摘要:

Summary1. The biometric approach to selection experiments has been outlined, and found to be rather deficient because it is based on excessively restrictive models which cannot take into account the complex architectures of quantitative traits as are being revealed today.2. The nature of polygenes is discussed in detail from the theoretical point of view. In out breeding species, some form of the balanced polygenic complex is likely, showing polymorphism for the constituent genes. Although polymorphism is implicit in the argument, definitive evidence for poiymorphisms has only just appeared.3. There is no evidence that polygenes differ from any other gene.4. Several artificial selection experiments are described, in particular inDrosophila. By means of appropriate breeding techniques after obtaining responses to selection, genetic activity controlling quantitative traits can be located to chromosomes, and even specific loci found. Such few studies as have been carried out reveal, in general, the types of genetic architecture predicted on theoretical grounds.5. Selection for behavioural traits is considered briefly and it appears that no new principles are needed, except that careful environmental control and objective measurement present problems.6. The results of selection for quantitative traits in micro‐organisms reveal similarities to results in higher organisms in the few cases where definitive work has been carried out.7. Work on the simulation of models by computers has not greatly advanced selection experiment theory, mainly because, with few exceptions, linkage has been ignored.8. The existing theory on which selection experiments are based is inadequate for several reasons. It cannot predict the rate of response to selection nor the ultimate limits to selection, the nature of correlated responses to selection, nor the nature of gene segregation underlying the observed variability.9. Strains set up from single inseminated founder females from the same population ofDrosophilahave been shown to differ genetically for several quantitative traits. Therefore the base population is polymorphic for genes controlling these traits. This was exploited by carrying out directional selection on lines derived from those strains showing a high incidence of scutellar chaetae. This led to far more rapid responses to selection than lines derived from strains where the incidence of scutellar chaetae was lower.10. Ultimately, one can envisage the selection experiment as it is known today being partly replaced by the manipulation of located genes controlling quantitative traits into certain combination

ISSN:1464-7931    

DOI:10.1111/j.1469-185X.1968.tb00957.x

出版商:Blackwell Publishing Ltd    

年代:1968

数据来源: WILEY

摘要:

Summary1. The caudal neurosecretory system is defined in teleosts as a complex of secretory neurones (Dahlgren cells) in the caudal spinal cord leading by a tract to neurohaemal tissue organized as a typical neurosecretory storage‐release organ: the urophysis.2. The teleost urophysis is generally a distinct, easily recognizable, lobate structure of variable external form. Significant morphological variations lie in the organization of the neurosecretory fibres in relation to the vascular bed and in the degree of penetration of the meninx by the neurosecretory fibres to form an organ external to the spinal cord proper.3. The elasmobranch caudal system is composed of large cells with short axons projecting to a diffuse vascular bed; there is no organized urophysis.4. The caudal neurosecretory system and its urophysis appear late in post larval development by comparison with the hypothalamic neurosecretory system. The Dahlgren cells originate from the ependyma in development and also during regeneration of the caudal system in adult life.5.The elasmobranch system may represent the more primitive condition, and stages in the evolution of the advanced urophysial types can be visualized. The particular histology shown by the caudal system appears to have taxonomic significance.6.The cytology of the Dahlgren cell and its neurosecretory material is described. The proteinaceous neurosecretory material has an affinity for acid stains but not for the Gomori stains or reagents demonstrating SH/SS groups. The inclusions visible at the light‐microscope level are aggregates of elementary neurosecretory granules, 800–2500A diameter, which originate from Golgi centres. The possible participation of preterminal axonal regions–and tubular systems evident therein—in the formation of neurosecretory material is considered.7.The structure of the axon terminals raises questions about the way in which neurohormone may be released into the blood. Small vesicles have been variously interpreted as cholinergic synaptic vesicles and as products of the fragmentation of membranes of elementary neurosecretory granules. Evidence for the release of ‘neuro‐secretion centripetally’ into the cerebrospinal fluid also exists.8.Functional analysis of the caudal neurosecretory system has proven most difficult, The bulk of earlier data and more recent information indicate a role in ionic regulation. Increased sodium uptake by the gills of goldfish has been reported, as a result of administration of urophysial extract, and electrophysiological studies indicate a responsiveness of the system to variations in blood sodium ion concentration. The urophysis also has a definite pressor effect in eels and will stimulate water retention in anurans. The early claim of Enami that the system was involved in buoyancy regulation has never been substantiated. It must be admitted that the function of this system, virtually ubiquitous in teleost and elasmobranch fishes at least, has been anything but established and still represents a major challenge to comparativ

ISSN:1464-7931    

DOI:10.1111/j.1469-185X.1968.tb00958.x

出版商:Blackwell Publishing Ltd    

年代:1968

数据来源: WILEY

摘要:

