Weak Barrier Research Articles

Fitness Consequences of Reciprocally Asymmetric Hybridization between Simultaneous Hermaphrodites

Depending on fitness consequences, hybridization may rescue Inbred populations; generate premating barriers, reproductive Interference, or hybrid species; or extinguish a species. However, the fitness of hybrids is unpredictable without direct quantification of their performance in fitness components across multiple generations. The land snails Bradybaena pellucida and B. similaris, which are Indigenous and non-Indigenous in Japan, respectively, copulate with each other simultaneously and reciprocally. However, only B. pellucida produces hybrids, because it ends mating by removing the penis before transferring a spermatophore, while B. similaris Inseminates B. pellucida. To evaluate the strength of an Intrinsic postzygotic barrier against the hybrids produced by B. pellucida, we conducted breeding experiments in the laboratory and measured six life-history traits: (1) growth rate, (2) body weight at maturity, (3) number of days to first oviposition after being permitted to mate, (4) clutch size, (5) fecundity, and (6) hatchability. We also calculated the relative intrinsic fitness based on five of these trait values (excluding clutch size). F(1) hybrids exhibited heterosis in growth rate, body weight at maturity and relative intrinsic fitness. F(2) hybrids also showed heterosis in body weight at maturity. Nevertheless, the F(2) hybrids produced significantly fewer progeny than the mid-point value of the parental species. Thus, the F(2) hybrids exhibited weak out-breeding depression in reproduction, offsetting their vigor in body size. These results indicate that only a weak postzygotic barrier, contrasting with strong F(1) heterosis, has evolved during genetic divergence of the two sibling species in allopatry.

MACROHETEROCERA БЕЗ GEOMETRIDAE И NOCTUIDAE s. lat. (INSECTA, LEPIDOPTERA) НИЖНЕГО ПРИАМУРЬЯ

A distribution of 182 species of the so-called bombycoid moths from superfamilies Uranioidea, Drepanoidea, Lasiocampoidea, Sphingoidea, Bombycoidea, Notodontoidea, Noctuoidea (excluding Noctuidae s. lat.), as well as Hepialidae, Cossidae, Limacodidae, Zygaenidae, Thyrididae, within the Lower Amur territory is discussed. Effectiveness of the distributional barriers between main collecting places in Lower Amur and South Primorye is discussed. For the entire species set, the most important barrier is located between Kiselevka-Tsimmermanovka (51˚ 20-25' N, 139˚ E) and the Amur river mouth. It coincides with the north-eastern limits of the polydominant broad-leaved forests. A weaker barrier is located between Khabarovsk and Komsomolsk areas. Several species, like Caligula japonica, don’t overpass a barrier along the river Gur. For obligatory spring species, the most effective barrier is detected between Khabarovsk and Komsomolsk areas; for obligatory autumn species – between South Primorye and Khabarovsk, and between Khabarovsk and Komsomolsk. These barriers act mainly as north-eastern limits of nemoral species; for boreal species these barriers are ten times less effective. The area of the Amur river mouth is inhabited by the subnemoral fauna sensu Dubatolov & Kosterin, 2000, it is transitional between East Asian (Palearchaearctic) and Euro-Siberian Subregions of the Palearctic. Another example of such a transitional territory is the Upper Amur basin, with Eastern Transbaikalia [Dubatolov, Vasilenko & Streltzov, 2003].

The role of poly(3,4-ethylenedioxythiophene):poly(styrenesulphonate) as a hole injection layer in a blue-emitting polymer light-emitting diode

The authors study the role of the conducting polymer poly(3,4-ethylenedioxythiophene): poly(styrenesulphonate) (PEDOT:PSS) in determining the transient and steady-state operation of a blue-emitting polymer light-emitting diode. Combining the results from photoemission spectroscopy, time-of-flight photocurrent measurements, and studies on hole-only devices reveals a significant barrier for the injection of holes into the polymer. Simulations with a numerical drift-diffusion model, however, show that the injection currents determined from single-carrier devices cannot account for the rapid transient luminance onset and the efficient steady-state luminance output of the corresponding bipolar light-emitting devices. It is shown that the transient electroluminescence traces measured at different external bias can be well reproduced when assuming the presence of a weak barrier for electron extraction at this interface, which is attributed to electron accumulation at a thin phase-segregated PSS-rich layer at the surface of PEDOT:PSS. In addition, interface conditioning, presumably due to electron trapping near PEDOT:PSS, renders the anode-polymer interface nearly Ohmic. This conditioning, however, occurs on intermediate time scales normally not addressed by either transient or steady-state measurements.

