S1 Population Research Articles

Effect of dichloromethylene diphosphonate on liver regeneration following thioacetamide-induced necrosis in rats

To study the effect of dichloromethylene diphosphonate (DMDP), a selective Kupffer cell toxicant in reference to liver damage and postnecrotic liver regeneration in rats induced by sublethal dose thioacetamide (TA). Rats, intravenously (iv) pre-treated with a single dose of DMDP (10 mg/kg), were intraperitoneally (ip) injected with TA 6.6 mmol/kg (per 500 mg/kg body weight). Hepatocytes were isolated from rats at 0, 24, 48 and 72 h following TA intoxication and blood and liver samples were obtained. To evaluate the mechanisms involved in the postnecrotic regenerative state, DNA distribution and ploidy time course were assayed in isolated hepatocytes. Circulating cytokine tumor necrosis factor-alpha (TNF-α) was assayed in serum and determined by reverse transcriptase-polymerase chain reaction in liver extract. The effect of DMDP induced noticeable changes in postnecrotic regeneration, causing an increased percentage of hepatocytes in the cell cycle S phase. The increase at 24 h in S1 population in rats pretreated with DMDP + TA was significantly (P < 0.05) different compared with that of the TA group (18.07% vs 8.57%). Hepatocytes increased their proliferation as a result of these changes. Also, TNF-α expression and serum level were increased in rats pre-treated with DMDP. Thus, DMDP pre-treatment reduced TA-induced liver injury and accelerated postnecrotic liver regeneration. These results demonstrate that Kupffer cells are involved in TA-induced liver, as well as in postnecrotic proliferative liver states.

Electron transfer from Cyt b 559 and tyrosine-D to the S2 and S3 states of the water oxidizing complex in photosystem II at cryogenic temperatures

The Mn(4)CaO(5) cluster of photosystem II (PSII) catalyzes the oxidation of water to molecular oxygen through the light-driven redox S-cycle. The water oxidizing complex (WOC) forms a triad with Tyrosine(Z) and P(680), which mediates electrons from water towards the acceptor side of PSII. Under certain conditions two other redox-active components, Tyrosine(D) (Y(D)) and Cytochrome b(559) (Cyt b(559)) can also interact with the S-states. In the present work we investigate the electron transfer from Cyt b(559) and Y(D) to the S(2) and S(3) states at 195 K. First, Y(D)(•) and Cyt b(559) were chemically reduced. The S(2) and S(3) states were then achieved by application of one or two laser flashes, respectively, on samples stabilized in the S(1) state. EPR signals of the WOC (the S(2)-state multiline signal, ML-S(2)), Y(D)(•) and oxidized Cyt b(559) were simultaneously detected during a prolonged dark incubation at 195 K. During 163 days of incubation a large fraction of the S(2) population decayed to S(1) in the S(2) samples by following a single exponential decay. Differently, S(3) samples showed an initial increase in the ML-S(2) intensity (due to S(3) to S(2) conversion) and a subsequent slow decay due to S(2) to S(1) conversion. In both cases, only a minor oxidation of Y(D) was observed. In contrast, the signal intensity of the oxidized Cyt b(559) showed a two-fold increase in both the S(2) and S(3) samples. The electron donation from Cyt b(559) was much more efficient to the S(2) state than to the S(3) state.

Exciton Dynamics at CuPc/C60 Interfaces: Energy Dependence of Exciton Dissociation

Donor–acceptor interfaces are critical for the operation of organic photovoltaic devices. Exciton dynamics at these interfaces play a significant role in determining efficiency and controlling open circuit voltage (VOC), short circuit current (JSC), or fill factor (FF). These fundamental interfacial dynamical processes are dependent on the interfacial electronic and molecular structure. In this report we use time-resolved two-photon photoemission (TR-2PPE) to investigate exciton dissociation, recombination, and relaxation processes occurring at well-characterized prototypical donor–acceptor interfaces of copper phthalocyanine (CuPc) layers on C60. S1 excitons are created by excitation in the CuPc Q-band. The excited S1 population is probed as a function of time via photoemission with a UV probe pulse. TR-2PPE measurements provide a picture of subpicosecond charge separation and recombination processes as a function of distance from the CuPc/C60 interface, starting with a CuPc single layer. Analysis via rate equation modeling reveals that the bulk intersystem crossing and intraband relaxation occur on picosecond to subpicosecond time scales, resulting in rapid relaxation of the exciton population. At the interface, these processes compete with electron transfer to C60. The rate constant governing exciton dissociation is energy-dependent, decreasing by orders of magnitude for excitons below the energy of interfacial charge-transfer states. Connections to semiclassical models of charge transfer and implications for device performance are discussed.

