Localization of quantitative trait loci (QTL) for agronomic important characters by the use of a RFLP map in barley (Hordeum vulgare L.)

Two hundred and fifty doubled haploid lines were studied from a cross between two 2-row winter barley varieties. The lines were evaluated for several characters in a field experiment for 3 years on two locations with two replications. From a total of 431 RFLP probes 50 were found to be polymorphic and subsequently used to construct a linkage map. Quantitative trait loci (QTLs) were determined and localized for resistance against Rhynchosporium secalis and Erysiphe graminis, for lodging, stalk breaking and ear breaking tendency, for the physical state before harvest, plant height, heading date, several kernel parameters and kernel yield. The heritability of the traits ranged from 0.56 to 0.89. For each trait except for kernel thickness, QTLs have been localized that explain 5–52% of the genetic variance. Transgressive segregation occurred for all of the traits studied.     

New insights into the genetics of in vivo induction of maternal haploids, the backbone of doubled haploid technology in maize

Haploids and doubled haploid (DH) inbred lines have become an invaluable tool for maize genetic research and hybrid breeding, but the genetic basis of in vivo induction of maternal haploids is still unknown. This is the first study reporting comparative quantitative trait locus (QTL) analyses of this trait in maize. We determined haploid induction rates (HIR) in testcrosses of a total of 1061 progenies of four segregating populations involving two temperate haploid inducers, UH400 (HIR = 8%) and CAUHOI (HIR = 2%), one temperate and two tropical inbreds with HIR = 0%, and up to three generations per population. Mean HIR of the populations ranged from 0.6 to 5.2% and strongly deviated from the midparent values. One QTL (qhir1) explaining up to p = 66% of the genetic variance was detected in bin 1.04 in the three populations involving a noninducer parent and the HIR-enhancing allele was contributed by UH400. Segregation ratios of loci in bin 1.04 were highly distorted against the UH400 allele in these three populations, suggesting that transmission failure of the inducer gamete and haploid induction ability are related phenomena. In the CAUHOI × UH400 population, seven QTL were identified on five chromosomes, with qhir8 on chromosome 9 having p > 20% in three generations of this cross. The large-effect QTL qhir1 and qhir8 will likely become fixed quickly during inducer development due to strong selection pressure applied for high HIR. Hence, marker-based pyramiding of small-effect and/or modifier QTL influencing qhir1 and qhir8 may help to further increase HIR in maize. We propose a conceptual genetic framework for inheritance of haploid induction ability, which is also applicable to other dichotomous traits requiring progeny testing, and discuss the implications of our results for haploid inducer development.     

Composite interval mapping reveals a major locus influencing embryonic development rate in rainbow trout (Oncorhynchus mykiss)

Little is known about the genetics controlling the rate of embryonic development in salmonids, despite the fact that this trait plays an important role in the life history of wild and cultured stocks. We investigated the genetics of embryonic development rate by performing an analysis of quantitative trait loci (QTL) on two families of androgenetically derived doubled haploid rainbow trout produced from a hybrid of two clonal lines with divergent embryonic development rates. A total of 170 doubled haploid individuals were genotyped at 222 marker loci [219 amplified fragment length polymorphism (AFLP) markers, 2 microsatellites, and p53]. A genetic linkage analysis resulted in a map consisting of 27 linkage groups with 21 of the markers remaining unlinked at a minimum LOD of 3.0 and maximum theta of 0.40. Eight of these linkage groups were matched to published rainbow trout linkage groups. Composite interval mapping (CIM) revealed evidence for two QTL influencing time to hatch, and suggestive evidence for a third. These QTL accounted for a total of 24.6% of the variation in time to hatch. One of these QTL had a large effect on development rate, especially in one family of doubled haploids, in which it explained 25.6% of the variance in time to hatch. QTL influencing embryonic length and weight at the commencement of exogenous feeding were also identified. The QTL with the strongest effect on embryonic length (lenR13) mapped to the same position as the QTL with the strongest effect on time to hatch (tthR13), suggesting a single QTL may have a pleiotropic effect on both these traits. These results suggest that the use of clonal lines with a doubled haploid crossing design is an effective way of analyzing the genetic basis of complex traits in salmonids.     

Genetic parameter estimates in a clonally replicated progeny test of teak (Tectona grandis Linn. f.)

