Statistical method for testing the neutral mutation hypothesis by DNA polymorphism.

Background: We aimed to investigate the influence of oligomeric forms of β-amyloid (Aβ) and the influence of the duration of exposure on the development of tau ph

/pdf/amyloid-oligomers-exacerbate-tau-pathology-in-a-mouse-model-3oe5hcjtk3.pdf

Amyloid Oligomers Exacerbate Tau Pathology in a Mouse Model of Tauopathy

Anisotropic water diffusion in neural fibres such as nerve, white matter in spinal cord, or white matter in brain forms the basis for the utilization of diffusion tensor imaging (DTI) to track fibre pathways. The fact that water diffusion is sensitive to the underlying tissue microstructure provides a unique method of assessing the orientation and integrity of these neural fibres, which may be useful in assessing a number of neurological disorders. The purpose of this review is to characterize the relationship of nuclear magnetic resonance measurements of water diffusion and its anisotropy (i.e. directional dependence) with the underlying microstructure of neural fibres. The emphasis of the review will be on model neurological systems both in vitro and in vivo. A systematic discussion of the possible sources of anisotropy and their evaluation will be presented followed by an overview of various studies of restricted diffusion and compartmentation as they relate to anisotropy. Pertinent pathological models, developmental studies and theoretical analyses provide further insight into the basis of anisotropic diffusion and its potential utility in the nervous system.

https://onlinelibrary.wiley.com/doi/pdf/10.1002/nbm.782

The basis of anisotropic water diffusion in the nervous system – a technical review

The polymerization dynamics of microtubules are central to their biological functions. Polymerization dynamics allow microtubules to adopt spatial arrangements that can change rapidly in response to cellular needs and, in some cases, to perform mechanical work. Microtubules utilize the energy of GTP hydrolysis to fuel a unique polymerization mechanism termed dynamic instability. In this review, we first describe progress toward understanding the mechanism of dynamic instability of pure tubulin and then discuss the function and regulation of microtubule dynamic instability in living cells.

/pdf/microtubule-polymerization-dynamics-27938flntt.pdf

Microtubule polymerization dynamics

Selection against deleterious alleles maintained by mutation may cause a reduction in the amount of genetic variability at linked neutral sites. This is because a new neutral variant can only remain in a large population for a long period of time if it is maintained in gametes that are free of deleterious alleles, and hence are not destined for rapid elimination from the population by selection. Approximate formulas are derived for the reduction below classical neutral values resulting from such background selection against deleterious mutations, for the mean times to fixation and loss of new mutations, nucleotide site diversity, and number of segregating sites. These formulas apply to random-mating populations with no genetic recombination, and to populations reproducing exclusively asexually or by self-fertilization. For a given selection regime and mating system, the reduction is an exponential function of the total mutation rate to deleterious mutations for the section of the genome involved. Simulations show that the effect decreases rapidly with increasing recombination frequency or rate of outcrossing. The mean time to loss of new neutral mutations and the total number of segregating neutral sites are less sensitive to background selection than the other statistics, unless the population size is of the order of a hundred thousand or more. The stationary distribution of allele frequencies at the neutral sites is correspondingly skewed in favor of rare alleles, compared with the classical neutral result. Observed reductions in molecular variation in low recombination genomic regions of sufficiently large size, for instance in the centromere-proximal regions of Drosophila autosomes or in highly selfing plant populations, may be partly due to background selection against deleterious mutations.

/pdf/the-effect-of-deleterious-mutations-on-neutral-molecular-49m5u8w40l.pdf

The effect of deleterious mutations on neutral molecular variation.

Published data on allele frequencies at isozyme loci in inbreeding and outbreeding plant species were analyzed to examine intraspecific variation in gene diversity and effective population size (Ne). Compared with outbreeders, inbreeding species showed markedly greater variation among populations in average values of Nei's gene diversity statistic. Effective population size was estimated by assuming that the variation observed at isozyme loci is selectively neutral. Inbreeding species showed greater levels of variation in Ne than did outbreeders, although the upper range of Ne was similar in the two classes of species. The results suggest that there may be considerable genetic variation and potential for evolutionary change in some but not all populations of inbreeders. Moreover, these findings are important with respect to the conservation of genetic resources. In particular, that the amount of intraspecific variation in population genetic diversity and Ne differs between inbreeding and outbreeding species should be taken into account in sampling efforts designed to optimize the diversity of germplasm collections.