In search of the dark state of 5-methyl-2-hydroxypyrimidine using a numerical DFT/MRCI gradient

In a recent publication, Lobsiger et al. [Phys. Chem. Chem. Phys. 12, 5032 (2010)] presented infrared and electronic absorption spectra of supersonic jet-cooled 5-methyl-2-hydroxypyrimidine (5M2HP), the enol form of deoxythymine. In addition, they reported on the fast nonradiative decay of the S1 population to a dark state. In the present paper, we have investigated the mechanism and rate constants of this nonradiative decay by means of quantum chemical multi-configuration methods. To this end, minima of the lowest excited singlet and triplet states as well as the minimum-energy crossing point of singlet and triplet potential energy hypersurfaces (PEHs) have been determined employing a numerical DFT/MRCI gradient where DFT/MRCI stands for a combination of density functional theory (DFT) and a semi-empirical multi-reference configuration interaction (MRCI) approach. Rate constants have been calculated in the Condon approximation using a time-dependent approach based on harmonic oscillator functions and electronic spin–orbit coupling matrix elements evaluated at the DFT/MRCI level. It is shown that the first excited triplet state possesses 3(n → π*) character in the gas phase. Fast intersystem crossing is mediated by the low-lying 3(π → π*) state whose PEH crosses both, the S1 1(n → π*) and T1 3(n → π*) PEHs.

IDENTIFICATION OF MAINTAINER LINES AND EVALUATION OF EXPERIMENTAL HYBRIDS DERIVED FROM A BRAZILIAN TROPICAL ONION POPULATION

The four to five-fold price of onion hybrid seeds accounts for the wide use of open pollinated onion populations in Northeastern Brazil. Here we report the identification of maintainer and male-sterile onion lines within the ‘Alfa Sao Francisco’ yellow onion population, after three years of work, associating random field pairing of male-fertile plants with selected male-sterile plants and a PCR-based marker cytoplasm monitoring system. Male-sterile plants produced flowers with light green anthers which were easily detected in the field. A 2.0% frequency of male-sterile plants was estimated in the ‘Alfa Sao Francisco’ sampled population. Male-sterile plants produced the 5’cob-marker 180-bp and the orfA501-marker 473-bp fragments, suggesting the T-cytoplasm type, while the maintainer line produced only the 5’cobmarker 180-bp. Pairing ‘A’ line with a red onion resulted in 100% F1 red bulbs produced from harvested seed in the ‘A line’, confirming that a stable CMS system was identified within the Brazilian tropical ‘Alfa Sao Francisco’ population. Hybrids produced with another ‘Baia periforme’ onion population (Line ‘C’) and the ‘A’ identified lines’ have resulted in a 38% bulb yield increase compared with the best parent, ‘Alfa Sao Francisco’. The identified ‘A’ and ‘B’ lines, associated with other developed S1 and S2 populations (‘C’ lines) will be used to produced commercial hybrids well adapted to Brazilian low latitudes and with affordable prices for growers.

Computational study on fluoride recognition by an urea-activated phthalimide chemosensor

The mechanism of the fluoride anion recognition by an urea–phthalimide chemosensor has been studied in detail by means of CASPT2//CASSCF calculations. Computational data are in well-agreement with changes in the photophysical properties previously reported experimentally in the presence or absence of fluoride anion. In both cases, fast relaxation from the S2 potential energy surface leads to S1 population. Deactivation of the excited state is in agreement with the experimental change in fluorescence observed when the anion is present. Mechanistic information obtained allows to provide an explanation of the experimental data and to suggest some modifications to improve the synthetic applicability of this type of chemosensors.

Genetic Control of White Flower Color in Scarlet Rosemallow (Hibiscus coccineus Walter)

Scarlet rosemallow (Hibiscus coccineus Walter) is a diploid, perennial, erect, and woody shrub. The species is a desirable inclusion in home landscapes because it is a native plant with attractive flowers and unusual foliage. The objective of these experiments was to determine the number of loci, number of alleles, and gene action controlling flower color (red vs. white) in scarlet rosemallow. Three white-flowered and 1 red-flowered parental lines were used to create S(1) and F(1) populations, which were self-pollinated or backcrossed to generate S(2), F(2), and BC(1) populations. Evaluation of these generations showed that flower color in these populations was controlled by a single diallelic locus with red flower color completely dominant to white. I propose that this locus be named "white flower" with alleles W and w.

