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At least 1,135 records · Page 63Linked to original sources
PubMed1999

Is redshift-dependent evolution of galaxies a theoretical artifact?

The physical validity of the hypothesis of (redshift-dependent) luminosity evolution in galaxies is tested by statistical analysis of an intensively studied complete high-redshift sample of normal galaxies. The necessity of the evolution hypothesis in the frame of big-bang cosmology is confirmed at a high level of statistical significance; however, this evolution is quantitatively just as predicted by chronometric cosmology, in which there is no such evolution. Since there is no direct observational means to establish the evolution postulated in big-bang studies of higher-redshift galaxies, and the chronometric predictions involve no adjustable parameters (in contrast to the two in big-bang cosmology), the hypothesized evolution appears from the standpoint of conservative scientific methodology as a possible theoretical artifact.

Astronomical Phenomena↗
PubMed1999

Effect of high pressure homogenization on microbial and chemico-physical characteristics of goat cheeses.

The objective of this work was to compare goat cheeses obtained from milk previously subjected to high pressure homogenization (1000 bar) with those produced from untreated milk and milk subjected to sanitization (61 degrees C; 20 min) or to pasteurization (72 degrees C; 15 s). The pressure homogenization treatment had both direct and indirect effects on cheese characteristics and their evolution during ripening. The direct effects were principally linked to the change in water-binding capacity of proteins as shown also by the lower whey separation. The indirect effects involved the microbial growth or activity and, particularly, modifications of the population of the lactic acid bacteria that occurred naturally and their evolution as well as a more precocious yeast and mold growth with a consequent rapid rise in pH. Although the treatment proved to enhance both proteolytic and lipolytic activities according to Fourier transform infrared analysis, which was used to obtain a rapid description of the biochemical modification, the cheeses homogenized under high pressure showed relevant qualitative differences only in the zone corresponding to amide I and amide II signals of proteins. The activation of these enzymatic activities observed in the homogenized cheeses could be either an indirect effect of the shift of the microbial population or a consequence of a different exposure of the macromolecules to the enzymatic activity. Scanning electron microscopy analyses of goat cheeses revealed that cheeses homogenized under high pressure had a more homogeneous microstructure than did the others.

Animals↗
PubMed2005

Testing hypotheses about ecological specialization using phylogenetic trees.

It is often assumed that ecological specialization represents an evolutionary "dead-end" that limits further evolution. Maximum-likelihood (ML) analyses on phylogenies for 15 groups of phytophagous insects revealed that high transition rates both to and from specialization occurred, but that the mean ratio of rates was significantly biased toward a higher rate to specialization. Here we explore the consequences of the fact that transition rates inferred by ML are affected not only by the distribution, but also by the frequency, of character states. Higher rates to the more common state were inferred in the analyses of Nosil (2002), in similar studies published since 2002, and in a small set of simulations. Thus, the ratio of the rate toward versus away from specialization was strongly, positively correlated with the proportion of specialist species at the tips of the phylogeny and whether transitions away from specialization occur remains unclear. Here we reexamine these data using methods that do not rely on directly comparing transition rates. Maximum-likelihood analyses show that models with no transitions in one direction (e.g., irreversible evolution as predicted by the "specialist as dead end" framework) are usually strongly rejected, independent of the proportion of specialists at the tips. Ancestral state reconstruction revealed two instances where generalists were unambiguously derived from specialists. Transition rates would need to biased 100-fold and 5000-fold toward specialization to reconstruct a history where these shifts from specialization toward generalization do not occur. The general conclusions of Nosil (2002) appear to hold; transitions in either direction likely occur such that specialization does not always limit further evolution. Most generally, inferences regarding character evolution can be strengthened by comparing models of character change and examining ancestor states, rather than only comparing parameter values.

Adaptation, Biological↗
PubMed1988

Posthatching changes in levels and molecular forms of acetylcholinesterase in slow and fast muscles of the chicken: effects of denervation and direct electrical stimulation.

