The benefits of interpopulation hybridization diminish with increasing divergence of small populations.

Interpopulation hybridization can increase the viability of small populations suffering from inbreeding and genetic drift, but it can also result in outbreeding depression. The outcome of hybridization can depend on various factors, including the level of genetic divergence between the populations, and the number of source populations. Furthermore, the effects of hybridization can change between generations following the hybridization. We studied the effects of population divergence (low vs. high level of divergence) and the number of source populations (two vs. four source populations) on the viability of hybrid populations using experimental Drosophila littoralis populations. Population viability was measured for seven generations after hybridization as proportion of populations facing extinction and as per capita offspring production. Hybrid populations established at the low level of population divergence were more viable than the inbred source populations and had higher offspring production than the large control population. The positive effects of hybridization lasted for the seven generations. In contrast, at the high level of divergence, the viability of the hybrid populations was not significantly different from the inbred source populations, and offspring production in the hybrid populations was lower than in the large control population. The number of source populations did not have a significant effect at either low or high level of population divergence. The study shows that the benefits of interpopulation hybridization may decrease with increasing divergence of the populations, even when the populations share identical environmental conditions. We discuss the possible genetic mechanisms explaining the results and address the implications for conservation of populations.

Predominance of outcrossing in Lymnaea stagnalis despite low apparent fitness costs of self-fertilization.

We have quantified the natural mating system in eight populations of the simultaneously hermaphroditic aquatic snail Lymnaea stagnalis, and studied the ecological and genetic forces that may be directing mating system evolution in this species. We investigated whether the natural mating system can be explained by the availability of mates, by the differential survival of self- and cross-fertilized snails in nature, and by the effects of mating system on parental fecundity and early survival. The natural mating system of L. stagnalis was found to be predominantly cross-fertilizing. Density of snails in the populations had no relationship with the mating system, suggesting that outcrossing rates are not limited by mate availability at the population densities observed. Contrary to expectations for outcrossing species, we detected no evidence for inbreeding depression in survival in nature with inferential population genetic methods. Further, experimental manipulations of mating system in the laboratory revealed that self-fertilization had no effect on parental fecundity, and only minor effects on offspring survival. Predominance of cross-fertilization despite low apparent fitness costs of self-fertilization is at odds with the paradigm that high self-fertilization depression is necessary for maintenance of cross-fertilization in self-compatible hermaphrodites.

Estimation of ECG signal of closely separated bipolar electrodes using thorax models.

New miniaturized portable ECG measuring devices may require reduced electrode size and distance. Modeling tools can be useful in predicting the behavior of electric field between electrodes. This work introduces a project where the effect of interelectrode distance (IED) of ECG precordial electrodes was studied with a model of the thorax as a volume conductor and with body surface potential map (BSPM) data. The objective was to study how the IED affects the signal strength and how well the modeling data corresponds to the clinical data. 2D and 3D finite difference method (FDM) torso models based on visible human man data were used. On these FDM models, the electrodes9 sensitivity to measure the electric field of the heart was derived. The results were compared to clinical 120 channel BSPM data. It was found out that reducing the IED obviously decreases the signal strength. According to the clinical data, the magnitude of this effect depends on the electrode location. This study indicates that modeling the volume conductor can predict the signal strength obtained with given electrode configurations. 3D modeling is more accurate in predicting the signal strength from clinical recordings; however, also simple and fast 2D modeling results show comparable values.

Prediction of implantable ECG lead systems by using thorax models.

New implantable ECG devices may provide more stable and noiseless measurements compared to body surface ECG measurements. When the electrodes are moved to inside of the body the way the ECG measurement is done is changing. Modeling can be an effective way to study effects of implantation to the capacity of electrodes to measure ECG compared to surface measurements. This work introduces a project where effects of electrode implantation to the magnitude and direction of lead sensitivity to detect cardiac source, lead field, was studied with a model of the thorax as a volume conductor. The study was based on 3D finite difference method (FDM) featuring visible human man. The results of the study indicate that the effect of electrode implantation under the skin (5-15 mm) to the way they measure ECG is rather small. Magnitude change is dependent of the studied lead and the change of the sensitivity to heart's equivalent sources in direction of lead field is minor.