Direct, genome-wide assessment of DNA mutations in single cells

DNA mutations are the inevitable consequences of errors that arise during replication and repair of DNA damage. Because of their random and infrequent occurrence, quantification and characterization of DNA mutations in the genome of somatic cells has been difficult. Random, low-abundance mutations are currently inaccessible by standard high-throughput sequencing approaches because they cannot be distinguished from sequencing errors. One way to circumvent this problem and simultaneously account for the mutational heterogeneity within tissues is whole genome sequencing of a representative number of single cells. Here, we show elevated mutation levels in single cells from Drosophila melanogaster S2 and mouse embryonic fibroblast populations after treatment with the powerful mutagen N-ethyl-N-nitrosourea. This method can be applied as a direct measure of exposure to mutagenic agents and for assessing genotypic heterogeneity within tissues or cell populations.

QTL mapping of resistance to leaf rust in Salix

Genetic mapping of quantitative trait loci (QTL) for resistance to Melampsora leaf rust was performed in two willow families: the progeny from a backcross between Salix viminalis and a hybrid S. viminalis × Salix schwerinii (population S1), and the F1 progeny of a cross between two S. viminalis (population S3). Disease levels were scored in the field for three consecutive years. In the laboratory, five different rust strains were sprayed onto leaf disks and the following resistance components were scored: latent period, diameter and number of uredinia, and flecking. One major QTL and 14 smaller were identified in the S1 host population. One rust strain, that represents a Melampsora form with limited incidence on S. viminalis, showed lower aggressiveness to the S1 host population together with a different pattern in QTLs. In the S3 host population, we detected 13 QTLs for rust resistance, of which two were located to the same genomic regions as those found for the S1 population. We showed that the strongest QTL co-segregated with a gene homologous to a known Toll/interleukin receptor-nucleotide binding site-leucine-rich repeat resistance gene in poplar. The importance of the identified QTLs is discussed in relation to breeding for durable resistance.

Photoisomerization around a Fulvene Double Bond: Coherent Population Transfer to the Electronic Ground State?

Photoisomerization around a central fulvene-type double bond is known to proceed through a conical intersection at the perpendicular geometry. The process is studied with an indenylidene-dihydropyridine model compound, allowing the use of visible excitation pulses. Transient absorption shows that 1) stimulated emission shifts to the red and loses oscillator strength on a 50 fs timescale, and 2) bleach recovery is highly nonexponential and not affected by solvent viscosity or methyl substitution at the dihydropyridine ring. Quantum-chemical calculations are used to explain point 1 as a result of initial elongation of the central C=C bond with mixing of S(2) and S(1) states. From point 2 it is concluded that internal conversion of S(1)→S(0) does not require torsional motion to the fully perpendicular state. The S(1) population appears to encounter a sink on the torsional coordinate before the conical intersection is reached. Rate equations cannot model the observed ground-state recovery adequately. Instead the dynamics are best described with a strongly damped oscillatory contribution, which could indicate coherent S(1)-S(0) population transfer.

Dark Excited States of Carotenoid Regulated by Bacteriochlorophyll in Photosynthetic Light Harvesting

In photosynthesis, carotenoids play important roles in light harvesting (LH) and photoprotective functions, which have been described mainly in terms of two singlet excited states of carotenoids: S(1) and S(2). In addition to the "dark" S(1) state, another dark state, S*, was recently identified and its involvement in photosynthetic functions was determined. However, there is no consistent picture concerning its nature or the mechanism of its formation. One particularly anomalous behavior obtained from femtosecond transient absorption (TA) spectroscopy is that the S*/S(1) population ratio depends on the excitation intensity. Here, we focus on the effect of nearby bacteriochlorophyll (BChl) on the relaxation dynamics of carotenoid in the LH complex. We performed femtosecond TA spectroscopy combined with pre-excitation of BChl in the reconstituted LH1 complex from Rhodospirillum rubrum S1. We observed that the energy flow from S(1), including its vibrationally excited hot states, to S* occurs only when nearby BChl is excited into Q(y), resulting in an increase in S*/S(1). We also examined the excitation-intensity dependence of S*/S(1) by conventional TA spectroscopy. A comparison between the pre-excitation effect and excitation-intensity dependence shows a strong correlation of S*/S(1) with the number of BChls excited into Q(y). In addition, we observed an increase in triplet formation as the S* population increased, indicating that S* is an electronic excited state that is the precursor to triplet formation. Our findings provide an explanation for observed spectroscopic features, including the excitation-intensity dependences debated so far, and offer new insights into energy deactivation mechanisms inherent in the LH antenna.