The evolution of acetylcholinesterase (AChE) activity and AChE molecular form distribution were studied in slow-tonic anterior latissimus dorsi (ALD) and in fast-twitch posterior latissimus dorsi (PLD) muscles of chickens 2-18 days of age. In ALD as well as in PLD muscles, the AChE-specific activity increased transiently from day 2 to day 4; the activity then decreased more rapidly in PLD muscle. During this period asymmetric AChE forms decreased dramatically in ALD muscle and the globular forms increased. In PLD muscle, the most striking change was the decline in A8 form between days 2 and 18 of development. Denervation performed at day 2 delayed the normal decrease in AChE-specific activity in PLD muscle, whereas little change was observed in ALD muscle. Moreover, A forms in these two muscles were virtually absent 8 days after denervation. Direct electrical stimulation depressed the rise in AChE-specific activity in denervated PLD muscle and prevented the loss of the A forms. Furthermore, the different molecular forms varied according to the stimulus pattern. In ALD muscle, electrical stimulation failed to prevent the effect of denervation. This study emphasizes the differential response of denervated slow and fast muscles to electrical stimulation and stresses the importance of the frequency of stimulation in the regulation of AChE molecular forms in PLD muscle during development.

Acetylcholinesterase↗
PubMed1986

How does a tadpole know when to metamorphose? A theory linking environmental and hormonal cues.

Tadpoles are unusual among free-living amphibians in having an atonic, non-acid secreting, underdeveloped stomach. Morphologically the typical tadpole foregut is most similar to the flaccid, non-acid secreting stomach of adult female of the gastric-brooding frog, Rheobatrachus, during brooding. In Rheobatrachus the brooding condition is induced by prostaglandin E2 secreted from the mouths of brooded larvae. I propose that typical, free-living tadpoles also excrete prostaglandins of the E family in their oral mucus and that these compounds are naturally swallowed with food particles by the tadpoles. According to this hypothesis, when food is abundant larvae swallow a large amount of mucus and, consequently, a lot of hormone, which retards differentiation of the adult, acid secreting, peristaltic stomach. However, when food is less abundant less food and mucus is swallowed. In this situation less prostaglandin passes down the alimentary tract and the gut proceeds to differentiate. If this theory is correct it provides a direct link between an environmental factor--the availability of food--and an endocrinological factor affecting metamorphosis. The theory is consistent with our current understanding of the endocrinology of metamorphosis, as well as the evolution of direct-development in anurans.

Animals↗
PubMed2002

Oxygen exchange in relation to carbon assimilation in water-stressed leaves during photosynthesis.

In a study on metabolic consumption of photosynthetic electrons and dissipation of excess light energy under water stress, O2 and CO2 gas exchange was measured by mass spectrometry in tomato plants using 18O2 and 13CO2. Under water stress, gross O2 evolution (E(O)), gross O2 uptake (U(O)), net CO2 uptake (PN), gross CO2 uptake (TPS), and gross CO2 evolution (Ec) declined. The ratio P(N)/E(O) fell during stress, while the ratios U(O)/E(O) and E(C)/TPS rose. Mitochondrial respiration in the light, which can be measured directly by 12CO2 evolution during 13CO2 uptake at 3000 microl l(-1) 13CO2, is small in relation to gross CO2 evolution and CO2 release from the glycolate pathway. It is concluded that PSII, the Calvin cycle and mitochondrial respiration are down-regulated under water stress. The percentages of photosynthetic electrons dissipated by CO2 assimilation, photorespiration and the Mehler reaction were calculated: in control leaves more than 50% of the electrons were consumed in CO2 assimilation, 23% in photorespiration and 13% in the Mehler reaction. Under severe stress the percentages of electrons dissipated by CO2 assimilation and the Mehler reaction declined while the percentage of electrons used in photorespiration doubled. The consumption of electrons in photorespiration may reduce the likelihood of damage during water deficit.

Carbon↗
PubMed1994

Geographic distribution and evolution of yellow fever viruses based on direct sequencing of genomic cDNA fragments.

We have compared the nucleotide sequence of an envelope protein gene fragment encoding amino acids 291 to 406 of 22 yellow fever (YF) virus strains of diverse geographic and host origins isolated over a 63 year time span. The nucleotide fragment of viral RNA was examined by direct sequencing of a PCR product derived from complementary DNA. Alignment with the prototype Asibi strain sequence showed divergence of 0 to 21.5% corresponding to a maximum of 5.2% divergence in the amino acid sequence. Taking 10% nucleotide divergence as a cut-off point, the 22 YF virus strains fell into three topotypes which corresponded to different geographical areas, namely West Africa, Central-East Africa, and South America. Two subgroups were defined in West Africa, a genotypic group circulating in the sylvatic zone of the western part of Africa, from western Ivory Coast-Mali to Senegal, and a group responsible for large outbreaks from eastern Ivory Coast-Burkina Faso to Cameroon. Strains from Central-East Africa showed a low ratio of transition:transversion of about 1 instead of 8 to 10 for other strains, when their nucleotide sequences were compared with those of other African strains. This may reflect a more distant relationship between the former strains and the others. No change was observed in the highly conserved amino acid domain encompassing the TGD sequence, an important determinant of flavivirus tropism and pathogenesis. Our results support earlier observations on the genetic relationships between YF isolates established by T1 oligonucleotide fingerprinting and offer a useful tool for the understanding of YF virus distribution and evolution.