Teak (Tectona grandis Linn. f.) has been planted extensively in the tropics for its highly valued timber. We analysed data from a 3.5-year-old teak progeny test with clonal replication located in northern Australia. Additive and non-additive genetic variances were estimated for commercially important traits. Trees originating from seedlings were on average 2% taller and 4% straighter than those of the same genotype originating from cuttings. Non-additive genetic variance represented 35–50% of total genetic variance for growth traits and 63% of total genetic variance for incidence of flowering. Narrow-sense heritability was 0.22 for diameter, 0.18 for height and volume, 0.07 for stem straightness, 0.05 for insect defoliation, 0.03 for epicormic sprouts and 0.30 for incidence of flowering (estimated on an assumed underlying continuous scale). Broad-sense heritability was 0.37 for diameter, 0.28 for height, 0.35 for volume, 0.12 for stem straightness, 0.06 for insect defoliation, 0.12 for epicormic sprouts and 0.71 for incidence of flowering. Positive correlations were found between tree volume and flowering and between tree volume and stem straightness. The presence of sizeable non-additive variance supports the selection and deployment of clones to capture the full extent of genetic variation in commercially important traits.     

Identification of associations between SSR markers and fiber traits in an exotic germplasm derived from multiple crosses among Gossypium tetraploid species

Genetic improvement in yield and fiber quality is needed for worldwide cotton production. Identification of molecular markers associated with fiber-related traits can facilitate selection for these traits in breeding. This study was designed to identify associations between SSR markers and fiber traits using an exotic germplasm population, species polycross (SP), derived from multiple crosses among Gossypium tetraploid species. The SP population underwent 11 generations of mixed random mating and selfing followed by 12 generations of selfing. A total of 260 lines were evaluated for fiber-related traits under three environments in 2005 and 2006. Large genotypic variance components in traits were identified relative to components of genotype × environment. Eighty-six primer pairs amplified a total of 314 polymorphic fragments among 260 lines. A total of 202 fragments with above 6% allele frequency were analyzed for associations. Fifty-nine markers were found to have a significant (P < 0.05, 0.01, or 0.001) association with six fiber traits. There were six groups identified within the population using structure analysis. Allele frequency divergence among six groups ranged from 0.11 to 0.27. Of the 59 marker–trait associations, 39 remained significant after correction for population structure and kinship using a mixed linear model. The effect of population sub-structure on associations was most significant in boll weight among the traits analyzed. The sub-structure among the SP lines may be caused by natural selection, the breeding method applied during development of inbred lines, and unknown factors. The identified marker–trait associations can be useful in breeding and help determine genetic mechanisms underlying interrelationships among fiber traits. Electronic supplementary material  The online version of this article (doi:) contains supplementary material, which is available to authorized users.     

THE OCCURRENCE AND SIGNIFICANCE OF EPISTATIC VARIANCE FOR QUANTITATIVE CHARACTERS AND ITS MEASUREMENT IN HAPLOIDS

Epistatic genetic variance for quantitative traits may play an important role in evolution, but detecting epistasis in diploid organisms is difficult and requires complex breeding programs and very large sample sizes. We develop a model for detecting epistasis in organisms with a free-living haploid stage in their life cycles. We show that epistasis is indicated by greater variance among families of haploid progeny derived from individual diploids than among clonally replicated haploid sibs from the same sporophyte. Simulations show that the power to detect epistasis is linearly related to the number of sporophytes and the number of haploids per sporophyte in the dataset. We illustrate the model with data from growth variation among gametophytes of the moss, Ceratodon purpureus. The experiment failed to detect epistatic variance for biomass production, although there was evidence of additive variance.     

The potential of aspen clonal forestry in alberta: breeding regions and estimates of genetic gain from selection

Background

Aspen naturally grows in large, single-species, even-aged stands that regenerate clonally after fire disturbance. This offers an opportunity for an intensive clonal forestry system that closely emulates the natural life history of the species. In this paper, we assess the potential of genetic tree improvement and clonal deployment to enhance the productivity of aspen forests in Alberta. We further investigate geographic patterns of genetic variation in aspen and infer forest management strategies under uncertain future climates.

Methodology/Principal Findings

Genetic variation among 242 clones from Alberta was evaluated in 13 common garden trials after 5–8 growing seasons in the field. Broad-sense heritabilities for height and diameter at breast height (DBH) ranged from 0.36 to 0.64, allowing 5–15% genetic gains in height and 9–34% genetic gains in DBH. Geographic partitioning of genetic variance revealed predominant latitudinal genetic differentiation. We further observed that northward movement of clones almost always resulted in increased growth relative to local planting material, while southward movement had a strong opposite effect.