SummarySome of the following propositions are to be read as suggestions or hypotheses, supported by circumstantial or direct evidence, but not yet rigorously demonstrated. An estimate of the significance to be attached to each should be gathered from the body of the paper rather than from the following brief statements.1.The problem is posed: how do endophagous parasitoids counteract the haemocytic defence reactions of their usual hosts?2.It has been demonstrated that the egg and young first‐instar larva ofNemeritis canescenshave a coating on their surface which enables them to escape the attention of the haemocytes of their usual host, and to develop without exciting a defence reaction. The coating is applied to the egg before it is laid, and to the cuticle of the larva before it hatches. A little evidence suggests that some other ichneumon wasps of the subfamily Ophioninae may use this mechanism of resistance.3.Older first‐instar larvae, and the second and later instars, of many parasitoids, both hymenopterous and dipterous, probably overcome the haemocytic reaction of their host by rapid feeding, which depletes its blood both of cells and of nutrients, and so drains its resources that haematopoiesis is prevented and encapsulation becomes impossible.4.The common habit of parasitoids of lingering in the first instar, before ingesting much food, while the host goes on developing to another stage or undergoes diapause, may enable the larva to retain a protective coating that would have become ineffective if it had grown. When at length the larva does feed and grow, the preceding mechanism (3) comes into play.5.The teratocytes and pseudogerms formed by many species in several families of Hymenoptera absorb nutrients on a large scale from the blood of the host. They act quickly, as soon as the larva hatches. I suggest that by their attrition of the host's reserves of food, and its consequent debility, they prevent an effective haemocytic reaction to the young parasitoid.6.Some dipterous and hymenopterous parasitoids first inhabit the intestine of their host, and do not penetrate the body cavity until they are ready to overwhelm the defence reactions by rapid and gross feeding.7.Parasitoids that live temporarily inside an organ of the host may there acquire a coating which protects them from reaction by the blood cells.8.Species of parasitoids that occupy an organ of the host for a long period, and develop inside it, escape a defence reaction because they live within the connective tissue covering the organ, to which the blood cells do not react.9.Eggs of hymenopterous parasitoids laid within the embryos of their hosts may be treated by the embryonic blood cells as a developing organ, and become covered with connective tissue as those organs are. Thereafter they would not be recognized as foreign bodies.10.Parasitoid eggs laid in the eggs or the young larvae of their host may be coated with host substances, or covered by connective tissue (9), before the blood of the host be comes capable of vigorousdefence reactions. They would there after escape recognition as foreign bodies. This may be the advantage of the habit of the so‐called egg‐larval parasitoids.11.Reasons have been given by Schneider (1950) for his belief that the serosa of the ichneumon waspDiplazon fissoriussecretes something that locally inhibits the defence reactions of its hosts. The trophamnion and pseudoserosa of some parasitoid eggs may have this function.12. Some parasitoids, especially second‐ and third‐instar larvae of Tachinidae, physically repulse the haemocytes of their host, moulding them into a capsule that serves the maggot as a respira

ISSN:1464-7931    

DOI:10.1111/j.1469-185X.1968.tb00959.x

出版商:Blackwell Publishing Ltd    

年代:1968

数据来源: WILEY

摘要:

Summary1.This article reviews the occurrence, mechanism, and functional significance of the cytoplasmic regulation of nuclear activity during cell differentiation and especially during early animal development.2.Nuclei from brain, and from other kinds of adult cell normally inactive in DNA synthesis, are rapidly induced to commence DNA synthesis by components or properties of intact egg cytoplasm.The components of egg cytoplasm which induce DNA synthesis are not species‐specific and they are likely to include DNA polymerase. It is known that DNA polymerase exists in egg cytoplasm before it becomes associated with nuclei in which it is effective.The induction of DNA synthesis in brain nuclei by living egg cytoplasm is always preceded by a pronounced nuclear swelling, a dispersion of chromosomes or chromatin, and the entry of cytoplasmic protein into the nucleus.3.RNA synthesis can be experimentally induced or repressed by living cytoplasm. The cytoplasm of unfertilized and fertilized eggs appears to contain components which can reversibly and independently repress the synthesis of ribosomal RNA, transfer RNA, and heterogeneous RNA. RNA synthesis can be induced by introducing nuclei inactive in this respect into the cytoplasm of cells very active in RNA synthesis. The induction and repression of RNA synthesis is preceded by a marked swelling of the nucleus and the dispersion of its chromosome material.4.The cytoplasmic control of chromosome condensation before division has been demonstrated by introducing sperm or adult brain nuclei into the cytoplasm of oocytes undergoing meiotic maturation.5.The evidence that regional differences in the composition of eggs and other cells are associated with changes in nuclear and gene activity is reviewed in Section 111. While it is certain that these regional differences are of great importance in cell differentiation, evidence that they have a direct effect on nuclear activity has been obtained in a few instances only.In some species it has been shown that the cytoplasmic components related to germ‐cell differentiation include RNA and, frequently, granules.6.It is concluded that whenever nuclei are introduced experimentally into the cytoplasm of another cell, they very quickly assume, in nearly every respect, the nuclear activity characteristic of the host cell. In many instances, altered function has been demonstrated in nuclei which subsequently support normal development. The induced nuclear changes are therefore regarded as normal and it is believed that they are achieved through the same mechanism as that by which the host cell nucleus originally came to function in its characteristic way.Examples are cited to show that changes in gene activity very frequently arise immediately after mitosis. The changes induced experimentally in transplanted nuclei resemble in very many respects those undergone by nuclei which are naturally reconstituted after mitosis, and it is argued that the two processes are functionally equivalent,It is suggested that during telophase of mitosis, chromosomes are reprogrammed in respect of potential gene activity by association with cytoplasmic proteins. Inter‐phase nuclei seem not to show changes of gene activity except when they undergo a pronounced enlargement after entering a new cytoplasmic enviro

ISSN:1464-7931    

DOI:10.1111/j.1469-185X.1968.tb00960.x

出版商:Blackwell Publishing Ltd    

年代:1968

数据来源: WILEY