Amino Acid Sequence↗
PubMed2007

Origin and evolution of 3'UTR of flaviviruses: long direct repeats as a basis for the formation of secondary structures and their significance for virus transmission.

The 3' untranslated regions (3'UTRs) of flaviviruses are reviewed and analyzed in relation to short sequences conserved as direct repeats (DRs). Previously, alignments of the 3'UTRs have been constructed for three of the four recognized flavivirus groups, namely mosquito-borne, tick-borne, and nonclassified flaviviruses (MBFV, TBFV, and NCFV, respectively). This revealed (1) six long repeat sequences (LRSs) in the 3'UTR and open-reading frame (ORF) of the TBFV, (2) duplication of the 3'UTR of the NCFV by intramolecular recombination, and (3) the possibility of a common origin for all DRs within the MBFV. We have now extended this analysis and review it in the context of all previous published analyses. This has been achieved by constructing a robust alignment between all flaviviruses using the published DRs and secondary RNA structures as "anchors" to reveal additional homologies along the 3'UTR. This approach identified nucleotide regions within the MBFV, NKV (no-known vector viruses), and NCFV 3'UTRs that are homologous to different LRSs in the TBFV 3'UTR and ORF. The analysis revealed that some of the DRs and secondary RNA structures described individually within each flavivirus group share common evolutionary origins. The 3'UTR of flaviviruses, and possibly the ORF, therefore probably evolved through multiple duplication of an RNA domain, homologous to the LRS previously identified only in the TBFV. The short DRs in all virus groups appear to represent the evolutionary remnants of these domains rather than resulting from new duplications. The relevance of these flavivirus DRs to evolution, diversity, 3'UTR enhancer function, and virus transmission is reviewed.

3' Untranslated Regions↗
PubMed2007

Ribosome-inactivation display system.

We present a novel strategy for the connection of phenotype and genotype in vitro that can be used for the selection of functional proteins. The strategy involves the generation of a stable complex among a ribosome, an messenger RNA and its translated protein, without removal of the termination codon, as a result of the action of the ricin A chain during translation. The technique requires no transfection, no chemical synthesis, no ligation, and no removal of the termination codon. Thus, our novel ribosome-inactivation display system should provide, without loss of the pool population, a reliable, simple, and robust selection system for the in vitro evolution of the properties of proteins in a predictable direction by a combination of randomization and appropriate selection strategies.

Amino Acid Sequence↗
PubMed1979

Chromosomal evolution in primates: tentative phylogeny from Microcebus murinus (Prosimian) to man.

The karyotypes of more than 60 species of Primates are studied and compared, with the use of almost all existing banding techniques. There is a very close analogy of chromosome banding between the Simians studied and man. The quantitative or qualitative variations detected all involve the heterochromatin. It is very likely that all the euchromatin (nonvariable R and Q bands) is identical in all the species. Approximately 70% of the bands are common to the Simians and to the Lemurs (Prosimians). In the remaining 30%, technical difficulties prevented a valuable comparison, but this does not exclude the possibility that a complete analogy may exist. Thus, it is very likely that chromosomal evolutions of the Simians, and probably of all the Primates, has occurred without duplication or deficiency of the euchromatin. Approximately 150 rearrangements could be identified and related to the human chromosomes. The types of rearrangement vary from one group (suborder, family, genus) to another. For instance, Robertsonian translocations are preponderant among the Lemuridae (44/57), but are nonexistent among the Pongidae. Chromosome fissions are very frequent amng the Cercopithecidae (10/23), but were not found elsewhere, and pericentric inversions are preponderant in the evolution of Pongidae and man (17/28). This suggest that the chromosomal evolution may be directed by the genic constitution (favouring the occurrence of a particular type of rearrangement, by enzymatic reaction), by the chromosomal morphology (the probability that Robertsonian translocation will be formed depends at least partially on the number of acrocentrics), and by the reproductive behaviour of the animals. Reconstitution of the sequence of the chromosomal rearrangements allowed us to propose a fairly precise genealogy of many Primates, giving the positions of the Catarrhines, the Platyrrhines, and the Prosimians. It was also possible to reconstruct the karyotypes of ancestors that died out several dozen million years ago. The possible role of chromosomal rearrangements in evolution is discussed. It appears necessary to consider different categories of rearrangements separately, depending on their behaviour. The 'nonfavoured' rearrangements, such as pericentric inversions, need to occur in an isolated small population for implanting, by an equivalent of genic derivation. The 'favoured' rearrangements, e.g., Robertsonian translocations, may occur and diffuse in panmictic populations, and accumulate. Their role of gametic barrier could be much more progressive. For discrimination between these two categories, it was necessary to differentiate the selective advantage or disadvantage of the rearrangement itself. It was not possible to show that chromosomal rearrangements play a direct role in modification of the phenotype by position effect. Comparison of the rearrangement that have occurred during evolution and those detected in the human population shows a strong correlation for some of them...

