The role of synaptic facilitation in spike coincidence detection

Using a realistic model of activity dependent dynamical synapse, which includes both depressing and facilitating mechanisms, we study the conditions in which a postsynaptic neuron efficiently detects temporal coincidences of spikes which arrive from N different presynaptic neurons at certain frequency f. A numerical and analytical treatment of that system shows that: (1) facilitation enhances the detection of correlated signals arriving from a subset of presynaptic excitatory neurons, and (2) the presence of facilitation yields to a better detection of firing rate changes in the presynaptic activity. We also observed that facilitation determines the existence of an optimal input frequency which allows the best performance for a wide (maximum) range of the neuron firing threshold. This optimal frequency can be controlled by means of facilitation parameters. Finally, we show that these results are robust even for very noisy signals and in the presence of synaptic fluctuations produced by the stochastic release of neurotransmitters.     

Extreme temperatures increase the deleterious consequences of inbreeding under laboratory and semi-natural conditions

The majority of experimental studies of the effects of population bottlenecks on fitness are performed under laboratory conditions, which do not account for the environmental complexity that populations face in nature. In this study, we test inbreeding depression in multiple replicates of inbred when compared with non-inbred lines of Drosophila melanogaster under different temperature conditions. Egg-to-adult viability, developmental time and sex ratio of emerging adults are studied under low, intermediate and high temperatures under laboratory as well as semi-natural conditions. The results show inbreeding depression for egg-to-adult viability. The level of inbreeding depression is highly dependent on test temperature and is observed only at low and high temperatures. Inbreeding did not affect the developmental time or the sex ratio of emerging adults. However, temperature affected the sex ratio with more females relative to males emerging at low temperatures, suggesting that selection against males in pre-adult life stages is stronger at low temperatures. The coefficient of variation (CV) of egg-to-adult viability within and among lines is higher for inbred flies and generally increases at stressful temperatures. Our results contribute to knowledge on the environmental dependency of inbreeding under different environmental conditions and emphasize that climate change may impact negatively on fitness through synergistic interactions with the genotype.     

Inbreeding depression on beef cattle traits: Estimates,linearity of effects and heterogeneity among sire-families

Records from up to 19054 registered cows and 10297 calves in 155 herds of the Alentejana cattle breed were used to study the effects of individual (F_i) and maternal (F_m) inbreeding on reproductive, growth and carcass traits, as well as assessing the importance of non-linear associations between inbreeding and performance, and evaluating the differences among sire-families in the effect of F_i and F_m on calf weight at 7 months of age (W7M). Overall, regression coefficients of performance traits on inbreeding were small, indicating a minor but still detrimental effect of both F_i and F_m on most traits. The traits with the highest percentage impact of F_i were total number of calvings through life and calf weight at 3 months of age (W3M), followed by longevity and number of calves produced up to 7 years, while the highest effect of F_m was on W3M. Inbreeding depression on feed efficiency and carcass traits was extremely small and not significant. No evidence was found of a non-linear association between inbreeding and performance for the traits analyzed. Large differences were detected among sire-families in inbreeding depression on W7M, for both F_i and F_m, encouraging the possibility of incorporating sire effects on inbreeding depression into selection decisions.     

Selfed embryo death in Pinus taeda: a phenotypic profile

Selective elimination of selfed embryos, or inbreeding depression, is shared among many members of the Pinaceae but it has not been fully characterized at the phenotypic level. Here, two death pattern model hypotheses are tested using 10 621 Pinus taeda embryos sampled in two cohorts. Cones from a single pedigree based on selfed, outbred, parent-offspring and offspring-parent matings were destructively sampled weekly before, during and after fertilization. Selfed embryo deaths adhered to two patterns over the course of development: death was linear with respect to days from fertilization; and a stage-specific death peak occurred during the early embryogeny stage. This death peak occurred from 23 to 36 d after fertilization in the 2004 cohort and from 27 to 34 d after fertilization in the 2006 cohort. Of those selfed embryos that died, 64-83% died at stages where a single dominant embryo was elongating inside the female gametophyte. Additional genetic models are needed to account for the stage-specific death component of selfed P. taeda embryos.     

