Volume 17, Issue 3
September 1992, pages 193-352
pp 193-215 September 1992
Early embryos of metazoan species are subject to the same set of physical forces and interactions as any small parcels of semi-solid material, living or nonliving. It is proposed that such “generic” properties of embryonic tissues have played a major role in the evolution of biological form and pattern by providing an array of morphological templates, during the early stages of metazoan phylogeny, upon which natural selection could act. The generic physical mechanisms considered include sedimentation, diffusion, and reaction-diffusion coupling, all of which can give rise to chemical nonuniformities (including periodic patterns) in eggs and small multicellular aggregates, and differential adhesion, which can lead to the formation of boundaries of non-mixing between adjacent cell populations. Generic mechanisms that produce chemical patterns, acting in concern with the capacity of cells to modulate their adhesivity (presumed to be a primitive, defining property of metazoa), could lead to multilayered gastrulae of various types, segmental organization, and many of the other distinguishing characteristics of extant and extinct metazoan body plans. Similar generic mechanisms, acting on small tissue primordia during and subsequent to the establishment of the major body plans, could have given rise to the forms of organs, such as the vertebrate limbs. Generic physical processes acting on a single system of cells and cell products can often produce a widely divergent set of morphological phenotypes, and these are proposed to be the raw material of the evolution of form. The establishment of any ecologically successful form by these mechanisms will be followed, under this hypothesis, by a period of genetic evolution, in which the recruitment of gene products to produce the “generically templated” morphologies by redundant pathways would be favoured by intense selection, leading to extensive genetic change with little impact on the fossil record. In this view, the stabilizing and reinforcing functions of natural selection are more important than its ability to effect incremental change in morphology. Aspects of evolution which are problematic from the standard neo-Darwinian viewpoint, or not considered within that framework, but which follow in a straightforward fashion from the view presented here, include the beginnings of an understanding of why organisms have the structure and appearance they’ do, why homoplasy (the recurrent evolution of certain forms) is so prevalent, why evolution has the tempo and mode it does (“punctuated equilibrium”), and why a “rapid” burst of morphological evolution occurred so soon after the origin of the metazoa.
pp 217-232 September 1992
The Mcr systems (previously known as Rgl systems) ofEscherichia coli recognize and cleave specific sequences carrying methylated or hydroxymethylated cytosines. We have cloned the mcrA gene and determined its nucleotide sequence. An 831 base pair sequence encodes the McrA protein. Analysis of the sequence data reveals that there arc additional ORFs internal to the above. A phage T7 expression system was used to determine the protein products encoded by the cloned mcrA gene. The results clearly show that a 31 kDa polypeptide is responsible for McrA activity. This is in agreement with the molecular weight deduced from sequence data. McrA protein was found to be localized in the outer membrane ofEscherichia coli. To our knowledge this is the first restriction enzyme localized in the outer membraneof Escherichia coli.
pp 233-239 September 1992
Genetic and molecular analyses of an unstable region encompassing the gene loci cml arg and a 5.7 kb amplifiable unit of DNA were done. Spontaneous mutants from Cm1R →CmlS and the revertants from CmlS →CmlR were analysed for mutations at arg locus and amplification of amplifiable unit of DNA. Twenty-one revertants were analysed. Two of these had large-scale amplification and one of these was also Arg-. Nine of the revertants which were Arg+ had low-level or intermediate-level amplification of the 5.7 kb DNA sequence but no deletions of the flanking sequences were detected. Five of the CmIR’ revertants, which were also Arg+, had lost one of the two copies from the doublet of amplifiable unit of DNA. The remaining five revertants did not show any other change. The amplifiable unit of DNA, therefore, not only undergoes amplification but can also suffer specific deletion of one copy. Thus, this region as a whole is characterized by instability and the events appear to take place at more than one locus concomitantly with a high frequency.
pp 241-251 September 1992
Sensitivity to the daily photoperiod, particularly with respect to pupal diapause induction, was studied during ovarian, embryonic, and larval development of the flesh flySarcophaga argyrostoma. Large flies were shown to have a greater number of primary follicles in their ovaries and to be capable of limited ovarian maturation in the absence of exogenous protein (autogeny). Such ovarian development occurred independently of photoperiod. However, long days experienced during embryogenesis caused more rapid development, and earlier larviposition, than short days. Short days during embryonic and subsequent larval development also induced pupal diapause, whereas long days led to continuous or non-diapause development of the pupae. Pupal diapause could not be induced by photoperiods during the vitellogenic phase of ovarian development. InSarcophaga argyrostoma, a maternal effect preventing pupal diapause among the progeny of files with a diapause history was not observed.
pp 253-273 September 1992
InGryllotalpa fossor (Orthoptera) (23, X0 male; 24, XX female) we have established the existence of random X chromosome inactivation for dosage compensation of X-linked genes. Both cytogenetical (DNA replication and transcription) and biochemical (X-linked glucose-6-phosphate dehydrogenase) studies have indicated that one of the two X chromosomes in the female soma (hepatic caeca) is late replicating and transcriptionally silent leaving the other X chromosome to remain active as in males thereby ensuring the production of almost the same amount of X-linked glucose-6-phosphate dehydrogenase in both sexes. Even in oogonia, one of the two X chromosomes continues to retain inactive. Only prior to their entry into meiosis the inactive X chromosome is reactivated. Accordingly, there is two-fold increase m the level of X-linked glucose-6-phosphate dehydrogenase in oocytes, From this it is implied that the restoration of X chromosome inactivation should occur some time during early embryogenesis. Thus, dosage compensation inGryllotalpa seems to be analogous to that in mammals. Our work bears testimony to the ancient origin of this mechanism.