Animals↗
PubMed2007

Rate of evolution in brain-expressed genes in humans and other primates.

Brain-expressed genes are known to evolve slowly in mammals. Nevertheless, since brains of higher primates have evolved rapidly, one might expect acceleration in DNA sequence evolution in their brain-expressed genes. In this study, we carried out full-length cDNA sequencing on the brain transcriptome of an Old World monkey (OWM) and then conducted three-way comparisons among (i) mouse, OWM, and human, and (ii) OWM, chimpanzee, and human. Although brain-expressed genes indeed appear to evolve more rapidly in species with more advanced brains (apes > OWM > mouse), a similar lineage effect is observable for most other genes. The broad inclusion of genes in the reference set to represent the genomic average is therefore critical to this type of analysis. Calibrated against the genomic average, the rate of evolution among brain-expressed genes is probably lower (or at most equal) in humans than in chimpanzee and OWM. Interestingly, the trend of slow evolution in coding sequence is no less pronounced among brain-specific genes, vis-à-vis brain-expressed genes in general. The human brain may thus differ from those of our close relatives in two opposite directions: (i) faster evolution in gene expression, and (ii) a likely slowdown in the evolution of protein sequences. Possible explanations and hypotheses are discussed.

Animals↗
PubMed1994

Recurrent origin of a sexually selected trait in Xiphophorus fishes inferred from a molecular phylogeny.

Darwin believed that sexual selection accounts for the evolution of exaggerated male ornaments, such as the sword-like caudal fin extensions of male fishes of the genus Xiphophorus, that appear detrimental to survival. Swordtails continue to feature prominently in empirical work and theories of sexual selection; the pre-existing bias hypothesis has been offered as an explanation for the evolution of swords in these fishes. Based upon a largely morphological phylogeny, this hypothesis suggests that female preference to mate with sworded males arose in ancestrally swordless species, thus pre-dating the origin of the sword itself and directly driving its evolution. Here we present a molecular phylogeny (based on mitochondrial and nuclear DNA sequences) of Xiphophorus which differs from the traditional one: it indicates that the sword originated and was lost repeatedly. Our phylogeny suggests that the ancestor of the genus is more likely to have possessed a sword than not, thus questioning the applicability of the pre-existing bias hypothesis as an explanation for the evolution of this sexually selected trait.

Animals↗
PubMed1986

The Relationship between H(2) Evolution and Acetylene Reduction in Pisum sativum-Rhizobium leguminosarum Symbioses Differing in Uptake Hydrogenase Activity.

Peas (Pisum sativum L.) were inoculated with strains of Rhizobium leguminosarum having different levels of uptake hydrogenase (Hup) activity and were grown in sterile Leonard jars under controlled conditions. Rates of H(2) evolution and acetylene reduction were determined for intact nodulated roots at intervals after the onset of darkness or after removal of the shoots. Hup activity was estimated using treatment plants or equivalent plants from the growth chamber, by measuring the uptake of H(2) or (3)H(2) in the presence of acetylene. In all cases, the rate of H(2) evolution was a continuous function of the rate of acetylene reduction. In symbioses with no demonstrable Hup activity, H(2) evolution increased in direct proportion to acetylene reduction and the slopes were similar with the Hup(-) strains NA502 and 128C79. Hup activity was similar in strains 128C30 and 128C52 but significantly lower in strain 128C54. With these strains, the slopes of the H(2) evolution versus acetylene reduction curves initially increased with acetylene reduction, but became constant and similar to those for the Hup(-) strains at high rates of acetylene reduction. On these parallel portions of the curves, the decreases in H(2) evolution by Hup(+) strains were similar in magnitude to their H(2)-saturated rates of Hup activity. The curvilinear relationship between H(2) evolution and acetylene reduction for a representative Hup(+) strain (128C52) was the same, regardless of the experimental conditions used to vary the nitrogenase activity.