Excited-State Dynamics in 6-Thioguanosine from the Femtosecond to Microsecond Time Scale

Patients treated with the immunosuppressant and anticancer drugs 6-thioguanine, azathioprine, or mercaptopurine can metabolize and incorporate them in DNA as 6-thioguanosine. The skin of these patients is sensitive to UVA radiation, and long-term treatment can result in extremely high incidence of sunlight-induced skin cancer. In this contribution the photophysics of 6-thioguanosine have been studied in aqueous buffer solution and in acetonitrile after excitation with UVA light to provide mechanistic insights about the origin of its phototoxicity. It is shown that most of the initial excited-state population in the S(2)(ππ*, L(a)) state decays by ultrafast intersystem crossing to the triplet manifold. A triplet quantum yield of 0.8 ± 0.2 is determined in aqueous buffer solution. A minor fraction of the S(2) population bifurcates on an ultrafast time scale to populate the S(1)(n(S)π*) state, which decays back to the ground state in tens of picoseconds. Quantum-chemical calculations that include solvent effects support the experimental results. The high triplet yield of 6-thioguanosine, which we argue can result in photosensitization of molecular oxygen and photooxidative DNA damage, is proposed to explain the high phototoxicity exhibited by these pro-drugs in patients upon sunlight exposure. Finally, the experimental and computational results for 6-thioguanosine are compared with those reported for the DNA/RNA guanine monomers.

Construction of intraspecific linkage maps, detection of a chromosome inversion, and mapping of QTL for constitutive root aerenchyma formation in the teosinte Zea nicaraguensis

The teosinte Zea nicaraguensis, a wild relative of maize, possesses a flooding tolerance-related trait: the formation of constitutive root aerenchyma under drained (non-flooded) soil conditions. A previous study suggested that the degree of constitutive aerenchyma formation varies within Z. nicaraguensis. The objectives of this study were to construct linkage maps, to determine the marker order in a region of chromosome 4 in which recombination between maize and Z. nicaraguensis is suppressed, and to identify quantitative trait loci (QTL) controlling constitutive root aerenchyma formation in two segregating populations of Z. nicaraguensis. A total of 236 simple sequence repeat (SSR) markers were screened for polymorphism in an S1 population of Z. nicaraguensis. Seventy-one polymorphic SSR markers were assigned to 10 chromosomes, and a linkage map was constructed covering 793.5 cM. In the S1 map, a paracentric inversion was detected on the long arm of chromosome 4; this rearrangement was confirmed in an S1 linkage map of a different Z. nicaraguensis accession. Composite interval mapping analysis in 96 S1 plants revealed QTL for aerenchyma formation on chromosomes 1 (bins 1.06–1.07) and 7 (bin 7.01), explaining 17 and 12% of the total phenotypic variance, respectively. The QTL on chromosome 1 was verified by using 156 S2 plants. Near-isogenic lines exhibiting the presence or absence of the aerenchyma QTL have been developed that should be useful for genetic and physiological analyses of root aerenchyma formation.

Demography of an endangered endemic rupicolous cactus

We compared the demography of two populations of Mammillaria huitzilopochtli, an endemic and threatened rupicolous cacti species with a narrow distribution in the semiarid Tehuacan-Cuicatlan region of central Mexico. Censuses were conducted over a 5-year period in two populations: a disturbed site (S1) and a well-preserved site (S2). Five annual size-based matrices and a mean transition matrix of each population were constructed to estimate demographic trends. Prospective time-invariant analyses were performed to calculate population growth rate and elasticities, whereas prospective stochastic analyses were performed to assess quasi-extinction probabilities and how simulated changes in recruitment, stasis and growth affected the population growth rate. Retrospective perturbation analyses (life table response experiments, LTREs) were used to explore the contributions of demographic processes, plant sizes, and temporal variability (years) to the observed variations in population growth rate. The species showed decreasing population growth rates for almost all years and sites, S1 showed lower population growth rates than S2. Quasi-extinction probabilities were 1 after 9 years for S1 and 17 years for S2. Elasticity values were highest for matrix entries corresponding to plants remaining in the same category (stasis) followed by growth and fecundity. LTREs showed that fecundity had negative contributions to population growth rates for all years in S1 population, while it had positive contributions in 4 out of 5 years in population S2. Prospective stochastic analyses showed that increasing recruitment by 50% could give population growth rates >1 in S2 while none of the simulations give this value in S1. Increasing survival also raises population growth rates but always below one. These results indicate that populations under the influence of human disturbance will eventually be lost. According to the models the most promising management strategy to conserve this species is to increase recruitment rates and to give special care to reproductive adults.

