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排序方式: 共有195条查询结果,搜索用时 31 毫秒
121.
López-Villavicencio M Genton BJ Porcher E Shykoff JA 《American journal of botany》2005,92(12):1995-2002
In gynodioecious plant species, females are expected to have more resources available for maturing seeds because pistillate flowers are smaller, do not produce pollen, and are thus less costly that perfect flowers. The potential female advantage arising from more abundant resources is, however, likely to vary depending on whether seed production is limited by resource or pollen availability. Here we experimentally investigated the influence of pollen and resource limitation on female advantage in a gynodioecious species using two levels of pollination. Total seed production of females was always greater than that of hermaphrodites: females produced more flowers and more fruits that contained similar numbers of seeds of similar mass. Under low pollination, female and hermaphrodite plants allocated resources to increased flower production rather than to increased seed size or quality. We did not detect any influence of pollen or resource limitation on female advantage, which remained similar under low (= abundant resources) and full pollination. Outcrossed fruits performed better than selfed fruits when the same plant received both selfed and outcrossed pollen on different flowers. These differences were not greater under high pollination, possibly because resources available for each fruit did not differ between our pollen intensity treatments. 相似文献
122.
Females of gynodioecious species need to have reproductive advantages to compensate for their lack of male function and assure their maintenance in natural populations. Females may be more fecund than hermaphrodites because they reallocate resources from male to female function, avoid selfing and inbreeding depression, or produce higher quality offspring by screening arriving pollen better. A novel type of female advantage is proposed that may be important for several species of gynodioecious Caryophyllaceae. The anther smut fungus Microbotryum violaceum generally sterilizes its infected hosts but even without infection, spore deposition can negatively affect the reproduction of healthy individuals. In Gypsophila repens, flowers of hermaphrodite plants experimentally pollinated with both fungal spores and pollen produced significantly fewer fruits and seeds than those that received pollen alone, whereas female reproduction was unaffected by spore deposition. This unexplored reproductive advantage of females is probably due to the larger stigmatic surface in pistillate flowers, that allows pollen germination despite the presence of spores. Because longer stigmas may also lead to increased spore deposition, these results raise questions regarding the infection probability for each sex, possible sex-specific defense mechanisms, and the general role of pathogens in the maintenance of females in natural gynodioecious populations. 相似文献
123.
Price DJ Campbell PG Sutton AG Grech ED Davies A Hall JA De Belder MA 《International journal of cardiovascular interventions》2001,4(1):15-20
BACKGROUND: The EPISTENT trial reported improved early outcomes with routine use of abciximab after coronary stenting. Increasing use of stents means that routine abciximab adds significantly to costs of percutaneous coronary intervention (PCI). This paper reports the results of a protocol encouraging restriction of abciximab therapy to high-risk patients only. METHODS: Data were collected prospectively over a 34-month period for patients undergoing PCI with stenting. In addition to those who fulfilled criteria for inclusion in the EPISTENT trial, patients treated in the setting of acute myocardial infarction (AMI) were studied. Demographic data, procedural details and early clinical outcomes were recorded. RESULTS: Of 808 patients studied, 601 fulfilled EPISTENT inclusion criteria and comprised 367 patients (45%) treated for stable angina and 234 (30%) treated for unstable or post-infarct angina. The additional 207 patients (25%) were treated during AMI. The 808 patients received a total of 981 stents. Abciximab was given in only 88 cases (10.9%). Major adverse clinical events occurred in 39 patients (4.8%). CONCLUSION: Selective use of abciximab for patients undergoing coronary stenting can be associated with outcomes equivalent to those reported for routine use, but with significant cost savings. 相似文献
124.
Salvaudon L Héraudet V Shykoff JA 《Evolution; international journal of organic evolution》2005,59(12):2518-2524
Simultaneous effects of host and parasite in determining quantitative traits of infection have long been neglected in theoretical and experimental investigations of host-parasite coevolution with the notable exception of gene-for-gene resistance studies. A cross-infection experiment, using five lines of the plant Arabidopsis thaliana and two strains of its oomycete pathogen Hyaloperonospora parasitica, revealed that three traits traditionally considered those of the parasite (number of infected leaves, transmission success, and time until 50% transmission), differed among specific combinations of host and parasite lines, being determined by the two protagonists of the infection. However, the two parasite strains did not differ significantly for most measured phenotypic traits of the infection. Globally, transmission increased with increasing virulence among the different host-parasite combinations, as assumed by most models of evolution of virulence. Surprisingly, however, there was no general relationship between parasite and host fitness, estimated respectively as transmission and seed production. Only one of the two strains showed the expected significant negative genetic correlation between these two variables. Our results thus highlight the importance of taking into account both host and parasite genetic variation because their interaction can lead to unexpected evolutionary outcomes. 相似文献
125.
