Molecular genetics of growth and development in populus. IV. Mapping QTLs with large effects on growth, form, and phenology traits in a forest tree.

We have mapped quantitative trait loci (QTLs) for commercially important traits (stem growth and form) and an adaptive trait (spring leaf flush) in a Populus F2 generation derived from a cross between interspecific F1 hybrids (P. trichocarpa x P. deltoides). Phenotypic data were collected over a 2-year period from a replicated clonal trial containing ramets of the parental, F1, and F2 trees. Contrary to the assumptions of simple polygenic models of quantitative trait inheritance, 1-5 QTLs of large effect are responsible for a large portion of the genetic variance in each of the traits measured. For example, 44.7% of the genetic variance in stem volume after 2 years of growth is controlled by just two QTLs. QTLs governing stem basal area were found clustered with QTLs for sylleptic branch leaf area, sharing similar chromosomal position and mode of action and suggesting a pleiotropic effect of QTLs ultimately responsible for stem diameter growth.

Molecular genetics of growth and development in Populus. I. Triploidy in hybrid poplars.

While constructing a genetic linkage map of a hybrid poplar genome (Populus trichocarpa x P. deltoides), we identified several restriction fragment length polymorphismus (RFLPs) for which the parental trees are heterozygous. Although 8 of the 11 F1 hybrid offspring inherited, as expected, single RFLP alleles from each parent, 3 F1 trees in the mapping pedigree inherited both maternal alleles along with a single paternal allele at some loci. Aneuploidy or polyploidy in these 3 F1 trees due to partial or complete nondisj unction during female gametogenesis is the simplest explanation for this finding. Of the 3 f1 offspring with supernumerary RFLP alleles 2 have triploid nuclear DNA contents as measured by fluorescence flow cytometry; the 3rd F1 with supernumerary alleles has a sub-triploid nuclear DNA content and is probably aneuploid. Among the tri/aneuploid hybrids, leaf quantitative traits either are skewed toward those values characteristic of the P. trichocarpa female parent (adaxial stomate density, petiole length: blade length ratio; abaxial color) or show transgressive variation (epidermal cell size). Abaxial leaf color was used to screen a large population of P. trichocarpa x P. deltoides hybrids for further evidence of tri/aneuploidy. In each case where a "white" abaxial leaf surface was observed and the nuclear DNA content measured, the hybrid proved to be tri/aneuploid. All sexually mature female triploids examined were sterile, although the inflorescences completed their development in the absence of embryo formation. The (probably) aneuploid F1 hybrid is a fertile female. Of 15 female P. trichocarpa parents used in crosses to P. deltoides, 10 produced one or more tri/aneuploid hybrid offspring. In an intraspecific cross using a P. trichocarpa female that had produced triploid hybrids with five different P. deltoides males, no tri/aneuploid offpsring were found.

Molecular genetics of growth and development in Populus. II. Segregation distortion due to genetic load.

Distortion of expected Mendelian segregation ratios, commonly observed in many plant taxa, has been detected in an experimental three-generation inbred pedigree of Populus founded by interspecific hybridization between P. trichocarpa and P. deltoides. An RFLP linkage map was constructed around a single locus showing severe skewing of segregation ratio against F2 trees carrying the P. trichocarpa allele in homozygous form. Several hypotheses for the mechanism of segregation distortion at this locus were tested, including directional chromosome loss, segregation of a pollen lethal allele, conflicts between genetic factors that isolate the parental species, and inbreeding depression as a result of genetic load. Breeding experiments to produce inbred and outcrossed progenies were combined with PCR-based detection of RFLPs to follow the fate of the deficient allele throughout embryo and seedling development. A recessive lethal allele, lth, inherited from the P. trichocarpa parent, was found to be tightly linked to the RFLP marker locus POP1054 and to cause embryo and seedling mortality. Heterozygotes (lth/+) appear to be phenotypically normal as embryos, seedlings, and young trees.

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.

The choice of genetic material for mechanistic studies of adaptation in forest trees.

There is considerable urgency to study the mechanisms by which forest trees track environmental change, given the prospect of possible rapid climatic changes. Environmental tracking is achieved through three basic processes: (1) expression of phenotypic plasticity at the level of the individual; and (2) evolutionary change and (3) migration, both expressed at the level of the population over generations. The current distribution and genetic architecture of a species reflect how these processes interacted in response to past climatic changes during and after the last glaciation. Part of that record is encoded in the DNA of the current generation of trees and, as a result of existing field tests, is accessible for study. These field tests include, in ascending order of genetic resolution, (1) provenance tests, (2) progeny tests, and (3) three-generation clonal tests; as well as (4) clone tests, with or without genetic structure. The suitability and limitations of these tests for mechanistic studies of environmental tracking are described, both as field installations and as sources of material for parallel in-depth studies. We conclude that they represent an important information resource, which deserves to be more effectively used by the scientific community.

Influence of soil water on the physiological and morphological components of plant water balance in Populus trichocarpa, Populus deltoides and their F(1) hybrids.