Overruling the energy gap law: fast triplet formation in 6-azauracil

The photophysical properties of 6-azauracil were studied by means of ab initio quantum chemical methods. On the basis of our calculations we propose here the following mechanism for the lack of fluorescence and the high triplet quantum yield that was observed experimentally after irradiation of this compound with UV light [Kobayashi et al., J. Phys. Chem. A, 2008, 112, 13308]. Multiple potential energy surface crossings between excited singlet states of π→π* and n→π* character lead to an ultrafast transfer of the S(2) ((1)π→π*) population to the lower-lying S(1) ((1)n→π*) state. This state acts as a doorway state from which the T(1) ((3)π→π*) state is formed approximately within 125 ps in the isolated 6-azauracil and within 30 ps in acetonitrile solution according to our calculations. The enhancement of the S(1)[radiolysis arrow - arrow with voltage kink] T(1) intersystem crossing in acetonitrile solution is noteworthy as it goes along with an increased adiabatic energy gap between the interacting states. Blue shift of the S(1) potential energy surface by about 0.2 eV in this polar, aprotic environment places the intersection between the S(1) and T(1) potentials close to the S(1) minimum, thus increasing the overlap of the vibrational wavefunctions and consequently speeding up the spin-forbidden nonradiative transition.

Intersystem Crossing and Characterization of Dark States in the Pyrimidine Nucleobases Uracil, Thymine, and 1-Methylthymine

The ground and low-lying excited states of the pyrimidine nucleo bases uracil, thymine, and 1-methylthymine have been characterized using ab initio coupled-cluster with approximate doubles (CC2) and a combination of density functional theory (DFT) and semiempirical multireference configuration interaction (MRCI) methods. Intersystem crossing rate constants have been determined perturbationally by employing a nonempirical one-center mean-field approximation to the Breit-Pauli spin-orbit operator for the computation of electronic coupling matrix elements. Our results clearly indicate that the S(2)((1)pi-->pi*)-->T(2)((3)n-->pi*) process cannot compete with the subpicosecond decay of the S(2) population due to spin-allowed nonradiative transitions, whereas the T(1)((3)pi-->pi*) state is populated from the intermediate S(1)((1)n-->pi*) state on a subnanosecond time scale. Hence, it is very unlikely that the S(1)((1)n-->pi*) state corresponds to the long-lived dark state observed in the gas phase.

Photophysics of Soret-Excited Tetrapyrroles in Solution. II. Effects of Perdeuteration, Substituent Nature and Position, and Macrocycle Structure and Conformation in Zinc(II) Porphyrins

The steady-state absorption, fluorescence, and excitation spectra and upper excited-state temporal fluorescence decay profiles of 11 tetrapyrroles in several fluid solvents are presented and analyzed to ascertain the factors that control their S2 population decay times. The S2 lifetimes, which vary by more than 2 orders of magnitude, are controlled exclusively by their rates of radiationless decay. The only important electronic relaxation path is S2-S1 internal conversion, the efficiency of which is near 1.0 in all compounds studied (except CdTPP where it is 0.69). The rate of S1 population rise equals the rate of S2 population decay in all cases. Among the compounds studied, only MgTPP exhibits S2-S1 decay behavior that corresponds to the weak coupling limit of radiationless transition theory; all zinc metalloporphyrins exhibit intermediate to strong coupling. Perdeuteration of ZnTPP produces no significant change in the rate of S2 decay or in the quantum yield of S2-S0 fluorescence, indicating that in-plane C-C and C-N vibrations are the accepting modes in S1 with the largest Franck-Condon factors. The initial vibrational energy content of the S2 states (0 < E(vib) < 3500 cm(-1) over the range of compounds) plays no significant role in determining their overall population decay rates in solution. The S2 population decay rates of these tetrapyrroles are controlled by two factors: the Franck-Condon factor, which is inversely proportional to the exponent of the S2-S1 electronic energy spacing and the S2-S1 coupling energy. The S2-S1 electronic energy spacing is determined in solution by the difference in the polarizabilities of the S2 and S1 states and can be controlled by varying the polarizability of the solvent. The S2-S1 coupling energy is influenced by the nature, location, and effect of the substituents, with beta-alkyl substitution and reduction of symmetry in the tetrapyrrole--for example by loss of planarity--increasing the interstate coupling energy.