Neuronal membrane potential is mildly depolarized in the anoxic turtle cortex

Neuronal membrane potential (E_m) regulates the activity of excitatory voltage-sensitive channels. Anoxic insults lead to a severe loss of E_m and excitotoxic cell death (ECD) in mammalian neurons. Conversely, anoxia-tolerant freshwater turtle neurons depress energy usage during anoxia by altering ionic conductance to reduce neuronal excitability and ECD is avoided. This wholesale alteration of ion channel and pump activity likely has a significant effect on E_m. Using the whole-cell patch clamp technique we recorded changes in E_m from turtle cortical neurons during a normoxic to anoxic transition in the presence of various ion channel/pump modulators. E_m did not change with normoxic perfusion but underwent a reversible, mild depolarization of 8.1 ± 0.2 mV following anoxic perfusion. This mild anoxic depolarization (MAD) was not prevented by the manipulation of any single ionic conductance, but was partially reduced by pre-treatment with antagonists of GABA_A receptors (5.7 ± 0.5 mV), cellular bicarbonate production (5.3 ± 0.2 mV) or K⁺ channels (6.0 ± 0.2 mV), or by perfusion of reactive oxygen species scavengers (5.2 ± 0.3 mV). Furthermore, all of these treatments induced depolarization in normoxic neurons. Together these data suggest that the MAD may be due to the summation of numerous altered ion conductance states during anoxia.     

INBREEDING AND FITNESS IN THE FRESHWATER SNAIL LYMNAEA PEREGRA: AN EVALUATION OVER TWO GENERATIONS OF SELF-FERTILIZATION

Inbreeding depression was estimated from an outbreeding population of the freshwater snail Lymnaea peregra, on the basis of two successive generations of enforced selling and outcrossing, and 70 maternal lineages. Outcrossing was analyzed under two treatments, groups of two and five individuals. The fitness parameters measured included fecundity, growth, and survival. In the first generation, we contrasted three treatments (selfers vs. paired outcrossers and group outcrossers). Very similar results were obtained between the two outcrossing treatments. A strong self-fertilization depression (which includes parental fecundity and progeny fitness) was detected in the selling treatment (about 90%). In the second generation, there was again marked evidence for self-fertilization depression, with the highest contributions coming from parental fecundity and progeny hatching rate. Our results suggest that the decreased parental fecundity is a consequence of the mating system in the previous generation, although the role of partial self-incompatibility and the copulation behavior could not be ruled out. Hatching rate and early survival data are suggestive of purging of lethal mutations. Significant variation in fitness among selfing lineages was found for most fitness traits. Our experimental design also allowed to test for interactions among fitness loci. Only one trait of the nine studied behaved as expected under synergistic interactions. However we cannot rule out some purging during the experiment, which could have biased results towards linearity. Inbreeding depression was also inferred from the change of inbreeding level across generations in the same population. We obtained a value similar to the experimental estimate.     

Effect of inbreeding on production of filled seed in Pinus radiata — experimental results and a model of gene action

Summary Pinus radiata D. Don was inbred to different degrees, commencing with a founder population of 8 clones. Yield of filled seed was determined for each mating type. Mean yields (%), relative to outcrossing, were: half-sib 102; full-sib 97; S₁ 43; S₂ 42. An epistatic model was developed to predict the empty seed yield following selfing, assuming that homozygosity for several recessive co-lethals is necessary to kill an embryo. Calculations were then extended to predict the yields following different degrees of inbreeding. The proposed model gave a better fit to present results, and to other published results of similar investigations, than did an alternative based on action of independent recessive lethals. Implications for breeding and seed production strategy are discussed.     

Drift load in populations of small size and low density

According to theory, drift load in randomly mating populations is determined by past population size, because enhanced genetic drift in small populations causes accumulation and fixation of recessive deleterious mutations of small effect. In contrast, segregating load due to mutations of low frequency should decline in smaller populations, at least when mutations are highly recessive and strongly deleterious. Strong local selection generally reduces both types of load. We tested these predictions in 13 isolated, outcrossing populations of Arabidopsis lyrata that varied in population size and plant density. Long-term size was estimated by expected heterozygosity at 20 microsatellite loci. Segregating load was assessed by comparing performance of offspring from selfings versus within-population crosses. Drift load was the heterosis effect created by interpopulation outbreeding. Results showed that segregating load was unrelated to long-term size. However, drift load was significantly higher in populations of small effective size and low density. Drift load was mostly expressed late in development, but started as early as germination and accumulated thereafter. The study largely confirms predictions of theory and illustrates that mutation accumulation can be a threat to natural populations.