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21.
Ethiopian mitochondrial DNA heritage: tracking gene flow across and around the gate of tears 总被引:5,自引:1,他引:5 下载免费PDF全文
Kivisild T Reidla M Metspalu E Rosa A Brehm A Pennarun E Parik J Geberhiwot T Usanga E Villems R 《American journal of human genetics》2004,75(5):752-770
Approximately 10 miles separate the Horn of Africa from the Arabian Peninsula at Bab-el-Mandeb (the Gate of Tears). Both historic and archaeological evidence indicate tight cultural connections, over millennia, between these two regions. High-resolution phylogenetic analysis of 270 Ethiopian and 115 Yemeni mitochondrial DNAs was performed in a worldwide context, to explore gene flow across the Red and Arabian Seas. Nine distinct subclades, including three newly defined ones, were found to characterize entirely the variation of Ethiopian and Yemeni L3 lineages. Both Ethiopians and Yemenis contain an almost-equal proportion of Eurasian-specific M and N and African-specific lineages and therefore cluster together in a multidimensional scaling plot between Near Eastern and sub-Saharan African populations. Phylogeographic identification of potential founder haplotypes revealed that approximately one-half of haplogroup L0–L5 lineages in Yemenis have close or matching counterparts in southeastern Africans, compared with a minor share in Ethiopians. Newly defined clade L6, the most frequent haplogroup in Yemenis, showed no close matches among 3,000 African samples. These results highlight the complexity of Ethiopian and Yemeni genetic heritage and are consistent with the introduction of maternal lineages into the South Arabian gene pool from different source populations of East Africa. A high proportion of Ethiopian lineages, significantly more abundant in the northeast of that country, trace their western Eurasian origin in haplogroup N through assorted gene flow at different times and involving different source populations. 相似文献
22.
Van Geest GJ Hessen DO Spierenburg P Dahl-Hansen GA Christensen G Faerovig PJ Brehm M Loonen MJ Van Donk E 《Oecologia》2007,153(3):653-662
A dramatic increase in the breeding population of geese has occurred over the past few decades at Svalbard. This may strongly
impact the fragile ecosystems of the Arctic tundra because many of the ultra-oligotrophic freshwater systems experience enrichment
from goose feces. We surveyed 21 shallow tundra ponds along a gradient of nutrient enrichment based on exposure to geese.
Concentrations of total phosphorus (P) and dissolved inorganic nitrogen (DIN) in the tundra ponds ranged from 2–76 to 2–23 μg l−1 respectively, yet there was no significant increase in phytoplankton biomass (measured as chlorophyll a; range: 0.6–7.3 μg l−1) along the nutrient gradient. This lack of response may be the result of the trophic structure of these ecosystems, which
consists of only a two-trophic level food chain with high biomasses of the efficient zooplankton grazer Daphnia in the absence of fish and scarcity of invertebrate predators. Our results indicate that this may cause a highly efficient
grazing control of phytoplankton in all ponds, supported by the fact that large fractions of the nutrient pools were bound
in zooplankton biomass. The median percentage of Daphnia–N and Daphnia–P content to particulate (sestonic) N and P was 338 and 3009%, respectively, which is extremely high compared to temperate
lakes. Our data suggest that Daphnia in shallow arctic ponds is heavily subsidized by major inputs of energy from other food sources (bacteria, benthic biofilm),
which may be crucial to the persistence of strong top–down control of pelagic algae by Daphnia. 相似文献
23.
Biomanipulation measures in lakes, taken to diminish algal blooms, have mainly been restricted to the reduction of zooplanktivorous
fish with the aim to stimulate the grazing pressure by native filter feeders such as Daphnia. However, larger filter feeders like the exotic zebra mussel, Dreissena polymorpha, have been suggested as an optional tool because of their high filtering capacity. We compared grazing by two filter feeders,
D. polymorpha and Daphnia galeata, offered seston from Lake IJsselmeer, the Netherlands in two consecutive years: 2002 and 2003. The seston in both years was
dominated by the colony-forming cyanobacterium Microcystis aeruginosa. The grazing studies were performed under controlled conditions in the laboratory and samples were analyzed on a flow cytometer,
making it possible to quantify grazing on different seston components and size fractions, including cyanobacteria, other phytoplankton
(green algae, diatoms, etc.), and detritus. No differences in clearance rates, on a per weight basis, were found between the
two grazer species. The clearance rate on cyanobacteria (especially <20 μm) was lower in 2003 than in 2002. In 2003, the microcystin
concentration of cyanobacteria was higher than in 2002, suggesting that the observed lower clearance rate in 2003 was due
to the enhanced toxin content of the cyanobacteria. Zebra mussels, although indiscriminately filtering all seston groups out
of the water, positively selected for phytoplankton in their mantle cavity, irrespective of its toxicity, and rejected detritus.
Since no differences in clearance rates were found between the two grazer species, we conclude that for biomanipulation purposes
of shallow lakes, native species like the daphnids should be preferred over exotic species like zebra mussels. When the seston
is dominated by phytoplankton that cannot be filtered out of the water column by Daphnia, however, the use of zebra mussels may be considered. Care should be taken, however, in the choice of the lakes since the
mussels may have severe ecological and economic impacts. 相似文献
24.