Air and oxygen flowmeter confusion

We wish to report an episode of air being administered to a critically ill patient rather than oxygen, due to a mix-up with wall-mounted flowmeters. An elderly gentleman in ventricular tachycardia was anaesthetized in a resuscitation area for emergency cardioversion. Afterwards, what was thought to be high flow oxygen was administered via facemask. Mild hypoxaemia (paO2 ~8kPa) persisted, but the cause was unclear until it was noticed that the mask tubing was connected to a piped air flowmeter. Once this was corrected, hypoxaemia improved and the patient suffered no adverse consequences. While there are standards for the design and colour of gas cylinders and pipeline outlets, there are none for flowmeters.1 In this case the error was caused by the anaesthetist (new to the hospital that week) viewing the flowmeter from the side ( Figure 1a) with another mask dangling from above obscuring the label (Figures 1b and c). Figure 1 (A) and (B) Two flowmeters in the resuscitation room: masks are dangling down and obscuring the flowmeter below. (C) When the mask is moved away, the ‘Air’ label becomes obvious The use of non-interchangeable connectors for anaesthetic gases has reduced the potential for administering the wrong gas, but mistakes like this are clearly happening frequently enough for the UK National Patient Safety Agency to issue guidance on the matter.2 They suggested potential solutions including capping air flowmeters, clearer labelling, and that air flowmeters should only be attached when required. Even they recognized that these are weak barriers based on human or administrative actions, which would be inadequate for the safe delivery of other drugs.3 The universal outlets of oxygen and air flowmeters – whether attached to pipeline supplies in hospitals or cylinders in surgeries and ambulances – both accept oxygen mask tubing, and while this persists we believe that this error will be made repeatedly, potentially to the detriment of many patients.

Root anatomy and spatial pattern of radial oxygen loss of eight true mangrove species

The root anatomical features of eight mangrove species in Hong Kong were similar, with large aerenchymal lacunae in the cortex for efficient internal oxygen transfer and an outer barrier consisting of an epidermis and hypodermis to prevent oxygen loss. The spatial pattern of radial oxygen loss (ROL) was also comparable, with more oxygen lost from the tip than that from the basal and mature zones. However, the aerenchyma in the cortex, the barrier and the extent of ROL varied along the root and these variations were species-specific. The whole root of Avicennia marina (Forsk.) Vierh., Acanthus ilicifolius L., Aegiceras corniculatum (Linn.) Blanco, Kandelia obovata Sheue, Liu & Yong (previously known as Kandelia candel (L.) Druce) and Heriteria littoralis Dryand. ex W. Ait. had schizogenous aerenchyma, while the aerenchyma of Lumnitzera racemosa Willd. and Bruguiera gymnorrhiza (L.) Poir changed from schizogenous in the root tip to lysigenous in the other parts of the root. Excoecaria agallocha L. displayed the opposite pattern, from lysigenous in the root tip to schizogenous further up. Among the eight species, the roots of A. marina and A. ilicifolius had the largest areas of aerenchyma air spaces, but they also had the weakest barrier. On the other hand, H. littoralis had the least longitudinal oxygen transfer because of its smaller area of aerenchyma air spaces in its root. The tolerance of mangrove species to waterlogged soil followed the order of A. marina (most foreshore species) > A. ilicifolius > K. obovata > A. corniculatum > B. gymnorrhiza > E. agallocha > L. racemosa > H. littoralis (most landward species), which is related to their anatomical features of root cortex, epidermis and hypodermis.

Dependence of Invadopodia Function on Collagen Fiber Spacing and Cross-Linking: Computational Modeling and Experimental Evidence