Conclusion/Significance

Aspen forests are an important natural resource in western Canada that is used for pulp and oriented strandboard production, accounting for ∼40% of the total forest harvest. Moderate to high broad-sense heritabilities in growth traits suggest good potential for a genetic tree improvement program with aspen. Significant productivity gains appear possible through clonal selection from existing trials. We propose two breeding regions for Alberta, and suggest that well-tested southern clones may be used in the northern breeding region, accounting for a general warming trend observed over the last several decades in Alberta.     

Analysis of NaCl stress response in Tunisian and reference lines of <Emphasis Type="Italic">Medicago truncatula</Emphasis>

We analyzed the effects of NaCl stress on nine lines of Medicago truncatula, including four lines from southern Tunisia, three from the north, and two references lines. Plants were cultivated in two treatments (0 and 45 mM NaCl) for a period of 45 days. At harvest, we measured seven quantitative traits of the shoot and root growth. The analysis of variance showed that responses of lines to NaCl stress depended on the effects of line, treatment, and their interaction. Treatment had the largest effect. All measured traits showed high broad-sense heritability (H ²) under both treatments. While the leaf area was the less affected trait, the length of stems was the most affected regarding the sensitivity index (SI) parameter. Estimated SI for total plant biomass showed that TN13.11 and TN11.0 were the most contrasting lines with the lowest and the highest values, respectively. Established correlations between measured traits and the hierarchical cluster relationships among lines depended on the treatment effect. The site-of-origin environmental factor that influenced more the response of lines to NaCl stress was the relative humidity.     

Identification of QTLs affecting traits of agronomic importance in a recombinant inbred population derived from a subspecific rice cross

To detect QTLs controlling traits of agronomic importance in rice, two elite homozygous lines 9024 and LH422, which represent the indica and japonica subspecies of rice (Oryza sativa), were crossed. Subsequently a modified single-seed-descent procedure was employed to produce 194 recombinant inbred lines (F₈). The 194 lines were genotyped at 141 RFLP marker loci and evaluated in a field trial for 13 quantitative traits including grain yield. Transgressive segregants were observed for all traits examined. The number of significant QTLs (LOD 2.0) detected affecting each trait ranged from one to six. The percentage of phenotypic variance explained by each QTL ranged from 5.1% to 73.7%. For those traits for which two or more QTLs were detected, increases in the traits were conditioned by indica alleles at some QTLs Japonica alleles at others. No significant evidence was found for epistasis between markers associated with QTLs and all the other markers. Pleitropic effects of single QTLs on different traits are suggested by the observation of clustering of QTLs. No QTL for traits was found to map to the vicinity of major gene loci governing the same traits qualitatively. Evidence for putative orthologous QTLs across rice, maize, oat, and barley is discussed.     

Identification of novel QTL for black tea quality traits and drought tolerance in tea plants (<Emphasis Type="Italic">Camellia sinensis</Emphasis>)

Tea (Camellia sinensis) contains polyphenols and caffeine which have been found to be of popular interest in tea quality. Tea production relies on well-distributed rainfall which influence tea quality. Phenotypic data for two segregating tea populations TRFK St 504 and TRFK St 524 were collected and used to identify the quantitative trait loci (QTL) influencing tea biochemical and drought stress traits based on a consensus genetic map constructed using the DArTseq platform. The populations comprised 261 F₁ clonal progeny. The map consisted of 15 linkage groups which corresponds to chromosome haploid number of tea plant (2n?=?2×?=?30) and spanned 1260.1 cM with a mean interval of 1.1 cM between markers. A total of 16 phenotypic traits were assessed in the two populations. Both interval and multiple QTL mapping revealed a total of 47 putative QTL in the 15 LGs associated with tea quality and percent relative water content at a significant genome-wide threshold of 5%. In total, six caffeine QTL, 25 catechins QTL, three theaflavins QTL, nine QTL for tea taster score, and three QTL for percent relative water contents were detected. Out of these 47 QTL, 19 QTL were identified for ten traits in three main regions on LG01, LG02, LG04, LG12, LG13, and LG14. The QTL associated with caffeine, individual catechins, and theaflavins were clustered mostly in LG02 and LG04 but in different regions on the map. The explained variance by each QTL in the population ranged from 5.5 to 56.6%, with an average of 9.9%. Identification of QTL that are tightly linked to markers associated with black tea quality coupled with UPLC assay may greatly accelerate development of novel tea cultivars owing to its amenability at seedling stage. In addition, validated molecular markers will contribute greatly to adoption of marker-assisted selection (MAS) for drought tolerance and tea quality improvement.     