Journal Article↗
PubMed1999

Antibiotic resistance versus small molecules, the chemical evolution.

Two discovery approaches directed to addressing the problem of increasing bacterial resistance are described. The first is a program to build activity against methicillin-resistant Staphylococcus aureus (MRSA) into the cephalosporin class of antibacterials, by enhancing affinity for PBP2a, the penicillin-binding protein responsible for this resistance. Through stepwise improvement in potency, human serum binding, solubility, and betalactamase stability, a stable of new compounds with excellent potential as anti-MRSA agents was realized. From this set was chosen MC-02, 479 (RWJ-54428), which is now undergoing extensive preclinical evaluation. The second approach explores the uridyl peptide family of antibiotics, inhibitors of bacterial translocase (mraY), whose members include the pacidamycins, mureidomycins, and napsamycins. Access to a diverse set of analogs by total synthesis was catalyzed by the discovery that hydrogenation of the 4'-exoenamidofuranosyl moiety causes no loss in biological activity. Indepth exploration of SAR required (1) establishment of the absolute stereochemistry of the central diaminobutyric acid (DABA) moiety and (2) determination of the stereochemistry of the 4'-substituent on the deoxyfuranose unit. The former was accomplished by comparison of DABA derived from degradation of a natural product pacidamycin with a sample synthesized from L-threonine. The biological activity of one member of a synthesized library of possible stereoisomers of the natural product established the absolute stereochemistry of the remaining centers. A variety of analogs of the natural product were prepared utilizing the synthetic methods developed, and their biological activities provide important insights into the specificity and spectrum of the antibiotic class.

Animals↗
PubMed1980

Inhibition of photosystem II by formate. Possible evidence for a direct role of bicarbonate in photosynthetic oxygen evolution.

In broken chloroplasts the presence of 100 mM sodium formate at pH 8.2 will specifically lengthen the Photosystem II relaxation times of the reactions S'2 leads to S3 and S'3 leads to S0. Rates of reactions S'0 leads to S1 and S'1 leads to S2 remain unaffected. Evidence is presented which indicates the discrimination among S-states by formate cannot be attributed to a block imposed on the reducing side of Photosystem II. The results are interpreted in context of the known interaction of formate and CO2 which is bound to the Photosystem II reaction center complex. It is proposed that those S-state transitions which show extended relaxation times in the presence of formate must result in the momentary release and rebinding of CO2. Furthermore since formate is acting on the oxygen-evolving side of Photosystem II, it would seem that CO2 is released in reactions that occur there. A chemical model of oxygen evolution is presented. It is based on the hypothesis that hydrated CO2 is the immediate source of photosynthetically evolved oxygen and explains why, under certain conditions formate slows only the S-state transitions S'2 leads to S3 and S'3 leads to S0.

Bicarbonates↗
PubMed2005

Factors affecting male song evolution in Drosophila montana.

D. montana (a species of the D. virilis group) has spread over the northern hemisphere, populations from different areas showing both genetic and phenotypic divergence. The males of this species produce an elaborate courtship song, which plays a major role both in species recognition and in intraspecific mate choice. The genetic architecture and physical constraints, as well as the importance of the signal for species recognition, set boundaries within which this signal can vary. Within these limits, courtship song parameters may change, depending on the males' physical condition and on the environment they inhabit. Females are likely to affect song evolution by exerting directional selection toward higher carrier frequencies. Given this complexity, only a comprehensive, multidisciplinary approach, starting with traditional field observation and combining controlled behavioral experiments, biometric measurements, and sophisticated molecular techniques, has the potential of shedding light on the past history and the evolution of this signal, and, eventually, adding to our understanding of the mechanisms, functions, and outcomes of sexual selection in acoustic communication systems.

Animals↗