosis.

Heterosis for yield and its related traits in Okra (Abelmoschus esculentus L. Moench)

Eight parental lines of okra and their 28 F 1 hybrids obtained from half diallel were studied to investigate the extent of heterosis for yield and yield attributing characters. The magnitude of heterosis varied from cross to cross for all the characters studied. Maximum positive heterosis for fruit yild per plant over better parent and standard check (JOH 2) was observed to be 62.12 and 44.11 per cent, respectively. The cause of heterosis may be due to its component traits, mainly, days to first flowering, nodes per plant, length of internode, fruit weight and fruits per plant. The best performing hybrid AOL 09-25 x AOL 09-26 which recorded 44.11 per cent heterosis for yield over standard check may be exploited for commercial cultivation.

Heterosis for water status in maize seedlings

Heterosis is one of the greatest practical achievements of plant breeding and has been extensively used in crop improvement in maize. However, the physiological basis of heterosis still remains poorly understood despite its manifestation at early stages of the plant life cycle. Therefore, a better understanding of the physiological mechanisms associated with heterosis may enable further exploitation of this phenomenon. Five maize hybrids and their parental lines were grown in a greenhouse under two water regimes (well watered and water stressed). Plant growth, different root traits, plant water use and water use efficiency and gas-exchange and chlorophyll fluorescence, nitrogen content and stable oxygen, carbon and nitrogen composition and stomatal density of leaves were measured. Plant height, shoot biomass and leaf area were higher in hybrids than in their respective parents in both control and stress conditions. Moreover, hybrids showed a better water use and water use efficiency for biomass than inbred lines. Significant heterosis was also found for photosynthetic rate, stomatal conductance, transpiration rate and the Ci/Ca ratio under water stress conditions, whereas for control conditions the differences were not significant. Likewise, root weight density and root length density were higher in hybrids than parents, especially underwater stress. No heterosis occurred for stable isotope composition under either water regime. The results do not support a constitutively better water status as a cause of heterosis under well watered conditions.

Accounting for expected linkage in biometric analysis of quantitative traits

The problem of accounting for a genetic estimation of expected linkage in the disposition of random loci was solved for the additive-dominant model. The Comstock-Robinson estimations for the sum of squares of dominant effects, the sum of squares of additive effects, and the average degree of dominance were modified. Also, the Wright's estimation for the number of loci controlling the variation of a quantitative trait was modified and its application sphere was extended. Formulas that should eliminate linkage, on average, were derived for these estimations. Nonbiased estimations were applied to the analysis of maize data. Our result showed that the most likely cause of heterosis is dominance rather than overdominance and that the main part of the heterotic effect is provided by dozens of genes.

Combining Ability Analysis to Identify Superior F1 Hybrids for Yield and Quality Improvement in Tomato (Solanum lycopersicum L.)

Combining ability and gene effects for yield and quality traits in tomato were studied by involving twenty four cross combinations obtained from crossing eight diverse lines with three testers in line×tester mating fashion. The analysis of variance revealed that the variance due to line×tester effects were highly significant for all the traits except lycopene content under study. Combining ability analysis revealed that magnitude of Specific Combining Ability (SCA) variance was greater than General Combining Ability (SCA) variance suggesting the predominance of non-additive gene action for yield per plant, pericarp thickness, TSS, titrable acidity, lycopene and shelf life. The degree of dominance revealed that over dominance is the cause of heterosis for these traits. Based on gca effects of parents, the lines LE-62 and LE-53 and the testers Arka Meghali and Arka Vikas were good general combiners for most of the traits under study. The crosses viz., EC-157568×Arka Vikas, EC-163611×Arka Alok, LE-62×Arka Alok and LE-64×Arka Vikas were found to be superior specific combiners for yield per plant. For quality traits, the cross EC-165749×Arka Alok was also superior specific combiner for yield per plant, TSS, ascorbic acid and shelf life and the cross EC-157568×Arka Alok was superior specific combiner for TSS, titrable acidity and lycopene.

Genetic basis of heterosis and prediction of hybrid performance

SUMMARY Overview and scope of the presentation: Efficient exploitation of heterosis is central for successful hybrid breeding in sunflower. Here we discussed important genetic hypotheses of the phenomenon of heterosis including dominance, overdominance, and epistasis. Moreover, we presented different approaches suggested for prediction of hybrid performance. Outline of recent major discoveries: Several experimental designs have been proposed to study the causes of heterosis. Among them, QTL analyses based on triple test cross in combination with the immortalized F2 design or the use of triple test cross designs based on near-isogenic lines are of particular interest. Several approaches have been used to predict hybrid performance using phenotypic, genomic, transcriptomic, and metabolomic data. Prediction accuracies have been studied using experimental data of maize, which clearly points towards the large potential for knowledge-based hybrid breeding. Perceived significant gaps in research, current debates, and perspectives for future directions of research: Experimental design and quantitative genetic theory is ready to be applied in sunflower to study the genetic basis of heterosis. Furthermore, we speculate that genomic selection possesses great potential to revolutionize hybrid prediction in sunflower.