25.
Bouazoune K Mitterweger A Längst G Imhof A Akhtar A Becker PB Brehm A 《The EMBO journal》2002,21(10):2430-2440
Drosophila Mi-2 (dMi-2) is the ATPase subunit of a complex combining ATP-dependent nucleosome remodelling and histone deacetylase activities. dMi-2 contains an HMG box-like region, two PHD fingers, two chromodomains and a SNF2-type ATPase domain. It is not known which of these domains contribute to nucleosome remodelling. We have tested a panel of dMi-2 deletion mutants in ATPase, nucleosome mobilization and nucleosome binding assays. Deletion of the chromodomains impairs all three activities. A dMi-2 mutant lacking the chromodomains is incorporated into a functional histone deacetylase complex in vivo but has lost nucleosome-stimulated ATPase activity. In contrast to dHP1, dMi-2 does not bind methylated histone H3 tails and does not require histone tails for nucleosome binding. Instead, the dMi-2 chromodomains display DNA binding activity that is not shared by other chromodomains. Our results suggest that the chromodomains act at an early step of the remodelling process to bind the nucleosome substrate predominantly via protein-DNA interactions. Furthermore, we identify DNA binding as a novel chromodomain-associated activity. 相似文献
26.
By examining adoptively transferred CSFE-labeled lymphocytic choriomeningitis virus (LCMV)-immune donor T cells in Thy-1 congenic hosts inoculated with viruses or with the cytokine inducer poly(I:C), strikingly different responses of bona fide memory T cells were found in response to different stimuli. Poly(I:C) (cytokine) stimulation caused a limited synchronized division of memory CD8 T cells specific to each of five LCMV epitopes, with no increase and sometimes a loss in number, and no change in their epitope hierarchy. Homologous LCMV infection caused more than seven divisions of T cells specific for each epitope, with dramatic increases in number and minor changes in hierarchy. Infections with the heterologous viruses Pichinde and vaccinia (VV) caused more than seven divisions and increases in number of T cells specific to some putatively cross-reactive but not other epitopes and resulted in substantial changes in the hierarchy of the LCMV-specific T cells. Hence, there can be memory T cell division without proliferation (i.e., increase in cell number) in the absence of Ag and division with proliferation in the presence of Ag from homologous or heterologous viruses. Heterologous protective immunity between viruses is not necessarily reciprocal, given that LCMV protects against VV but VV does not protect against LCMV. VV elicited proliferation of LCMV-induced CD8 and CD4 T cells, whereas LCMV did not elicit proliferation of VV-induced T cells. Thus, depending on the pathogen and the sequence of infection, a heterologous agent may selectively stimulate the memory pool in patterns consistent with heterologous immunity. 相似文献
27.
28.
Cizeron G; Lemeunier F; Loevenbruck C; Brehm A; Biemont C 《Molecular biology and evolution》1998,15(12):1589-1599
Copy numbers of sequences homologous to the Drosophila melanogaster
retrotransposable element 412, their distribution between the chromosome
arms and the chromocenter, and whether they contain full- size copies were
analyzed for 55 species of the Drosophila genus. Element 412 insertion
sites were detected on the chromosome arms of D. melanogaster, Drosophila
simulans, and a few species of the obscura group, but the chromocenter was
labeled in almost all species. The presence of element 412 sequences in the
majority of species shows that this element has a long evolutionary history
in Drosophilidae, although it may have recently invaded the chromosomes in
some species, such as D. simulans. Differences in copy number between
species may be due to population size or specific endogenous or
environmental factors and may follow the worldwide invasion of the species.
Putative full-length copies were detected in the chromocenters of some
species with no copies on the chromosome arms, suggesting that the
chromocenter may be a shelter for such copies and not only for deleted
ones.
相似文献
29.
Improved tests for heterogeneity across a region of DNA sequence in the ratio of polymorphism to divergence 总被引:15,自引:9,他引:6
The neutral theory of molecular evolution predicts that the ratio of
polymorphisms to fixed differences should be fairly uniform across a region
of DNA sequence. Significant heterogeneity in this ratio can indicate the
effects of balancing selection, selective sweeps, mildly deleterious
mutations, or background selection. Comparing an observed heterogeneity
statistic with simulations of the heterogeneity resulting from random
phylogenetic and sampling variation provides a test of the statistical
significance of the observed pattern. When simulated data sets containing
heterogeneity in the polymorphism-to-divergence ratio are examined,
different statistics are most powerful for detecting different patterns of
heterogeneity. The number of runs is most powerful for detecting patterns
containing several peaks of polymorphism; the Kolmogorov-Smirnov statistic
is most powerful for detecting patterns in which one end of the gene has
high polymorphism and the other end has low polymorphism; and a newly
developed statistic, the mean sliding G statistic, is most powerful for
detecting patterns containing one or two peaks of polymorphism with reduced
polymorphism on either side. Nine out of 27 genes from the Drosophila
melanogaster subgroup exhibit heterogeneity that is significant under at
least one of these three tests, with five of the nine remaining significant
after a correction for multiple comparisons, suggesting that detectable
evidence for the effects of some kind of selection is fairly common.
相似文献
30.