Genetic Control of Floral Morph in Tristylous Pickerelweed (Pontederia cordata L.)

Pickerelweed (Pontederia cordata L.) is a diploid (2n = 2x = 16) tristylous aquatic perennial. Populations usually contain 3 floral morphs that differ reciprocally in style length and anther height (referred to as the long-, mid-, and short-styled morphs, hereafter L-, M-, and S-morphs). The floral polymorphism promotes disassortative mating among the 3 floral morphs and is maintained in populations by negative frequency-dependent selection. The objective of this study was to determine the number of loci, number of alleles, and gene action controlling floral morph in pickerelweed. Three parental lines (one each of the L-, M-, and S-morph) were used to create S1 and F1 populations. F2 populations were produced through self-pollination of F1 plants. Progeny ratios of S1, F1, and F2 generations revealed that tristyly is controlled by 2 diallelic loci (S and M) with dominant gene action. The S locus is epistatic to the M locus, with the S-morph produced by plants with the dominant S allele (genotype S _ _ _). Plants with recessive alleles at the S locus were either L-morph (ssmm) or M-morph (ssM_). The results of this experiment demonstrate that the inheritance of tristyly in pickerelweed is the same as previously reported for several tristylous species in the Lythraceae and Oxalidaceae.

Fate of Excited States in Jet-Cooled Aromatic Molecules: Bifurcating Pathways and Very Long Lived Species from the S1 Excitation of Phenylacetylene and Benzonitrile

Pump-probe delayed ionization studies on phenylacetylene and benzonitrile in a supersonic beam reveal the production of a low-ionization-potential (approximately 5.7 eV) species lasting more than hundreds of microseconds after excitation to the S1 state. Excitation of the molecules was done with a frequency-doubled, Fourier transform-limited, pulse-amplified cw laser, and the rotationally resolved structure of the S1-S0 transition ensures that the excited molecules are monomers. Excited-state photoelectron spectroscopy shows that the long-lived species are formed during the light pulse but not by transfer from the fluorescing S1 population after the pulse, even though the S1 spectral signature is present in the long-lived action spectrum. This behavior differs greatly from that found in benzene and with most commonly held pictures of radiationless transitions in large molecules.

Inheritance of Phytophthora root rot resistance in red raspberry determined by generation means and molecular linkage analysis

Classical and molecular methodologies were used to determine the inheritance of Phytophthora root rot (PRR) resistance in red raspberry. The varieties 'Latham' and 'Titan,' resistant and susceptible, respectively, were used to create F(1), F(2), B(1), B(2), and S(1) populations for analysis. Generational means analysis was used to calculate the components of genetic variation and estimates of narrow and broad sense heritability for the plant disease index and the incidence of petiole lesions. The plant disease index showed additive genetic variation with additional significant interactions, but the incidence of petiole lesions was non-additive. A dominant, two-gene model was shown to be the best fit for the observed segregation ratios when classification for resistance was based on a combination of all criteria measured. Molecular linkage maps were generated from the segregating B(2) population. Linkage maps of both parents were constructed from amplified fragment length polymorphism (AFLP), Random amplified polymorphic DNA (RAPD), and uncharacterized resistant gene analog polymorphism (RGAP) markers with seven linkage groups each totaling 440 and 370 cM of genetic distance, respectively. An analysis of the distributional extremes of the B(2) population identified several RAPD markers clustered on two linkage groups associated with PRR resistance. QTL analysis identified two similar genomic regions on each map that explained significant percentages of phenotypic variation observed for the disease assessment criteria. Genetic mapping supports the dominant two-gene model developed from generational means analysis. The results reconcile conflicting reports on inheritance of PRR resistance, provide a basis for further investigation of durable resistance to Phytophthora caused diseases, and indicates that recurrent selection is the appropriate approach for the development of new resistant cultivars.