High-density linkage mapping of yield components and epistatic interactions in maize with doubled haploid lines from four crosses

Grain yield (GY) is a genetically complex and physiologically multiplicative trait which can be decomposed into the components kernel number (KN) and 100-kernel weight (HKW). Genetic analysis of these less complex yield component traits may give insights into the genetic architecture and predictive ability of complex traits. Here, we investigated how the incorporation of component traits and epistasis in quantitative trait locus (QTL) mapping approaches influences the accuracy of GY prediction. High-density genetic maps with 7,000–10,000 polymorphic single nucleotide polymorphisms were constructed for four biparental populations. The populations comprised between 99 and 227 doubled haploid maize lines which were phenotyped in field trials in two environments. Heritability was highest for HKW (88–89 %), intermediate for KN (72–80 %), and lowest for GY (64–83 %). Mapped QTL explained in total 21–55 %, 22–67 %, and 24–75 % of the genotypic variance for GY, KN, and HKW, respectively. Support intervals of QTL were short, indicating that QTL were located with high precision. Co-located QTLs with same parental origin of favorable alleles were detected within populations for different traits and between populations for the same traits. Using GY predictions based on the detected QTL, prediction accuracies (r) determined by cross validation ranged from 0.18 to 0.52. Epistatic models did not outperform the corresponding additive models. In conclusion, models based on QTL positions of component traits support the identification of favorable alleles for multiplicative traits and provide a basis to select superior inbred lines by marker-assisted breeding.     

Genetic Variation in Growth and Cone Traits of <i>Pinus Koraiensis</i> Half-Sib Families in Northeast China

Genetic parameters were evaluated for growth and cone characteristics (tree height, diameter at breast height, volume, cone number, thousand seeds weight and single cone seeds weight) on 86 half-sib families of Pinus koraiensis aged 31 years. Analyses of variance revealed significant differences (p < 0.001) in all growth and cone traits among families while no significant differences were detected among blocks and the interaction between blocks and families. The average family values for growth traits were 17.22 m, 8.67 cm and 0.43 m3 for tree height, diameter at breast height and volume, respectively. The average cone number, thousand seeds weight and single cone seeds weight were 17.57, 748.91 g and 77.25 g, respectively. Genotypic additive variance and phenotypic variances ranged from 0.00009 to 3.820 and from 0.0005 to 23.066, while genotypic and phenotypic coefficients of variation ranged from 2.693% to 37.196% and 4.963% to 60.595%, respectively. Heritability at the individual and family level ranged from 0.152 to 0.215 and 0.611 to 0.862, respectively. Growth traits were significantly positively correlated with each other, but cone traits showed a weak correlation with growth traits. Based on 10% selection rate, nine families each were selected as elite materials in terms of high performance in volume and cone numbers, with 22.16% and 43.82% genetic gain in volume and cone number, respectively. These results provide beneficial information to select excellent families and establish orchards of P. koraiensis from improved seeds.     

A QUANTITATIVE-GENETIC ANALYSIS OF LARVAL LIFE-HISTORY TRAITS IN HYLA CRUCIFER

We used a half-sib design to examine the genetic components of phenotypic variance in several life-history traits in Hyla crucifer. Egg viability, hatchling size, larval growth rate, length of larval period, and size at metamorphosis play critical roles in determining survivorship and are subject to persistent selection. Egg viability varied among families considerably, with most embryo mortality occurring between gastrulation and neurulation. Hatchling size was the only trait in which maternal effects were influential. Dominance genetic variance played the predominant role in determining phenotypic variance in hatchling size, growth rate, and length of larval period, accounting for, respectively, 70, 63, and 47% of the total variance. Size at metamorphosis displayed little dominance genetic variance and, unlike the other traits, displayed a high heritability. All additive genetic correlations between traits were positive. The directions of environmental correlations were the same as the directions of changes that have been induced in previous experimental work. The correlations due to dominance effects described a principal axis that independent ecological studies indicate to be directly correlated with fitness. These results agree with theoretical expectations for traits under consistent directional selection.     

