摘要:

Summary1. The osmoregulatory strategies of crocodilians in both saline and fresh‐water environments are discussed and dissected into their separate components.2. Contrasts between members of the Alligatoridae and the Crocodylinae emerge repeatedly in aspects such as integumental permeabilities, functioning of the renal/cloacal system, and the presence of lingual salt‐secreting glands.3. These contrasts contribute to the view that the alligatorid and crocodyline stocks are more divergent than has been suspected previously. In particular, there is cogent evidence of a significant marine phase in the evolution of the Crocodylinae but not of the Alligatoridae.4. The physiological evidence to support this view of a very basic dichotomy among the eusuchians is reviewed in detail and avenues which would contribute most to its critical evaluation are identif

ISSN:1464-7931    

DOI:10.1111/j.1469-185X.1988.tb00721.x

出版商:Blackwell Publishing Ltd    

年代:1988

数据来源: WILEY

摘要:

Summary1. The most coherent body of information on the malarial life‐cycles comes from studies onP. bergheiandP. falciparum.For both species there is an extensive and accurate description of the life‐cycle available based on synchronized in‐vitro and in‐vivo infections (the latter only forP. berghei).2. The trophozoites preceding the young gamonts are already sexually differentiated, that is, they are micro‐ and macrogamontoblasts. They are, however, morphologically indistinguishable from the asexual trophozoites which will develop into meronts. This is in contrast with the gamontoblasts of the closely related eimerian species, which are already morphologically differentiated at this stage of their sexual development.Experiments with synchronizedP. bergheiinfections suggest that the induction towards asexual or sexual development of intra‐erythrocytic parasites occurs between 8–12 h after erythrocyte invasion.3. DNA measurements by direct fluorometry of individual Feulgen‐stained stages ofP. bergheiandP. falciparum, has shown that all stages are haploid, except for the zygote. Hence, meiosis occurs within the zygote. The latter has been confirmed in EM studies of ookinete (zygote) development ofP. bergheiwithin its mosquito vector.4. EM studies also provided evidence for the fact that the malarial genome is organized in 10–14 chromosomes, which is in line with the estimates based on PFG analysis of molecular karyotypes of different malarial species.5. Infections with clones of the asexual intra‐erythrocytic stages resulted in the formation of both micro‐ and macrogamonts. Since these asexual stages are haploid, gamonts develop from genetically identical cells and their development must be due to selective gene expression. How many genes are involved in this process, how they are induced and regulate differentiation is still an open question.6. Macrogamonts contain an extra amount of nuclear DNA in excess of the haploid value. Roughly the same amount of extra DNA is still present in the zygote, indicating that it is probably due to a selective amplification of certain genes or repetitive sequences. No indications have been found for an amplification of rRNA genes in macrogamonts or in cloned lines ofPlasmodiumparasites producing high numbers of infective macrogamonts.7. Mature microgamonts possess DNA values between1C and2C and are not octaploid as has been generally accepted. DNA replication in microgamonts, necessary for the production of the 8 microgametes, starts at the activation of the microgamont. During this process, microgamonts perform three mitotic divisions in which they replicate their entire genome in about 3 min. It is assumed that microgamonts activate simultaneously approximately 1300 replication origins to enable such a fast genome replication. Direct evidence for this hypothesis in the form of direct ultrastructural evidence of DNA molecules of replicating microgamonts is, however, lacking.8. In‐vitro fertilization of macrogametes ofP. bergheitakes place within 1 h of gametogony. The formed zygote (2C) immediately starts to synthesize DNA and within 3 h a tetraploid value is reached. Thereafter, DNA synthesis ceases for at least 21 h. EM studies of this process in vivo showed that synaptonemal complexes are formed at 25 h after fertilization. Taken together, these results strongly indicate that genome duplication precedes the first meiotic division, i.e. meiosis inPlasmodiumfollows the normal eukaryotic pattern. Thus, as expectedPlasmodiumshows recombination between genes determining characters such as enzymes, antigens, drug resistance and virulence.9. DNA synthesis in malaria parasites is regulated by a DNA polymerase‐a like enzyme that differs in some respect in the corresponding enzyme of the host and this could become a target for chemotherapeutic control o

ISSN:1464-7931    

DOI:10.1111/j.1469-185X.1988.tb00722.x

出版商:Blackwell Publishing Ltd    

年代:1988

数据来源: WILEY

摘要:

Summary1. Three basic hatching methods are described together with one subsidiary method. The symmetrical method is characterized by rotation of the chick in the egg during hatching climax; a line of damage around the circumference of the egg is evident at the beginning of a pushing phase which causes a fairly symmetrical cap to be broken from the shell. The asymmetrical method is used by a few long‐billed species; it involves little or no rotation of the chick in the egg, and produces asymmetrical shell remains. The megapodes have developed a unique hatching method in response to their unusual incubation conditions. Parental assistance has been observed in some species, but only as an auxiliary to either the symmetrical or asymmetrical method. Approximately 150 species have been categorized according to the hatching method(s) they use.2. Among those species adopting the symmetrical method, there is considerable variability as to how far the chick turns in the egg during hatching climax. On this basis, a spectrum of behaviour, expressed in terms of angle of rotation (θ), has been proposed. At one end lie species such as the bobwhite quail and little owl (θ≥ 360°), and at the other, the ostrich and rhea (θ≤ 90°).3. The theory that, with the exception of the megapodes, there is only one basic hatching method is examined. The tenets of this theory are found to be inconsistent with recent observations of species differences in hatching behaviour. It is concluded that hatching behaviour is governed by an intrinsic species‐specific programme, in turn influenced by mechanical or other external factors.4. The literature contains several suggestions as to the external factors that might influence hatching technique, but only one detailed investigation. It is proposed that interspecific differences in the mechanical properties of the egg integument (the shell and its underlying membranes) can be regarded as forming a spectrum from very brittle to comparatively tough. The amount of climax rotation by the chick is seen as an adaptive response to brittleness or toughness of the egg integument. The hatching technique of the chick is further modified as a response to the effect of moisture content on the integument.5. The selective pressures leading to the evolution of the symmetrical and asymmetrical hatching methods are discussed. The previous model for the development of the asymmetrical method is amended to account for those species of gull which may adopt e

ISSN:1464-7931    

DOI:10.1111/j.1469-185X.1988.tb00723.x

出版商:Blackwell Publishing Ltd    

年代:1988

数据来源: WILEY

摘要:

Summary1. It is generally agreed that the blackening of osmium tetroxide by unsaturated lipid is too unpredictable to demonstrate lipid in tissues.2. At neutral pH osmium tetroxide combines with the double bonds in the lipoproteins of cellular membranes (mitochondria, etc.) and the deep colour reaction of ethyl gallate with this osmium provides good staining of lipid for the light microscope.3. Osmium taken up by tissue proteins at neutral pH is only a small fraction of that taken up by the lipid. (After acid fixatives osmium tetroxide is a general protein stain.)4. The uptake of Sudan black B by partition from dilute solution is a specific test for lipid, but in normally fixed tissue most of the structural lipid is ‘bound’ and is not accessible to the dye.5. Cautious treatment of fixed tissue with dilute sodium hypochlorite will unmask this lipid for viewing by the light microscope.6. Direct fixation with neutral osmium tetroxide is an effective method for visualizing lipid for the electron microscope (as in the ethyl gallate method for the light microscope). But the poor penetration of osmium limits its use in this way.7. After formol/glutaraldehyde fixation much of the lipid in the tissues is ‘bound’ and does not take up osmium. It can be unmasked by a saturated aqueous solution of thymol.8. The unmasked lipid can then be rendered more osmiophil by partition in a solution of the highly unsaturated terpene farnesol, thus increasing the uptake of osmium in a renewed application.9. Some of the novel observations on tissue lipids made by these methods are r

ISSN:1464-7931    

DOI:10.1111/j.1469-185X.1988.tb00724.x

出版商:Blackwell Publishing Ltd    

年代:1988

数据来源: WILEY

摘要:

Summary(1) N added to decomposing organic matter often has no effect or a negative effect on microbial activity, at least in the long term. More than 60 papers are cited in support of this statement.(2) The negative effect of N is mainly found with recalcitrant organic matter with a high C/N ratio (straw, wood, etc.), whereas a positive effect of N is common for easily degradable organic material with low C/N ratio.(3) The negative effect of N could be explained by: (i) N disturbs the outcome of competition between potent and less potent decomposers; (ii) through ‘ammonia metabolite repression’, N blocks production of certain enzymes, at least in basidiomycetes, and enhances breakdown of the most available cellulose, whereby recalcitrant lignocellulose accumulates; (iii) amino compounds condense with polyphenols and other decomposition products, forming ‘browning precursors’ which are toxic or inhibitory.(4) The effect of adding N may depend on the microflora present.(5) There are indications that some microorganisms have a ‘luxury uptake’ of N when it is present in sufficient amounts, thereby delaying N mineralization.(6) The addition of N seems to increase the formation ofwater‐soluble, brown, recalcitrant compounds, but to decrease the amount of

ISSN:1464-7931    

DOI:10.1111/j.1469-185X.1988.tb00725.x

出版商:Blackwell Publishing Ltd    

年代:1988

数据来源: WILEY

摘要:

A. R. KORNBLIHTT and A. GUTMAN. Molecular biology of the extracellular matrix proteinsM. RIDLEY. Mating frequency and fecundity in insectsJ. G. ALTIN and F. L. BYGRAVE. Second messengers and the regulation of Ca2fluxes by Ca2‐mobilizing agonists in rat liv

ISSN:1464-7931    

DOI:10.1111/j.1469-185X.1988.tb00726.x

出版商:Blackwell Publishing Ltd    

年代:1988

数据来源: WILEY