Invadopodia are subcellular organelles thought to be critical for extracellular matrix (ECM) degradation and the movement of cells through tissues. Here we examine invadopodia generation, turnover, and function in relation to two structural aspects of the ECM substrates they degrade: cross-linking and fiber density. We set up a cellular automaton computational model that simulates ECM penetration and degradation by invadopodia. Experiments with denatured collagen (gelatin) were used to calibrate the model and demonstrate the inhibitory effect of ECM cross-linking on invadopodia degradation and penetration. Incorporation of dynamic invadopodia behavior into the model amplified the effect of cross-linking on ECM degradation, and was used to model feedback from the ECM. When the model was parameterized with spatial fibrillar dimensions that closely matched the organization, in real life, of native ECM collagen into triple-helical monomers, microfibrils, and macrofibrils, little or no inhibition of invadopodia penetration was observed in simulations of sparse collagen gels, no matter how high the degree of cross-linking. Experimental validation, using live-cell imaging of invadopodia in cells plated on cross-linked gelatin, was consistent with simulations in which ECM cross-linking led to higher rates of both invadopodia retraction and formation. Analyses of invadopodia function from cells plated on cross-linked gelatin and collagen gels under standard concentrations were consistent with simulation results in which sparse collagen gels provided a weak barrier to invadopodia. These results suggest that the organization of collagen, as it may occur in stroma or in vitro collagen gels, forms gaps large enough so as to have little impact on invadopodia penetration/degradation. By contrast, dense ECM, such as gelatin or possibly basement membranes, is an effective obstacle to invadopodia penetration and degradation, particularly when cross-linked. These results provide a novel framework for further studies on ECM structure and modifications that affect invadopodia and tissue invasion by cells.

Internet-based Interventions Have Potential to Affect Short-term Mediators and Indicators of Dietary Behavior of Young Adults

Objective Evaluate a theory-based, Internet-delivered nutrition education module. Design Randomized, treatment-control design with pre-post intervention assessments. Setting and Participants Convenience sample of 160 young adults (aged 18-24) recruited by community educators in 4 states. Study completers (n = 96) included a mix of racial/ethnic groups and family demographics, but were predominately white females without children. Intervention A Transtheoretical Model (TTM)-based, stage-tailored Internet program, F&V (Fruit & Vegetable) Express Bites, was delivered to treatment group participants; controls received nontailored messages in a comparable format. Main Outcome Measures and Analysis TTM constructs at baseline, immediately postviewing, and at 30-day follow-up were compared via chi-square, McNemar's test, and logistic regression. Qualitative use and acceptability indicators were collected postviewing (treatment only). Results Of baseline respondents, 86.5% completed the study. At follow-up, self-efficacy, decisional balance, and selected processes improved for both groups, with only marginal advantages for the theory-based version. Fruit and vegetable consumption changes were similar for both groups. Qualitative measures of knowledge, confidence, and motivation improved (not measured for controls). Favorable ratings of treatment program acceptability and personal relevance were reported. Conclusions and Implications The Internet is a favorable option for reaching young adults with nutrition messages. This study did not identify strong advantages of theory-based tailoring, especially for fruit consumption, which had weaker barriers than vegetable consumption among this target audience.

Homoepitaxial growth of Bi(111)

Homoepitaxial growth of Bi(111) at temperatures between 80--300 K has been studied using spot profile analyzing low-energy electron diffraction (SPA-LEED) and scanning tunneling microscopy (STM). From the intensity oscillations of the (00)-spot with Bi coverage and the STM topography of two-dimensional (2D) islands at low coverage, a pure 2D nucleation followed by a quasi bilayer-by-bilayer growth mode has been confirmed. The oscillation amplitude decays slowly with coverage, indicating a slow kinetic roughening due to a weak Ehrlich-Schwoebel step edge barrier. From the Arrhenius behavior of the average island separation an intraterrace diffusion barrier of ${E}_{d}=0.135\text{ }\text{eV}$ is estimated. Regularly ordered quasidendritic shape islands reflect an asymmetry in adatom diffusion along the steps and the corners of the islands.

Age‐Related Immunohistochemical and Ultrastructural Changes in Rat Oculomotor Nerve

During ageing process, multiple changes occur on nervous tissue composed of cells and extracellular matrix. Changes on nervous tissues are usually known as degenerative changes on axon structure and connective tissue covering the nerve such as a decrease in the number of fibre or general structural changes. For this purpose, we have studied age-dependent ultrastructural changes in the rat oculomotor nerve with electron microscopy and also demonstrated collagen structure of the neural sheaths with immunohistochemical techniques. This study was conducted in Gazi University Faculty of Medicine, Department of Anatomy with a total of nine Wistar albino rats. We observed strong collagen type I immunoreactivity in endoneurium and slight to moderate reactivity in fibroblast cytoplasm in 3-month- and 12-month-old groups and mild reactivity in 24-month-old group. Collagen type IV immunoreactivity was stronger in endoneurium and perineurium in the 3-month- and 12-month-old groups compared with collagen type I and fibroblast cytoplasm showed a very strong reactivity. On the other hand, in the 24-month-old group, there was slight reactivity in endoneurium and a strong reactivity in perineurium. NGF staining showed moderate to strong reactivity on Schwann cells of the 3-month-old group. The immunoreactivity decreased in the 12-month- and 24-month-old groups. In the 3-month-old rat group, Schwann cell cytoplasm, mitochondrial structure and neurofilaments were normal. In the 12-month-old group, there were no changes in organelle distribution, mitochondrial structure and neurofilaments, but there was an increase in the connective tissue. An inconsiderable number of degenerated myelinated nerves were observed. We detected an important decrease in the collagen type I immunoreactivity, which could suggest that the endoneurium, perineurium and epineurium are less resistant to the age-related collagen loss and that the peripheral nerve is protected by a weaker barrier in the old group. The collagen type IV immunoreactivity was significantly decreased with age. NGF synthesis decreases with age because of Schwann cell structural degeneration or for different reasons. Thus, this could explain the diminished capacity of regeneration and damage of the myelination of the peripheral nerve.