Genetic determination of variability of barley doubled haploids inoculated with Fusarium culmorum (W.G.Sm.) Sacc. with regard to mycotoxin accumulation and reduction in yield traits

The genetic determination of variability of barley doubled haploid (DH) lines in regard of their susceptibility to Fusarium head blight caused by Fusarium culmorum was studied. The susceptibility was evaluated in 3-year field experiment on the basis of reduction in yield traits and myotoxin accumulation in infected kernels. The following traits were analysed in inoculated and control plants: kernel number and weight per ear, 1000-kernel weight, percentage of plump kernels (>2.5 mm), deoxynivalenol (DON) content and nivalenol (NIV) content of kernels. On the basis of the obtained data, heritability coefficient (ratio of genotypic to phenotypic variance) was assessed, and genetic parameters as well as the number of effective factors were estimated. Heritability coefficients calculated from two-way analysis of variance, i.e.regarding the influence of years and year x genotype interaction, appeared to be exceptionally low and ranged from 5.2% for the reduction in plump kernels to 38.2% for the reduction in 1000-kernel weight. In the case of mycotoxin accumulation about 60% of the observed variability in NIV concentrations and 30% in DON concentration resulted form genetic differences among lines. Additive effects of genes were important for all the analysed traits. Significant effects of dominance and dominance x dominance were observed for 1000-kernel weight and percentage of plump kernels. Moreover, it was found that the observed variability in yield trait reduction resulted from segregation of 5-6 effective factors, DON contents from 4 factors, while NIV content from 5 factors.     

Estimating genetic and environmental components of variance using sexual and clonal Artemia

Reproductive and life span traits were measured for two obligately parthenogenetic (Artemia parthenogenetica) and three sexual (two A. franciscana and one A. sinica) brine shrimp populations. For each population, clonal lineages or single mating pairs were followed through one life cycle. The relative contributions of environmental and genetic components to total phenotypic variation for 10 life-history traits in response to environmental stress (0, 10, 25 ppb Cu) were estimated. Within treatment variation (CV_W) was 39% higher for sexual populations than parthenogenetic populations, with significant (p<0.05) differences in total number of offspring and number of nauplii. CV_A (the change in variance due to rearing in different environments), when averaged for all traits and all populations, increased variability by 9.9%. CV_A was 44.2% higher for sexual than parthenogenetic populations, with significant differences in number of broods, total number of offspring, and number of nauplii. The average genetic component of variation for the 10 traits was 23.44%, ranging from 5.26% for number of cysts to 44.87% for number of nauplii. For all traits, the environmental component of variance is greater than the genetic component measured, but every trait has a genetic component, which can potentially be acted upon by selection.     

Detection of quantitative trait loci controlling bud burst and height growth in <Emphasis Type="Italic">Quercus robur</Emphasis> L.

Genetic variation of bud burst and early growth components was estimated in a full-sib family of Quercus robur L. comprising 278 offspring. The full sibs were vegetatively propagated, and phenotypic assessments were made in three field tests. This two-generation pedigree was also used to construct a genetic linkage map (12 linkage groups, 128 markers) and locate quantitative trait loci (QTLs) controlling bud burst and growth components. In each field test, the date of bud burst extended over a period of 20 days from the earliest to the latest clone. Bud burst exhibited higher heritability (0.15–0.51) than growth components (0.04–0.23) and also higher correlations across field tests. Over the three tests there were 32 independent detected QTLs (P5% at the chromosome level) controlling bud burst, which likely represent at least 12 unique genes or chromosomal regions controlling this trait. QTLs explained from 3% to 11% of the variance of the clonal means. The number of QTLs controlling height growth components was lower and varied between two and four. However the contribution of each QTL to the variance of the clonal mean was higher (from 4% to 19%). These results indicate that the genetic architecture of two important fitness-related traits are quite different. On the one hand, bud burst is controlled by several QTLs with rather low to moderate effects, but contributing to a high genetic (additive) variance. On the other hand, height growth depends on fewer QTLs with moderate to strong effects, resulting in lower heritabilities of the trait.     

