Quantifying stochastic introgression processes with hazard rates

Introgression is the permanent incorporation of genes from one population into another through hybridization and backcrossing. It can have large environmental consequences, such as the spread of insecticide or herbicide resistant genes, the escape of transgenes from genetically modified crops, and the invasion of exotic genes into new habitats. Introgression usually involves strong random components, such as rare hybridization and backcrossing events, and demographic variation in reproduction and survival. Most introgression studies ignore these random effects, and consequently fail to accurately assess the risk of introgression. This paper presents a methodology for quantifying stochastic introgression processes, based on multitype branching process models. We derive a quantity called the hazard rate, which can be used to investigate how the risk of introgression depends on crop management and life history.     

Establishment versus population growth in spatio-temporally varying environments

We consider situations where repeated invasion attempts occur from a source population into a receptor population over extended periods of time. The receptor population contains two locations that provide different expected offspring numbers to invaders. There is demographic stochasticity in offspring numbers. In addition, temporal variation causes local invader fitnesses to vary. We show that effects of environmental autocorrelation on establishment success depend on spatial covariance of the receptor subpopulations. In situations with a low spatial covariance this effect is positive, whereas high spatial covariance and/or high migration probabilities between the subpopulations causes the effect to be negative. This result reconciles seemingly contradictory results from the literature concerning effects of temporal variation on population dynamics with demographic stochasticity. We study an example in the context of genetic introgression, where invasions of cultivar plant genes occur through pollen flow from a source population into wild-type receptor populations, but our results have implications in a wider range of contexts, such as the spread of exotic species, metapopulation dynamics and epidemics.     

Seasonal dynamics and standing crops of biomass and nutrients in a subarctic tundra vegetation

Standing crops of biomass and nutrients were measured in Eriophorum vaginatum tussock tundra and on a north-facing slope, called the camp site, with similar species composition during the summer of 1976 at Eagle Creek, Alaska. These data were then compared to similar data collected at Meade River, Alaska in 1975. Four species are compared: Ledum palustre, Salix pulchra, Betula nana , and Eriophorum vaginatum . The density of aboveground individuals was greater at the tussock site than at the camp site. The total late season above- and belowground standing crop of organic matter and of biomass was greater at the camp site. The nitrogen and calcium contents of new leaves usually increased during the season while phosphorus and potassium contents decreased. Most of the nutrients were in the mosses and lichen compartments rather than in vascular plants.     

Specific attachment of collagen to cardiac myocytes: In vivo and in vitro

The formation and attachment of collagen to the sarcolemma of cardiac myocytes were examined in vivo in neonatal rats and hamsters and in vitro in cultures of neonatal rat myocytes. Scanning, transmission, and high-voltage electron microscopy were used to show that the collagen struts attach to specific sites on the sarcolemma just lateral to the Z band of neonatal animals. In vitro, collagen preferentially attaches to distal end of myocytes at a site where internal stress fibers also attach to the sarcolemma. Formation of the collagen struts appeared to be a multistep process involving several components of the extracellular matrix. The role of the collagen struts is involved in the distribution of force generated by muscle contraction.