Hybridization of glaucous gull (Larus hyperboreus) and herring gull (Larus argentatus) in Iceland: mitochondrial and microsatellite data.

Large white-headed gulls provide an interesting group of birds for studies of hybridization. The group is composed of 20 species of recent origin, often with weak reproductive barriers. Here we report the results from a study on the glaucous gull Larus hyperboreus, an Arctic species which has been breeding in Iceland for centuries, and the herring gull Larus argentatus which has a wide distribution in Europe but colonized Iceland in 1920s. Previous studies, based on morphological variation indicated hybridization between the two species in Iceland, have been questioned as it may just reflect variation within the species. Here we evaluate whether hybridization has occurred between the two species in Iceland by studying variation in microsatellites and mtDNA. The analysis is based on feathers taken from wings sampled in Iceland over a period of 40 years. The results are compared with samples obtained from East Greenland and published sequences of samples obtained throughout Europe. The genetic analysis reveals a distinctive grouping of the two species, although they present a shallow genealogy and an extensive sharing of the genetic variants between the two species. Several individuals show admixture for molecular markers, which may result from an incomplete lineage sorting although geographical patterns of both mtDNA haplotypes and microsatellites strongly indicate a recent hybridization in Iceland.

Transduction of Full-length Dystrophin to Multiple Skeletal Muscles Improves Motor Performance and Life Span in Utrophin/Dystrophin Double Knockout Mice

Duchenne muscular dystrophy (DMD) is a fatal, progressive, muscle-wasting disease caused by defects in the dystrophin. No viral vector except the helper-dependent adenovirus vector (HDAdv) can package 14-kilobase (kb) full-length dystrophin complementary DNA (cDNA), and HDAdv is considerably safer than old-generation adenovirus vectors because of the large-size deletion in its genome. We have generated HDAdv that carries myc-tagged murine full-length dystrophin cDNA (HDAdv-myc-mFLdys). We injected it into multiple proximal muscles of 7-day-old utrophin/dystrophin double knockout mice (dko mice) (which typically show symptoms quite similar to human DMD) because the proximal muscles are affected in DMD patients. Eight weeks after the injections, the transduced dystrophin was widely expressed, and we found a significant reduction in centrally nucleated myofibers and the restoration of the dystrophin-associated proteins, beta-dystroglycan (beta-DG) and alpha-sarcoglycan (alpha-SG), as well as neuronal nitric oxide synthase (nNOS). The injected dko mice also showed an increase in body weight, an improvement in motor performance, and a prolongation of life span. Using HDAdv, we could treat DMD model mice even by transferring the therapeutic gene into multiple skeletal muscles. Our results suggest that multiple intramuscular administrations of HDAdv carrying full-length dystrophin cDNA may reduce symptoms and compensate for lost functions in DMD patients.

Cell culture models of the human cornea — a comparative evaluation of their usefulness to determine ocular drug absorption in-vitro