pp 275-291 September 1992
The experimental study of plant growth and architecture requires two consecutive and complementary approaches. The first one is qualitative and botanical. Through this qualitative analysis, it is possible to divide a tree into botanically homogeneous subunits. The second one is quantitative. Modeling of the functioning of meristems based on stochastic processes has been carried out in the Laboratoire de Modelisation of CIRAD (1980 1990) in combination with extensive experimental work on temperate and tropical plants. Calculations involved in the tree simulations from field data are based on the probabilistic Monte Carlo method for the topological part and on analytical geometry for the morphological part. Data, such as length, angles, elasticities, tropism, are necessary for geometric construction of the plant. The growth model, together with the geometric calculations enables a random plant simulation in 3-D according to geometry for the morphological one. Computer graphics methods are then used to visualize the computed plant (perspective, colours, texture, shadows); they require special programs and graphic computers.
pp 293-303 September 1992
Thermoregulatory thermogenesis was evaluated in chronically undernourished subjects as well as normally nourished controls by 60 min infusions of norepinephrine at a calculated dose of 0.15 μg/kg fat free mass/min. There was a significant (P< 0.05) reduction in the thermogenic response to norepinephrine in the undernourished labourers. When the characteristics of response were evaluated, the baseline and peak responses were comparable in both groups. However, the steepness of the response was greater (P< 0.05) in the undernourished subjects. The time to register a threshold response of 5 ml per min increase in oxygen consumption over the baseline after the start of infusion was longer in undernourished (about 20 min) as compared to the controls (about 10 min). It appears that, while the undernourished subjects have a reduced thermogenic component in their energy output, their peak capacity or potential for regulatory thermogenesis is the same as that in control subjects, since their thermogenic response has a greater slope. It is possible that chronically undernourished subjects may have a suppressed response over shorter periods of stimulation (less than 20 min), that is, before steady state thermogenic responses are achieved.
pp 305-312 September 1992
McGlynn and her co-workers have reported that among the Vietnamese refugees in Philadelphia and among Alaskan natives who are hepatitis B carriers, there is a statistically significant association between anegative tuberculin test and the presence of hepatitis B e antigen. A repetition of this work among the population of Bangalore did not yield any significant results because of the very low incidence of hepatitis found among this population. However, on the basis of available data that hepatitis B infection is more prevalent among the Mongolian population than among people of other populations, the work was repeated among Tibetans who had settled down in Karnataka. This set of experiments showed that, contrary to the report of McGlynnet al, there is a statistically significant association between apositive tuberculin test and the presence of hepatitis B e antigen and that those individuals who showed the presence of hepatitis B e antigen exhibited less severe form of the disease than those who were negative to this antigen. These findings suggested that immunity to tuberculosis and hepatitis B infections may have a common underlying principle. Data bank search revealed a stretch of amino acid sequences which is common to hepatitis B e antigen and 19 kDa antigen ofMycobacterium tuberculosis. The significance of these results is discussed.
pp 313-323 September 1992
A novel series of murine monoclonal antibodies to islet cells (1–45, 1–51, 1–52 and 1–39) have been generated using human insulinoma homogenate as the immunogen in order to characterize pathogenetically relevant islet cell autoantigen(s). Differentiation antigens recognized by these islet cell monoclonal antibodies displayed varied cytological distribution (pan-islet or peripheral mantle only). Monoclonal antibody 1–45 reacted with all endocrine subsets of the pancreatic islet, similar to the reactivity of islet cell autoantibody positive sera from type I diabetes subjects. Preexposure to pH2 abolished the immunoreactivity of the autoantigen; 1–45 antigen was also sensitive to low pH. Preexposure to 100° C for 1 h did not significantly alter the immunoreactivity of islet antigens recognized by ICAb positive patient sera and monoclonal antibody 1–39, thus demonstrating the extraordinary heat stability of the corresponding epitopes; those recognized by 1–45 were less heat stable. Islet cells were found to share 1–45 differentiation antigen(s)/epitope(s) with other neuroendocrine cells,viz. amerior pituitary, adrenal medulla and gut endocrine cells.
pp 325-352 September 1992
Phenological observations on tree species in tropical moist forest of Uttara Kannada district (13ℴ55′ to 15ℴ31′ N lat; 74ℴ9′ to 75ℴ10′ E long) during the years 1983–1985 revealed that there exists a strong seasonality for leaf flush, leaf drop and reproduction. Young leaves were produced in the pre-monsoon dry period with a peak in February, followed by the expansion of leaves which was completed in March. Abscission of leaves occurred in the post-monsoon winter period with a peak in December. There were two peaks for flowering (December and March), while fruit ripening had a single peak in May–June, preceding the monsoon rainfall. The duration of maturation of leaves was the shortest, while that of full ripening of fruits was the longest. Mature flowers of evergreen species lasted longer than those of deciduous species; in contrast the phenophase of ripe fruits of deciduous species was longer than that of evergreen species.
Volume 44 | Issue 6
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