摘要:

Summary(1) The movements are only expressed in motor cells, regardless of the nature of the stimulation or its point of application. Therefore, these cells have structures capable of traducing the different stimulation‐induced messages which are received in parts incapable of movement.(2) K+, Cl‐and Ca2+are the major ions. Their fluxes have been followed during nyctinastic movements as well as during stimuli‐induced movements. At the moment, the location and the role of these ions are being studied.(3) The movement results from the integrated activity of all (n) motor cells in the pulvini (i.e.n>350 × 103in primary pulvini 3 mm long and 1·9 mm thick).(4) The motor cell is a full‐grown cell whose osmotic activity induces turgor variations allowing foliar movements.(5) The motor cell is a highly differentiated cell, which, up to now, has never been able to dedifferentiate in order to produce callus.(6) The motor cell has original features in its apoplastic compartment (large meatuses, wall foldings, large periplasm with membranes) and in its symplastic compartment (double vacuolar apparatus, morphological polarity given by the tannin vacuole location near the nucleus, abundant mitochondria).(7) Its cytoskeleton includes microtubules, cytoplasmic and vacuolar fibrils (in particular in the tannin vacuole), and a wall with special properties.(8) The motor cell is supposed to contain contractile proteins, whose nature and location are being investigated.(9) The shape change of the motor cell is obvious after pulvinar bending. This change is probably associated with a volume change in several intracellular compartments (vacuole, mitochondria, vesicles).(10) At the cellular and subcellular level the same general features are observed in motor cells of non‐seismonastic and of seismonastic species. Probably, functional differences depend upon differences occurring at the molecular level.(11) The motor cell is an interesting model for the study of the osmoregulation mechanism in plant cells, to test the effect of toxic products, in particular to find their optimal efficiency in the circ

ISSN:1464-7931    

DOI:10.1111/j.1469-185X.1988.tb00467.x

出版商:Blackwell Publishing Ltd    

年代:1988

数据来源: WILEY

摘要:

Summary1. Microarthropods (Acari and Collembola) are dominant components of the terrestrial fauna in the Antarctic. Their cold tolerance, which forms the mainspring of their adaptational strategy, is reviewed against a background of their structure and function, and by comparison with other arthropods.2. Two species, the isotomid collembolanCryptopygus antarcticusWillem and the oribatid miteAlaskozetes antarcticus(Michael), are examined in detail, and afford a comparative approach to the mechanisms underlying cold tolerance in insect and arachnid types.3. All microarthropods appear to be freezing‐susceptible (unable to tolerate tissue ice), and they utilize varying levels of supercooling to avoid freezing. Gut contents are considered to be the prime nucleation site in most arthropods when supercooled, particularly for Antarctic species. Moulting also increases individual supercooling ability especially in Collembola, and the activity of ice‐nucleating bacteria in cold‐hardy arthropods may be important.4. Sources of ice nucleators are many and varied, originating externally (motes) or internally (ice‐nucleating agents). They act either extracellularly (mainly in the haemolymph) to promote freezing in ice‐tolerant life stages, or intracellularly in freezing‐susceptible forms. Thermal hysteresis proteins, acting colligatively, occur in many arthropods including Collembola; they depress both the freezing point of body fluids and the whole‐body supercooling point of freezing‐ susceptible and freezing‐tolerant species.5. Bimodal supercooling point distributions are a feature of microarthropods and water droplets. Samples of field populations of Antarctic mites and springtails show significant seasonal changes in these distributions, which in some respects are analogous to purely physical systems of water droplets. Supercooling points are confirmed as accurate measures of cold‐hardiness and survival for Antarctic species, but not necessarily for other arthropods. The effects of constant sub‐zero temperatures approaching the limit of the supercooling ability of arthropods require study.6. Desiccation and dehydration influence microarthropod physiology in several ways; inAlaskozetesit triggers glycerol synthesis. Glycerol may aid binding of water in severely dehydrated insects, but the relationship of such ‘bound’ water to cold‐hardiness is unclear.7. Sugar alcohols (polyols) and sugars are accumulated as potential cryoprotectants in many arthropods at low temperatures, and antifreeze systems may be single or multi‐component in structure. Cryoprotectant synthesis and regulation have been studied principally in insects, and fresh weight concentrations of 0–3‐5 M of polyols have been found. Trehalose accumulation may also influence cold‐hardiness.8. Microarthropods fall within the spectrum of cold tolerance observed for arthropods and other invertebrates. No special adaptations are found in Antarctic species, and similar strategies and mechanisms are present in both insects and arachnids. The colonization and maintenance of microarthropod populations of polar land habitats seem not to have required the evolution of any novel feat

ISSN:1464-7931    

DOI:10.1111/j.1469-185X.1988.tb00468.x

出版商:Blackwell Publishing Ltd    

年代:1988

数据来源: WILEY

摘要:

SummaryAn equation of Lighthill's is used to calculate sperm thrusts. They have values in the range 5–350 pN, depending on species. The limitations of this approach are discussed and comparison is made with the measured thrust for human sperm. The effect of sperm thrusts of this magnitude on covalent bonds and reversible bonds is discussed. Sperm cannot break covalent bonds, but can reduce the lifetime of reversible bonds.The structure and physical properties of the zona pellucida are examined in relation to sperm penetration. The evidence suggests that sperm cannot penetrate it solely by force. A model for sperm penetration is elaborated in which the conjunctive application of thrust and a soluble enzyme leads to strain‐induced proteolysis and the formation of the penetration slit. The potential mechanism of the zona block is discussed, as is the site of the acrosome reaction. The effects of other mechanical inputs into fertilization such as stirring and swimming are examined briefly. Evidence suggests that sperm penetration of the cumulus oophorus and cervical mucus is mechanical, but that in the case of cervical mucus, it is affected by changes in the physical properties of the mu

ISSN:1464-7931    

DOI:10.1111/j.1469-185X.1988.tb00469.x

出版商:Blackwell Publishing Ltd    

年代:1988

数据来源: WILEY

ISSN:1464-7931    

DOI:10.1111/j.1469-185X.1988.tb00470.x

出版商:Blackwell Publishing Ltd    

年代:1988

数据来源: WILEY