Cell culture models of the cornea are continually developed to replace the isolated animal cornea for transcorneal drug absorption studies. The aim of this study was to determine and compare epithelial tightness and permeability of currently available corneal cell culture models to avoid interlaboratory variability and to assess their usefulness for in-vitro permeation studies. Pure epithelial cell culture models (CEPI, SIRC and HCE-T cell lines), primary cultures of human corneal epithelium (HCEpiC) and the two commercially available models (RHC and Epiocular), as well as organotypic human cornea constructs (HCC, HCC-HCE-T), were investigated and data were compared with those obtained from the excised bovine cornea. Barrier properties were assessed by measurements of transepithelial electrical resistance (TEER) and permeability of three passively absorbed substances (mannitol, testosterone and timolol maleate) with different physico-chemical properties. TEER experiments revealed weak barrier functions for all of the investigated epithelial models (<or=100-200 Omega cm2), except the HCE-T cell line. Transport studies confirmed TEER results insofar that models showing low TEER values also had higher permeation rates in comparison with the excised bovine cornea. However, models based on HCE-T cells demonstrated similar barrier properties to isolated corneal tissue. The corneal models investigated in our laboratory show clear differences in epithelial barrier function. In-vitro systems comprising the HCE-T cell line seem to be most appropriate to replace excised animal cornea for assessing corneal permeability.

Perception of sting and response to irritants is associated with a weak barrier

Perception of sting and response to irritants is associated with a weak barrier

Transport in narrow minigap superlattices with inter‐Wannier‐Stark level tunneling

AbstractLow temperature electron transport under interminiband Zener tunneling and LO‐phonon scattering in the second and higher minibands in GaAs/Alx Ga1¡x As superlattices with weak barriers has been studied. At low electric fields multiple negative differential low‐frequency conductivity regions and effects of complicated electron transport have been found. In stronger fields peculiarities of current ‐ voltage curves attributed to resonant tunneling between Wannier‐Stark levels of different minibands have been separated basing on calculated wavefunctions and energies of the levels. Calculated value of optical matrix element for inter‐Wannier‐Stark transitions is promising to achieve negative conductivity at these transitions. (© 2008 WILEY‐VCH Verlag GmbH &amp; Co. KGaA, Weinheim)

Globally nonsimultaneous Forbush decrease events and their implications

Forbush decrease (FD) events are supposed to happen simultaneously over the globe of the Earth. However, there have been several reports on nonsimultaneous FD events. We investigate for the first time the properties of nonsimultaneous FD events and the solar wind conditions causing such events in detail in order to determine what solar wind conditions lead to global simultaneity of FD events. We examined the hourly data of the Oulu Neutron Monitor (NM) station from 1998 to 2002. We have selected 49 FD events that have greater than 3.5% intensity reductions. Global simultaneity was determined by comparing the time profiles of these FD events with those recorded by other NM stations at Inuvik and Magadan. These three NM stations are located at close magnetic latitude (cutoff rigidity) but fairly evenly spaced in longitudes. The solar wind parameters driving FD event main phase are studied. Most FD event onsets (37 out of 49) are observed simultaneously by each NM station in universal time (UT) regardless of the location of the station, whereas some other FD events are not simultaneously detected but are at similar local times (LT). The stronger FD events tend to be simultaneous events, but the weaker FD events are nonsimultaneous. The simultaneous FD events are caused by the solar wind of higher speed and stronger IMF, whereas the nonsimultaneous events are driven by relatively lower speed and weaker IMF solar wind. The nonsimultaneous FD event may occur only if the main phase of the FD is superposed in phase with the declining phase of diurnal variation. Our interpretation is that the simultaneous FD events might occur when the high‐speed strong magnetic barrier (interplanetary shock sheath and magnetic cloud) overtakes the Earth, whereas the nonsimultaneous FD events may occur only when the slow‐moving weak magnetic barrier passes by on the duskside of the Earth. The global simultaneity of FD events depends on speed and IMF strength of solar wind overtaking Earth's magnetosphere and its propagation direction. This model of FD simultaneity can be tested by the STEREO mission.

Coherent zero-state and π-state in an exciton–polariton condensate array

The effect of quantum statistics in quantum gases and liquids results in observable collective properties among many-particle systems. One prime example is Bose-Einstein condensation, whose onset in a quantum liquid leads to phenomena such as superfluidity and superconductivity. A Bose-Einstein condensate is generally defined as a macroscopic occupation of a single-particle quantum state, a phenomenon technically referred to as off-diagonal long-range order due to non-vanishing off-diagonal components of the single-particle density matrix. The wavefunction of the condensate is an order parameter whose phase is essential in characterizing the coherence and superfluid phenomena. The long-range spatial coherence leads to the existence of phase-locked multiple condensates in an array of superfluid helium, superconducting Josephson junctions or atomic Bose-Einstein condensates. Under certain circumstances, a quantum phase difference of pi is predicted to develop among weakly coupled Josephson junctions. Such a meta-stable pi-state was discovered in a weak link of superfluid 3He, which is characterized by a 'p-wave' order parameter. The possible existence of such a pi-state in weakly coupled atomic Bose-Einstein condensates has also been proposed, but remains undiscovered. Here we report the observation of spontaneous build-up of in-phase ('zero-state') and antiphase ('pi-state') 'superfluid' states in a solid-state system; an array of exciton-polariton condensates connected by weak periodic potential barriers within a semiconductor microcavity. These in-phase and antiphase states reflect the band structure of the one-dimensional polariton array and the dynamic characteristics of metastable exciton-polariton condensates.