ENVIRONMENTAL AND GENETIC CONSTRAINTS ON ADAPTIVE POPULATION DIFFERENTIATION IN ANTHOXANTHUM ODORATUM

Maximum-likelihood estimates of environmental and broad sense genetic (co)variance components were obtained for the growth and reproductive output of clones of the grass Anthoxanthum odoratum. The clones were transplanted between a mesic and a xeric field site and across-environment genetic correlations were used to estimate the strength of genotype-environment interaction. Significant across-environment clonal covariance matrices were found for several traits, including lifetime reproductive output in one population. None of the matrices differed significantly between populations. Significant within-site clonal variation was found, but there was no significant across-environment clonal covariation. Most broad sense heritability estimates of character states within sites were small (median = 0.12), suggesting that only a slow response to selection is possible. All significant within-site clonal correlations between growth and reproductive output were positive, although the pattern of negative clonal correlations suggests that there may be a cost to first year reproduction, which might constrain future selection response.     

The demographic effects of functional traits: an integral projection model approach reveals population‐level consequences of reproduction‐defence trade‐offs

Quantitatively linking individual variation in functional traits to demography is a necessary step to advance our understanding of trait‐based ecological processes. We constructed a population model for Asclepias syriaca to identify how functional traits affect vital rates and population growth and whether trade‐offs in chemical defence and demography alter population growth. Plants with higher foliar cardenolides had lower fibre, cellulose and lignin levels, as well as decreased sexual and clonal reproduction. Average cardenolide concentrations had the strongest effect on population growth. In both the sexual and clonal pathway, the trade‐off between reproduction and defence affected population growth. We found that both increasing the mean of the distribution of individual plant values for cardenolides and herbivory decreased population growth. However, increasing the variance in both defence and herbivory increased population growth. Functional traits can impact population growth and quantifying individual‐level variation in traits should be included in assessments of population‐level processes.     

Genetic variation in populations of the threatened seagrass Halophila beccarii (Hydrocharitaceae)

Halophila beccarii (Hydrocharitaceae), a small monoecious seagrass, has been listed as a threatened species. In this study, a total of 106 samples were collected from four Chinese populations located at the northern limit of its distribution range. Using six polymorphic microsatellites, we found low genetic variation in this species, in which the mean number of alleles per locus was 2.8, and 16 multi-locus genotypes were revealed. In the four populations, the mean number of alleles per locus ranged from 1.2 to 2, one to eight genotypes were found, and clonal diversity ranged from 0 to 0.23; observed and expected heterozygosity ranged from 0.17 to 0.40 and from 0.09 to 0.24, respectively. Strong genetic differentiation was found among the populations, and the standardized fixation index (F′_ST) was 0.787. Species traits (i.e., clonal growth) and bottleneck effects due to drastic population fluctuation may contribute to the observed low genetic variation.     

Quantitative genetics of growth and development in Populus. I. A three-generation comparison of tree architecture during the first 2 years of growth

One approach to gain an insight into the genetics of tree architecture is to make use of morphologically divergent parents and study their segregating progeny in the F₂ and backcross (B₁) generations. This approach was chosen in the present study in which material of a three-generation pedigree growing side by side in a replicated plantation, was analyzed. The pedigree included Populus trichocarpa (T) and P. deltoides (D) parents, their F₁ and F₂ hybrids and their B₁ hybrids to the D parent. The trees were grown in the environment of the T parent and measured for the first 2 years of growth. Nine quantitative traits were studied at the stem, branch and leaf levels of tree architecture, in which the original parents differed. Strong F₁ hybrid vigor relative to the better parent (T) was expressed in growth and its components. Most quantitative traits in the F₂ and B₁ hybrids were intermediate between the T and D parents but displayed a wide range of variation due to segregation. The results from the analysis of variance indicated that all morphometric traits were significantly different among F₂ and B₁ clones, but the B₁ hybrids were more sensitive to replicates than the F₂. Broad-sense heritabilities (H ²) based on clonal means ranged from moderately high to high (0.50–0.90) for the traits studied, with H ² values varying over age. The H ² estimates reflected greater environmental noise in the B₁ than in the F₂, presumably due to the greater proportion of maladaptive D alleles in those hybrids. In both families, sylleptic branch number and length, and leaf size on the terminal, showed strong genetic correlations with stem growth. The large divergence between the two original parents in the traits studied, combined with the high chromosome number in Populus (2n=38), makes this pedigree well suited for the estimation of the number of quantitative trait loci (QTLs) underlying quantitative variation by Wright's biometric method (1968). Variation in several traits was found to be under the control of surprisingly few major QTLs: 3–4 in 2nd-year height and diameter growth, a single QTL in stem diameter/height ratio.