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61.
62.
Gorelick R 《American journal of botany》2005,92(6):979-984
Dioecy and sex chromosomes almost certainly evolved from ancestral hermaphrodites that only possessed autosomes. There is a growing body of evidence that genes for female or male function were then epigenetically suppressed in some of these hermaphrodites, creating the first males or females and nascent sex chromosomes. The incipient sex-determining epigenetic signals, such as cytosine methylation, then drove Muller's ratchet in many animals, resulting in shorter Y chromosomes. Based on this theory of sex chromosome evolution and limited data on gametophyte gene expression, I argue that plants should be largely immune from Muller's ratchet and therefore retain their ancestral state of equal length sex chromosomes, unless they incur chromosomal rearrangements or large-scale insertions of duplicated genomes. Usually heteromorphic sex chromosomes canalize dioecy, but extensive polyploidy or polysomy can provide an escape from this canalized dioecy. This theory implies that dioecy due to heteromorphic sex chromosomes should be evolutionarily ephemeral in bryophytes and homosporous pteridophytes because of their extraordinarily high incidences of polyploidy. And, if anything, these very high incidences of polyploidy are responsible for translocation or gradual addition of beneficial genes, rather than gradual reduction in the length of a sex chromosome. 相似文献
63.
The molecular mechanism of the powerstroke in muscle is examined by resonance energy transfer techniques. Recent models suggesting a pre-cocking of the myosin head involving an enormous rotation between the lever arm and the catalytic domain were tested by measuring separation distances among myosin subfragment-2, the nucleotide site, and the regulatory light chain in the presence of nucleotide transition state analogs. Only small changes (<0.5 nm) were detected that are consistent with internal conformational changes of the myosin molecule, but not with extreme differences in the average lever arm position suggested by some atomic models. These results were confirmed by stopped-flow resonance energy transfer measurements during single ATP turnovers on myosin. To examine the participation of actin in the powerstroke process, resonance energy transfer between the regulatory light chain on myosin subfragment-1 and the C-terminus of actin was measured in the presence of nucleotide transition state analogs. The efficiency of energy transfer was much greater in the presence of ADP-AlF(4), ADP-BeF(x), and ADP-vanadate than in the presence of ADP or no nucleotide. These data detect profound differences in the conformations of the weakly and strongly attached cross-bridges that appear to result from a conformational selection that occurs during the weak binding of the myosin head to actin. 相似文献
64.
Calisher CH Mills JN Sweeney WP Root JJ Reeder SA Jentes ES Wagoner K Beaty BJ 《Journal of wildlife diseases》2005,41(1):12-28
We followed seasonal and year-to-year population dynamics for a diverse rodent assemblage in a short-grass prairie ecosystem in southeastern Colorado (USA) for 6 yr. We captured 2,798 individual rodents (range, one to 812 individuals per species) belonging to 19 species. The two most common species, deer mice (Peromyscus maniculatus) and western harvest mice (Reithrodontomys megalotis), generally had population peaks in winter and nadirs in summer; several other murid species demonstrated autumn peaks and spring nadirs; heteromyids were infrequently captured in winter, and populations generally peaked in summer or autumn. Inter-annual trends indicated an interactive effect between temperature and precipitation. Conditions associated with low rodent populations or population declines were high precipitation during cold periods (autumn and winter) and low precipitation during warm periods (spring and summer). Severity of adverse effects varied by species. Heteromyids, for example, were apparently not negatively affected by the hot, dry spring and summer of 2000. Cross-correlations for the temporal series of relative population abundances between species pairs (which are affected by both seasonal and interannual population dynamics) revealed positive associations among most murids and among most heteromyids, but there were negative associations between murids and heteromyids. These results have important implications for those attempting to model population dynamics of rodent populations for purposes of predicting disease risk. 相似文献
65.
66.
Leslie L Root Gary D. Shipley 《In vitro cellular & developmental biology. Animal》1991,27(10):815-822
Summary We investigated the regulation of expression of bFGF and aFGF in cultures of normal human dermal fibroblasts grown in a defined,
serum-free medium which did not contain FGF. Under these conditions we detected three molecular weight forms of bFGF protein
[18.0, 23.0, and 26.6 kiloDaltons (kD)] and three molecular weight forms of aFGF protein (18.4, 19.2, and 28.6 kD) in these
cells using western blot analysis. The addition of fetal bovine serum (FBS) to these cultures caused an accumulation of all
three molecular weight forms of bFGF protein with a more dramatic accumulation of the 23.0 and 26.6 kD forms. In contrast,
the addition of FBS to the cultures had no effect on the level of aFGF proteins. Analysis of mRNA isolated from cells grown
in serum-free medium revealed multiple species of both bFGF and aFGF RNA with molecular weights that correlated with our previous
observations. The abundance of all bFGF mRNA species increased dramatically after serum treatment while the abundance of aFGF
mRNA species increased only slightly. Our observations demonstrate that factor(s) present in FBS elevate the levels of bFGF
mRNA and protein beyond the levels already present in the cultures growing in serum-free medium. Moreover, both bFGF and aFGF
protein are present in these cells as multiple molecular weight species. Some of these forms are higher in apparent molecular
weight than would be predicted from ATG-initiated primary translation products of these genes. We also show that the cells
used for this study proliferate in response to bFGF and aFGF, thus, it is possible that the growth of these cells could be
subject to autocrine/paracrine control in certain conditions. 相似文献
67.
Bactericidal and opsonic properties of C4-deficient guinea pig serum 总被引:17,自引:0,他引:17
68.
69.
For over a century, the paradigm has been that sex invariably increases genetic variation, despite many renowned biologists asserting that sex decreases most genetic variation. Sex is usually perceived as the source of additive genetic variance that drives eukaryotic evolution vis-à-vis adaptation and Fisher's fundamental theorem. However, evidence for sex decreasing genetic variation appears in ecology, paleontology, population genetics, and cancer biology. The common thread among many of these disciplines is that sex acts like a coarse filter, weeding out major changes, such as chromosomal rearrangements (that are almost always deleterious), but letting minor variation, such as changes at the nucleotide or gene level (that are often neutral), flow through the sexual sieve. Sex acts as a constraint on genomic and epigenetic variation, thereby limiting adaptive evolution. The diverse reasons for sex reducing genetic variation (especially at the genome level) and slowing down evolution may provide a sufficient benefit to offset the famed costs of sex. 相似文献
70.
James B. Gerken Colin W. Anson Yuliya Preger Peter G. Symons J. David Genders Yang Qiu Wenzhen Li Thatcher W. Root Shannon S. Stahl 《Liver Transplantation》2020,10(20)
Quinones are appealing targets as organic charge carriers for aqueous redox flow batteries (RFBs), but their utility continues to be constrained by limited stability under operating conditions. The present study evaluates the stability of a series of water‐soluble quinones, with redox potentials ranging from 605–885 mV versus NHE, under acidic aqueous conditions (1 m H2SO4). Four of the quinones are examined as cathodic electrolytes in an aqueous RFB, paired with anthraquinone‐2,7‐disulfonate as the anodic electrolyte. The RFB data complement other solution stability tests and show that the most stable electrolyte is a tetrasubstituted quinone containing four sulfonated thioether substituents. The results highlight the importance of substituting all C–H positions of the quinone in order to maximize the quinone stability and set the stage for design of improved organic electrolytes for aqueous RFBs. 相似文献