[1] A model of field-aligned currents is derived from highprecision magnetic field measurements from the Ørsted and Magsat satellites, being parameterized by the interplanetary magnetic field strength and direction for summer, winter and equinox. The high-precision data allow the model to be determined directly by a simple 2-D curl technique combined with fitting of spherical harmonic functions. New elements of the model are: (a) the FAC patterns are determined separately for both polar regions, resolving the seasonal dependence of interhemispheric asymmetries, (b) the IMF $ 0, ground-state patterns are also resolved; these elements are obtained for the first time. From the model, the total upward/downward currents have been determined for various IMF conditions. The ratio of the summer/winter currents is $1.35 and the equinox currents $1. The model allows FAC mapping for IMF |B| 12 nT, except during magnetic storms and substorms.
The theory of evolution at a selectively neutral locus that controls the recombination between two major loci that are under selection is studied. If the major loci are at a stable equilibrium in linkage disequilibrium under selection and recombination, then a mutation at the modifier locus will increase in frequency when rare if and only if it decreases the recombinationlfraction. If the major loci are in disequilibrium at a balance between selection against deleterious alleles and mutation towards them, then two new phenomena are observed. First, a recombination increasing mutation will succeed if the disequilibrium is negative and the modifier is sufficiently tightly linked to the major oci. Second, depending on the strength of selection, even if the disequilibrium is negative, recombination reduction may occur for looser linkage between the major and modifier loci.Most mathematical models for the evolution of recombination (often used interchangeably with sex in this context) in effectively infinite populations fall into one of two major classes. In the first, individuals are haploid and there are two loci with alleles A and a at the first and B and b at the second. There is recurrent mutation from A to a at rate IA and B to b at rate lB. Each of a and b are fitter than their alleles, and our interest is in the evolutionary rate of incorporation of the favorable double mutant ab into the population. Maynard Smith (1) showed that if the two loci are initially in linkage equilibrium and if the fitness of ab is the product of the fitnesses of Ab and aB, then recombination has no effect on the rate of change of the genotype frequencies. Eshel and Feldman (2) proved that if the fitness of ab is greater than the product of those of the advantageous single mutants Ab and aB and if the initial linkage disequilibrium is not negative, then from the first generation on there will always be more of the favored double mutants in the absence of recombination than in its presence. In this sense, recombination is disadvantageous. When the initial linkage disequilibrium is negative and mutation rates are sufficiently small, Karlin (3) showed that, for recombination rates bounded above by a specified function of the fitnesses, there can be more of the advantageous double mutants in the presence of recombination than in its absence and that this depends on the ordering of the fitnesses. In this class of deterministic models the criterion by which the success of recombination is evaluated is at the level of the population. That is, recombination succeeds if it produces a population with more of the favored double mutants.In a second approach to the evolution of recombination initiated by Nei (4), a selectively neutral recombination-modifying gene, with alleles M and m, controls the recombination between two major loci A/a and B/b that are under selection. Recom-bination evolves as the alleles at the modifying locus change in frequency. Nei (4, 5) suggested that, in general, recombination would tend to be reduced as l...
Optimality arguments and modifier theory are reviewed as paradigms for the study of the evolution of recombination. Optimality criteria (such as maximization of mean fitness) may agree with results from models developed in terms of the evolution of recombination at modifier loci. Modifier models demonstrate, however, that equilibrium mean fitness can decrease during the evolution of recombination rates and is not always maximized. Therefore, optimality arguments do not successfully predict the conditions under which increased or decreased recombination will evolve. The results from modifier models indicate that decreased recombination rates are usually favored when the population is initially near a polymorphic equilibrium with linkage disequilibrium. When the population is subject to directional selection or to deleterious mutations, increased recombination may be favored under certain conditions, provided that there is negative epistasis among alleles.
In a genome alignment of five individuals of the ascomycete fungus Zymoseptoria pseudotritici, a close relative of the wheat pathogen Z. tritici (synonym Mycosphaerella graminicola), we observed peculiar diversity patterns. Long regions up to 100 kb without variation alternate with similarly long regions of high variability. The variable segments in the genome alignment are organized into two main haplotype groups that have diverged ∼3% from each other. The genome patterns in Z. pseudotritici are consistent with a hybrid speciation event resulting from a cross between two divergent haploid individuals. The resulting hybrids formed the new species without backcrossing to the parents. We observe no variation in 54% of the genome in the five individuals and estimate a complete loss of variation for at least 30% of the genome in the entire species. A strong population bottleneck following the hybridization event caused this loss of variation. Variable segments in the Z. pseudotritici genome exhibit the two haplotypes contributed by the parental individuals. From our previously estimated recombination map of Z. tritici and the size distribution of variable chromosome blocks untouched by recombination we estimate that the hybridization occurred ∼380 sexual generations ago. We show that the amount of lost variation is explained by genetic drift during the bottleneck and by natural selection, as evidenced by the correlation of presence/ absence of variation with gene density and recombination rate. The successful spread of this unique reproductively isolated pathogen highlights the strong potential of hybridization in the emergence of pathogen species with sexual reproduction. species dynamics | incipient species | population genomics | genome mosaic | genome scans
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