Cobaltocene as a spin filter

In the context of investigating organic molecules for molecular electronics, doping molecular wires with transition metal atoms provides additional means of controlling their transport behavior. The incorporation of transition metal atoms may generate spin dependence because the conduction channels of only one spin component align with the chemical potential of the leads, resulting in a spin polarized electric current. The possibility to create such a spin polarized current is investigated here with the organometallic moiety cobaltocene. According to our calculations, cobaltocene contacted with gold electrodes acts as a robust spin filter: Applying a voltage less than 0.2 V causes the current of one spin component crossing the molecular bridge to be two orders of magnitude larger than the other. We address the key issue of sensitivity to molecule-lead geometry by showing that a weak barrier generated by CH(2) groups between the cobaltocene and the leads is crucial in reducing the sensitivity to the contact geometry while only reducing the current modestly. These results suggest cobaltocene as a robust basic building block for molecular spintronics.

Hydrography and biogeochemistry of the north western Bay of Bengal and the north eastern Arabian Sea during winter monsoon

The north eastern Arabian Sea and the north western Bay of Bengal within the Indian exclusive economic zone were explored for their environmental characteristics during the winter monsoons of 2000 and 2001 respectively. The two regions were found to respond paradoxically to comparable intensities of the atmospheric forcing. There is an asymmetry in the net heat exchange of these two basins with atmosphere because of the varying thickness of barrier layer. During winter, the convective mixing in the Arabian Sea is driven by net heat loss from the ocean, whereas the Bay of Bengal does not contribute to such large heat loss to the atmosphere. It appears that the subduction of high saline Arabian Sea water mass is the mechanism behind the formation of a barrier layer in the northeast Arabian Sea; whereas that in the Bay of Bengal and the southeast Arabian Sea are already established as due to low saline water mass. The weak barrier layer in the Arabian Sea yields to the predominance of convective mixing to bring in nitrate-rich waters from the deeper layers to the surface, thereby supporting enhanced biological production. On the other hand, the river discharge into the Bay of Bengal during this period results in the formation of a thick and stable barrier layer, which insulates vertical mixing and provide oligotrophic condition in the Bay.

Weak Barriers in Nonlinear Potential Theory

We characterize regular boundary points for p-harmonic functions using weak barriers. We use this to obtain some consequences on boundary regularity. The results also hold for $\mathcal{A}$ -harmonic functions under the usual assumptions on $\mathcal{A}$ , and for Cheeger p-harmonic functions in metric spaces.

Re-thinking the embryo lethal system within the Pinaceae

Outcrossing, wind-pollinated members of the Pinaceae have high self-pollination rates yet produce few selfed seedlings. How are selfs selectively eliminated? Barriers to selfing have long been considered to be (i) avoidance of self-pollen capture, (ii) competition via simple polyembryony, and (iii) lower viability of selfed embryos. Here, reviews — and some revisions — are offered in addition to future research needs. First, avoiding self-pollen capture is largely ineffectual. Second, simple polyembrony is also a weak or nonexistent barrier against selfed embryos. Third, selfed embryos die at all stages, but a stage-specific death peak occurs during early embryogeny in some taxa and this death peak, if prevalent across a wider range of taxa, deserves a closer look. The death peak does not preclude the prevailing genetic model for embryo viability loci, but could indicate that other genetic mechanisms are operative. Molecular dissection shows that zygotic lethals are distributed across different chromosomal segments. To date, these are mostly semi-lethal rather than lethal, and they range from partially dominant to overdominant. A literature survey suggests that only 5 of the 10 genera within the Pinaceae ( Abies , Larix , Picea , Pinus , and Pseudotsuga ) have been well characterized with respect to selfing. Extreme inbreeding depression during embryo development may not be a shared feature among conifers as a group. A critical research question is whether dual death patterns are present in other genera, and if they are, alternative genetic models to account for the death peak. Addressing these questions has broad relevance to conservation, domestication, and management of closed populations, not just conifers or gymnosperms.