Intraspecific variation in population gene diversity and effective population size correlates with the mating system in plants.

Wild crop relatives are an important source of genetic variation for improving domesticated species. Given limited resources, methods for maximizing the genetic diversity of collections of wild relatives are needed to help spread protection over a larger number of populations and species. Simulations were conducted to investigate the optimal strategy of sampling materials from populations of wild relatives, with the objective of maximizing the number of alleles (allelic richness) in collections of fixed size. Two methods, based on assessing populations for variation at marker loci (e.g., allozymes, restriction fragment length polymorphisms), were developed and compared with several methods that are not dependent on markers. Marker-assisted methods yielded higher overall allelic richness in the simulated collections, and they were particularly effective in conserving geographically localized alleles, the class of alleles that is most subject to loss.

https://www.pnas.org/content/pnas/90/22/10623.full.pdf

Conservation of allelic richness in wild crop relatives is aided by assessment of genetic markers.

Rapid Movement of Microtubules in Axons

Efforts to observe the slow axonal transport of cytoskeletal polymers during the past decade have yielded conflicting results, and this has generated considerable controversy. The movement of neurofilaments has now been seen, and it is rapid, infrequent and highly asynchronous. This motile behaviour could explain why slow axonal transport has eluded observation for so long.

Slow axonal transport: stop and go traffic in the axon.

Articles C urrent extinction rates of plant and animal species are estimated to be as much as 100-to 1000-fold higher than during the recent geological past, a phenomenon that conservation biologists attribute to wide-scale destruction of natural habitats (Pimm et al. 1995). As natural habitats continue to disappear, there have been increasing efforts to stockpile wild plant species in large, centralized seed banks—a form of conservation that falls under the general category of ex situ conservation , or conservation outside the native habitat. Seed banks are facilities where seeds are stored under cold and dry conditions. This prolongs seed viability and thereby preserves plants for future use. Traditionally, seed banks have played their largest role in the conservation of domesticated plant varieties (Plucknett 1987), though some agricultural seed banks such as those maintained by US National Plant Germplasm System have kept collections of nondomesti-cated species, particularly the wild relatives of crop plants. During the past two decades many botanical gardens began to establish seed banks for the purpose of conservation. Most noteworthy is the Millennium Seed Bank Project at the Royal Botanic Gardens, Kew, in Great Britain (Smith et al. 1998). This massive undertaking aims to stockpile 10% of the world's plant diversity, targeting species of the dry tropics, as well as all plant species native to Great Britain. Similar, though less ambitious, efforts are under way in North America, sponsored by the Center for Plant Conservation at the Missouri Botanical Garden in St. Louis, and regional initiatives (e.g., the New England Plant Conservation Program) are being carried out in many parts of the world. As well, over 700 botanical gardens maintain seed collections of mostly wild, ornamental, medicinal, and in some cases crop and crop-related species (FAO 1996). In this article we focus on seed banks as tools for the conservation of wild plant species, as opposed to plant species having direct, explicit use to humans (e.g., crops, pasture species, trees, and medicinal plants). The increasing popularity of the seed bank as a tool in the conservation of wild plant species can be attributed to a number of factors. First, seed banks provide immediate access to plant samples, allowing researchers and conservation biologists to evaluate them for properties such as new sources of medicines, nutrition, and genes. Second, the availability of plant germplasm in seed banks facilitates scientific study that could provide helpful information for conserving the remaining natural …

https://www.jstor.org/stable/10.1641/0006-3568(2001)051[0960:TCOWPS]2.0.CO;2

The Conservation of Wild Plant Species in Seed Banks

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