A rate-independent technique for analysis of nucleic acid sequences: evolutionary parsimony 总被引:24,自引:1,他引:23
The method of evolutionary parsimony--or operator invariants--is a
technique of nucleic acid sequence analysis related to parsimony analysis
and explicitly designed for determining evolutionary relationships among
four distantly related taxa. The method is independent of substitution
rates because it is derived from consideration of the group properties of
substitution operators rather than from an analysis of the probabilities of
substitution in branches of a tree. In both parsimony and evolutionary
parsimony, three patterns of nucleotide substitution are associated
one-to-one with the three topologically linked trees for four taxa. In
evolutionary parsimony, the three quantities are operator invariants. These
invariants are the remnants of substitutions that have occurred in the
interior branch of the tree and are analogous to the substitutions assigned
to the central branch by parsimony. The two invariants associated with the
incorrect trees must equal zero (statistically), whereas only the correct
tree can have a nonzero invariant. The chi 2-test is used to ascertain the
nonzero invariant and the statistically favored tree. Examples, obtained
using data calculated with evolutionary rates and branchings designed to
camouflage the true tree, show that the method accurately predicts the
tree, even when substitution rates differ greatly in neighboring peripheral
branches (conditions under which parsimony will consistently fail). As the
number of substitutions in peripheral branches becomes fewer, the parsimony
and the evolutionary-parsimony solutions converge. The method is robust and
easy to use.
相似文献
126.
127.
128.
We investigated the reproductive system of the threatened taxon Dombeya acutangula ssp. acutangula Cav. (Sterculiaceae), an endemic tree of the Mascarene archipelago (Indian Ocean). A controlled crossing experiment was performed
in two natural populations located in the remnants of the low-elevation dry forest on the island of La Réunion. Active pollination,
probably mainly by insects, was necessary for reproduction in this species. Individuals varied in their degree of self-sterility
from 0 to 100%. Outcrossing between nearby individuals produced lower seed set than did crosses between more distant individuals
within one of the two tested populations. The variation in reproductive success on selfing and in the different types of crosses
could result from inbreeding depression causing embryo death, and we provide evidence that progenies from selfing have lower
seed size and quality. However, for inbreeding depression to account for the dramatic variation in seed set found in our crossing
experiment, the distribution of genetic load and number of lethal factors required appear unrealistic. We favour an alternative
interpretation, that D. acutangula possesses an incompatibility system similar to that found in other Sterculiaceae species such as Theobroma cacao L. Such an incompatibility system allows a certain amount of selfing, and different individuals vary in their degree of self-incompatibility.
The low success of crosses among close neighbours in one population suggests that there was spatial structure for incompatibility
alleles in that population. This could partly explain the decline of the species in fragmented and disturbed habitats, since
relatedness at incompatibility loci may increase in small or isolated population and thus reduce mate availability.
Received: 2 March 1998 / Accepted: 3 August 1998 相似文献
129.
Carbamoylphosphate synthetase (CPS) catalyzes the first committed step in
pyrimidine biosynthesis, arginine biosynthesis, or the urea cycle.
Organisms may contain either one generalized or two specific CPS enzymes,
and these enzymes may be heterodimeric (encoded by linked or unlinked
genes), monomeric, or part of a multifunctional protein. In order to help
elucidate the evolution of CPS, we have performed a comprehensive
phylogenetic analysis using the 21 available complete CPS sequences,
including a sequence from Sulfolobus solfataricus P2 which we report in
this paper. This is the first report of a complete CPS gene sequence from
an archaeon, and sequence analysis suggests that it encodes an enzyme
similar to heterodimeric CPSII. We confirm that internal similarity within
the synthetase domain of CPS is the result of an ancient gene duplication
that preceded the divergence of the Bacteria, Archaea, and Eukarya, and use
this internal duplication in phylogenetic tree construction to root the
tree of life. Our analysis indicates with high confidence that this
archaeal sequence is more closely related to those of Eukarya than to those
of Bacteria. In addition to this ancient duplication which created the
synthetase domain, our phylogenetic analysis reveals a complex history of
further gene duplications, fusions, and other events which have played an
integral part in the evolution of CPS.
相似文献
130.
Formation of enlarged mitochondria in a liver cell line in response to a synthetic glucocorticoid
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JA Berliner 《The Journal of cell biology》1975,64(3):711-716
For a number of years it has been recognized that glucocorticoids cause alterations in liver cell morphology (6, 9). Several investigators have shown that in liver in vivo mitochondria can be enlarged to many times their normal volume by treatment with cortisone (13, 15). There is a concomitant decrease in mitochondrial number, and the results of Kimberg and Loeb suggest that this is due to mitochondrial fusion (7). However, the exact mechanism whereby mitochondrial volume is altered and whether in fact cortisone is the direct causal agent are not known due to the complexity of studying these questions in a whole animal system. We have found that dexamethasone sodium phosphate (dex), a synthetic glucocorticoid, causes the formation of enlarged mitochondria in a liver cell line RLC-GAI, which grows in defined medium. In this paper we present our observations on the amount of enlargement that occurs after 5 days of treatment. The formation of enlarged mitochondria is reversible upon removal of the hormone from the medium, and we have attempted to determine whether "mitochondrial" or "nonmitochondrial" inhibitors are more effective in blocking the return of mitochondria to their normal size when the hormone is removed. 相似文献