Heterosis in plants: Manifestation in early seed development and prediction approaches to assist hybrid breeding

Heterosis, or hybrid vigour, describes the superior performance of heterozygous hybrid individuals compared with their homozygous parental inbred lines. Although heterosis has been intensively used in plant breeding, the molecular and genetic mechanism underlying the phenomenon remains largely unknown. During the last years numerous laboratories initiated genomic approaches with new, often genome wide tools toward the elucidation of the molecular basis of heterosis. Various studies described differences in genome organization and gene expression of hybrids and their parental inbred lines. In maize, a considerable loss of co-linearity at many loci between different inbred lines was observed. Expression profile comparisons between inbred lines and hybrids revealed complex transcriptional networks specific for different developmental stages and tissues mainly in maize (Zea mays), rice (Oryza sativa) and Arabidopsis (Arabidopsis thaliana). Allele-specific expression data resolved the relative parental contributions and allowed figuring out the regulatory basis for expression variation in hybrids. Integrating all these complex expression data might help to get an idea about the molecular basis of heterosis. Thereby molecular processes during early seed development shortly after fertilization might be of particular importance, because the allelic interplay has to be coordinated after the unification of two diverse genomes and these processes might contribute to establish the basis for future performance of the sporophyte. Besides these fundamental interests in the molecular basis and manifestation of hybrid vigour applied aspects of the phenomenon are of high importance to support plant breeding and agriculture. Prediction methods are of special interest to identify the most promising parental lines of hybrid varieties, greatly reducing the financial effort and increasing the efficiency to develop new hybrid cultivars. Until now, most prediction approaches were based on genomic markers. The available heterosis associated expression data provide evidence to support the idea that gene regulatory networks at the level of transcription are involved in the control of hybrid vigour. If so, the transcriptome characteristics of inbred lines resulting from their individual genomic constitution should be useful as quantitative markers to predict the performance of hybrids generated by crosses of these inbred lines. Additionally, expression based prediction models promise to contribute substantially to the understanding of the genetic causes of heterosis by establishing a direct link between the transcriptome and hybrid performance.

Heterosis for Biomass-Related Traits in Arabidopsis Investigated by Quantitative Trait Loci Analysis of the Triple Testcross Design With Recombinant Inbred Lines

Arabidopsis thaliana has emerged as a leading model species in plant genetics and functional genomics including research on the genetic causes of heterosis. We applied a triple testcross (TTC) design and a novel biometrical approach to identify and characterize quantitative trait loci (QTL) for heterosis of five biomass-related traits by (i) estimating the number, genomic positions, and genetic effects of heterotic QTL, (ii) characterizing their mode of gene action, and (iii) testing for presence of epistatic effects by a genomewide scan and marker x marker interactions. In total, 234 recombinant inbred lines (RILs) of Arabidopsis hybrid C24 x Col-0 were crossed to both parental lines and their F1 and analyzed with 110 single-nucleotide polymorphism (SNP) markers. QTL analyses were conducted using linear transformations Z1, Z2, and Z3 calculated from the adjusted entry means of TTC progenies. With Z1, we detected 12 QTL displaying augmented additive effects. With Z2, we mapped six QTL for augmented dominance effects. A one-dimensional genome scan with Z3 revealed two genomic regions with significantly negative dominance x additive epistatic effects. Two-way analyses of variance between marker pairs revealed nine digenic epistatic interactions: six reflecting dominance x dominance effects with variable sign and three reflecting additive x additive effects with positive sign. We conclude that heterosis for biomass-related traits in Arabidopsis has a polygenic basis with overdominance and/or epistasis being presumably the main types of gene action.

Analysis of a Triple Testcross Design With Recombinant Inbred Lines Reveals a Significant Role of Epistasis in Heterosis for Biomass-Related Traits in Arabidopsis

Primary causes of heterosis are still unknown. Our goal was to investigate the extent and underlying genetic causes of heterosis for five biomass-related traits in Arabidopsis thaliana. We (i) investigated the relative contribution of dominance and epistatic effects to heterosis in the hybrid C24 x Col-0 by generation means analysis and estimates of variance components based on a triple testcross (TTC) design with recombinant inbred lines (RILs), (ii) estimated the average degree of dominance, and (iii) examined the importance of reciprocal and maternal effects in this cross. In total, 234 RILs were crossed to parental lines and their F1's. Midparent heterosis (MPH) was high for rosette diameter at 22 days after sowing (DAS) and 29 DAS, growth rate (GR), and biomass yield (BY). Using the F2-metric, directional dominance prevailed for the majority of traits studied but reciprocal and maternal effects were not significant. Additive and dominance variances were significant for all traits. Additive x additive and dominance x dominance variances were significant for all traits but GR. We conclude that dominance as well as digenic and possibly higher-order epistatic effects play an important role in heterosis for biomass-related traits. Our results encourage the use of Arabidopsis hybrid C24 x Col-0 for identification and description of quantitative trait loci (QTL) for heterosis for biomass-related traits and further genomic studies.