Patterns of leaf growth, transpiration and whole-plant water balance in Populus trichocarpa, P. deltoides and their F(1) hybrids were studied during a soil drying cycle. Plant responses were analyzed during three distinct stages of dehydration. In stage I, the transpiration rate of drought-stressed plants remained constant and equal to that of well-watered plants even though soil water content declined by more than 40%. Stage II began as soil and plant water deficits induced stomatal closure. When soil water was expressed as a fraction of transpirable soil water, the transition from stage I to stage II occurred at soil water fractions of 0.35, 0.45 and 0.60 for P. trichocarpa, P. deltoides and their F(1) hybrids, respectively. Reductions in leaf growth coincided with the shift from stage I to stage II. As soil water declined further, decreases in relative transpiration and whole-plant leaf area were significantly greater in parental species than in F(1) hybrids. Inherent feedbacks controlling stomatal water loss and the maintenance and growth of leaf tissue appeared to differ between F(1) and parental genotypes in a pattern characteristic of an overdominant mode of inheritance.Stage III began once the ability of stomata to compensate for water loss had been exhausted. Substantial differences were found in plant survival during stage III, with F(1) hybrids surviving longer than parental species. Survival was more strongly correlated with the hydraulic conductivity of xylem tissues than with the dehydration tolerance of leaf tissues. Collectively, these responses suggest that F(1) hybrids were more drought resistant than either parental species and highlight the importance of whole-plant studies of functional relationships between plant growth, water balance and hydraulic conductivity.

Interspecific crossability studies in poplars.

Pollen-stigma interactions were studied with scanning electron microscopy in intraspecific and intersectional crosses of Populus tremuloides (Sect. LEUCE); P. deltoides, P. nigra (Sect. AIGEIROS); and P. trichocarpa (Sect. TACAMAHACA). Intraspecific variation in hydration, size of hydrated pollen grains, and tube morphology is described. Exine sculpture in P. tremuloides was densely spinulate or scabrate, in P. trichocarpa densely reticulate, in the other species somewhat intermediate. In six of seven intersectional cross combinations pollen-tube behavior on the stigmatic surface at 5-22 hours after pollination was the same as in conspecific matings; in P. tremuloides x P. trichocarpa crosses, the percentage of twisted and bulging tubes was significantly higher. Irradiated mentor pollen did not affect pollentube behavior of adjacent incompatible pollen, but increased hybrid seedling production in interspecific crosses. By contrast, hexane treatment of pollen or stigma, and the spraying of catkins with gibberellic acid or naphtalene acetic acid were ineffective. The results are discussed in relation to several mechanisms presumed to be involved in restricting gene exchange among poplar species; and in relation to Hogenboom's (1973) incongruity concept.

Crown architecture of Populus clones as determined by branch orientation and branch characteristics.

Crown architecture, including branching pattern, branch characteristics and orientation of proleptic and sylleptic branches was studied in five poplar clones (Populus deltoides, P. trichocarpa and P. trichocarpa x P. deltoides hybrids), grown under intensive culture in the Pacific Northwest, USA. Branch characteristics measured were number, length, diameter, biomass and the angles of origin and termination. The results suggest that genotype has a major influence on crown architecture in Populus. Clonal differences in branch characteristics and branching patterns were found that resulted in striking differences in crown form and architecture. Branch angle and curvature differed significantly among clones, and among height growth increments within clones. Branch length and diameter were significantly correlated in all clones. Sylleptic branches and the considerable leaf area they carry have important implications for whole tree light interception, and thus, play a critical role in the superior growth and productivity of certain hybrid poplar clones. The considerable variation in branch characteristics implies a strong justification for including them in selection and breeding programs for Populus.

Leaf growth characteristics of fast-growing poplar hybrids Populus trichocarpa x P. deltoides.

Fast-growing hybrid poplar trees (Populus trichocarpa Torr. & A. Gray x P. deltoides Bartr. ex Marsh.) were compared with slower-growing parental types in both field and laboratory experiments to determine physiological components of leaf growth that could be closely related to biomass production. Stem volume was correlated with individual leaf area (r = 0.81) and leaf growth rate (r = 0.82). Hybrids had a greater total leaf area, not because they produced more leaves, but because they had larger leaves than either parental type. The greater leaf size of the hybrids may be explained by inheritance of larger cell number from P. deltoides and larger cell size from P. trichocarpa. Rates of enlargement of isolated leaf discs in liquid culture were approximately 50% of those observed in intact leaves of field-grown plants.

Molecular genetics of growth and development in Populus. III. A genetic linkage map of a hybrid poplar composed of RFLP, STS, and RAPD markers.

We have evaluated three DNA-based marker types for linkage map construction in Populus: RFLPs detected by Southern blot hybridization, STSs detected by a combination of PCR and RFLP analysis, and RAPDs. The mapping pedigree consists of three generations, with the F1 produced by interspecific hybridization between a P. trichocarpa female and a P. deltoides male. The F2 generation was made by inbreeding to the maximum degree permitted by the dioecious mating system of Populus. The applicability of STSs and RAPDs outside the mapping pedigree has been investigated, showing that these PCR-based marker systems are well-suited to breeding designs involving interspecific hybridization. A Populus genome map (343 markers) has been constructed from a combination of all three types. The length of the Populus genome is estimated to be 2400-2800 cM.