Transcriptomic analysis of growth heterosis in larval Pacific oysters ( Crassostrea gigas )

Compared with understanding of biological shape and form, knowledge is sparse regarding what regulates growth and body size of a species. For example, the genetic and physiological causes of heterosis (hybrid vigor) have remained elusive for nearly a century. Here, we investigate gene-expression patterns underlying growth heterosis in the Pacific oyster (Crassostrea gigas) in two partially inbred (f = 0.375) and two hybrid larval populations produced by a reciprocal cross between the two inbred families. We cloned cDNA and generated 4.5 M sequence tags with massively parallel signature sequencing. The sequences contain 23,274 distinct signatures that are expressed at statistically nonzero levels and show a highly positively skewed distribution with median and modal counts of 9.25 million and 3 transcripts per million, respectively. For nearly half of these signatures, expression level depends on genotype and is predominantly nonadditive (hybrids deviate from the inbred average). Statistical contrasts suggest approximately 350 candidate genes for growth heterosis that exhibit concordant nonadditive expression in reciprocal hybrids; this represents only approximately 1.5% of the >20,000 transcripts. Patterns of gene expression, which include dominance for low expression and even underdominance of expression, are more complex than predicted from classical dominant or overdominant explanations of heterosis. Preliminary identification of ribosomal proteins among candidate genes supports the suggestion from previous studies that efficiency of protein metabolism plays a role in growth heterosis.

Heterosis for biomass yield and related traits in five hybrids of Arabidopsis thaliana L. Heynh.

Heterosis is of utmost economic importance in plant breeding. However, its underlying molecular causes are still unknown. Given the numerous advantages of Arabidopsis thaliana as a model species in plant genetics and genomics, we assessed the extent of heterosis in this species using five hybrids derived from five ecotypes. Parents, F(1) and F(2), generations in both reciprocal forms were grown in a greenhouse experiment with four replications. Mid-parent heterosis (MPH) and best-parent heterosis (BPH) averaged across hybrids were surprisingly high for biomass yield (MPH: 60.3%; BPH: 32.9%) and rosette diameter (MPH: 49.4%; BPH: 34.8%), but smaller for flowering date (MPH: 27.5%; BPH: 18.5%), seed yield (MPH: 18.9%; BPH: 1.7%), and yield components. Individual hybrids varied considerably in their MPH and BPH values for all traits, one cross displaying 140.1% MPH for biomass yield. MPH was not associated with parental genetic distance determined from molecular markers. Reciprocal effects were significant only in a few cases. With a proper choice of hybrids, our results encourage the use of Arabidopsis as a model species for investigating the molecular causes of heterosis.

Estimating within-locus nonadditive coefficient and discriminating dominance versus overdominance as the genetic cause of heterosis.

Testing (over)dominance as the genetic cause of heterosis and estimating the (over)dominance coefficient (h) are related. Using simulations, we investigate the statistical properties of Mukai's approach, which is intended to estimate the average (h) of hi across loci by regression of outcrossed progeny on the sum of the two corresponding homozygous parents. A new approach for estimating h is also developed, utilizing data on families formed by multiple selfed genotypes from each outcrossed parent, thus not requiring constructing homozygotes. Assuming constant mutation effects, h can be estimated accurately by both approaches under dominance. When rare alleles have low frequencies at any polymorphic locus, Mukai's approach can estimate h accurately under over(under)dominance. Therefore, the (over)dominance hypothesis for heterosis can be tested by estimating h, under either dominance or overdominance at all genomic loci. However, this is invalid with more plausible mixed dominance and overdominance at different loci. Estimating the variance of hi across loci is also investigated. In self-compatible outcrossing populations with mutations of variable effects and lethals, our new approach is better than Mukai's, not only because of not requiring homozygotes but also because of the better statistical performance reflected by the smaller mean square errors of the estimates.

Genetic causes of heterosis in juvenile aspen:a quantitative comparison across intra- and inter-specific hybrids

The genetic causes of heterosis in tree growth were investigated by a comparative genetic analysis of intra- and inter-specific crosses derived from Populus tremuloides and P. tremula. A new analytical method was developed to estimate the effective number of loci affecting a quantitative trait and the magnitudes of their additive and dominant effects across loci. The method combines the assumption of multiple alleles, as frequently found in outcrossing species, and the family structure analysis at different hierarchical levels. During the first 3 years of growth, interspecific hybrids displayed strong heterosis in stem growth, especially volume index, over intraspecific hybrids. By a series of joint analyses on the combining ability and the genetic component, we found that F1 heterosis might be due to overdominant interaction between two alleles, one from the P. tremuloides parent and the other from the P. tremula parent, at the same loci. This inference was derived from the finding that heterozygotes, newly formed through species combination, showed much greater growth than the heterozygotes from intraspecifc crosses at a reference locus. Heterosis in aspen growth appeared to be under multi-genic control, with a slightly larger number of loci for stem diameter and volume (9-10) than for height (6-8). For traits with non-significant heterosis, such as stem allometry and internode number and length, the number of underlying loci seemed to be much fewer (3-4). While additive effects appeared to influence seedling traits collectively across loci, a few major dominant loci had much larger effects on stem growth.