摘要:

Summary1. Plastid inheritance in most green algae and land plants is uniparental. In oogamous species, plastids are usually derived from the maternal parent; even when inheritance is biparental, maternal plastids usually predominate. Only a few species of conifer are known to have essentially paternal plastid inheritance. In spite of the overall strong maternal bias, there exists a spectrum of species in which plastid inheritance ranges from purely maternal to predominantly paternal.2. Factors that influence the pattern of plastid inheritance operate both before (often long before) and after fertilization. For example, several different mechanisms for exclusion of plastids from particular cells, none of which is completely effective on its own, may operate sequentially during both gametogenesis and embryo‐genesis. There appears to exist a general trend such that the more highly evolved the organism, the more numerous the mechanisms employed and the earlier they first come into operation. The pattern of plastid inheritance shown by a species represents the efficiency or lack of efficiency of these combined mechanisms.3. In the newly‐formed zygote of many unicellular algae, the plastids from both gametes are present and there is direct competition between them. Often the plastid from one mating type (usually the ‘invading’ male gamete, where this can be identified) quickly degenerates. Species such asChlamydomonasare unusual in that the plastids from the two gametes fuse. In spite of this, inheritance of plastid DNA is normally uniparental. How this is accomplished remains unclear. In oogamous algae, the paternal plastids which enter the egg cell are frequently fewer in number and smaller in size than those contributed by the female gamete. The reduced contribution of paternal plastids can result from asymmetrical cell division or from differential timing of cell and plastid division during spermatogenesis.4. In species ranging from unicellular algae to angiosperms, plastids may be partially or completely debarred from particular cells at critical stages during the reproductive cycle. An important factor in this form of plastid elimination is their postioning with respect to the nucleus prior to a cell division. When plastids closely encircle the nucleus, they are usually incorporated equally into the two daughter cells; when the plastids are concentrated at some distance from the nucleus, they are frequently excluded from one daughter cell.5. Elimination of plastids from a gamete prior to plasmogamy prevents direct competition between the two types of plastid in the zygote or embryo. Perhaps the most effective method of excluding paternal plastids from the egg cell has been achieved by some lower land plants; the plastids migrate to the posterior part of the spermatozoid, and are discarded from there in a discrete vesicle before the egg is reached.6. Plastid inheritance in conifers appears to be unique. In those species in which the derivation of plastids in the pro‐embryo can be determined, it has been found that they come only from the male gamete. Maternal plastids are positively excluded from the pro‐embryo and later degenerate.7. In most angiosperm species plastid inheritance is maternal; in only a few species is it regularly biparental. The first step towards exclusion of paternal plastids often takes place in the uninucleate pollen grain where the plastids may be concentrated at the pole of the cell farthest from the site of the future generative cell. Any plastids that succeed in entering the generative cell may degenerate before the gametes are released from the pollen tube. Even if paternal plastids reach the egg, they are at a disadvantage because they are (a) entering an environment that is essentially alien, and (b) normally present in much smaller numbers than maternal plastids. Later, when the zygote divides, the few paternal plastids may fail to become incorporated in the small terminal cell which gives rise to the embryo proper.8. There appears to be no consistent evolutionary progression in the use of more efficient mechanisms to influence plastid inheritance; most of the mechanisms associated with exclusion of paternal plastids in angiosperms, for example, can also be found in one or other species of green alga. The primary factors that influence plastid inheritance appear to be (I) direct competition in the zygote between plastids of the two parental types – the principal mechanism operating in isogamous algae, but also operating in some angiosperms; and (2) the divergent evolution of the two types of gamete ‐ on the one hand a small male gamete with a minimum of cytoplasm which is capable of moving (spermatozoid) or being moved (pollen) efficiently, and, on the other hand, a large egg cell with numerous organelles, which is well able to act as ‘host’ for the future zygote. Many of the additional mechanisms that influence the pattern of plastid inheritance seem to be the more or less ‘accidental’ result of ot

ISSN:1464-7931    

DOI:10.1111/j.1469-185X.1982.tb00373.x

出版商:Blackwell Publishing Ltd    

年代:1982

数据来源: WILEY

摘要:

SummaryI. The dispersive role of mite phoresy, which has merely been presumed, is presented in the light of modern theories of migration with the aim of its characterization behaviourally, ecologically and physiologically.II. Data on phoresy accords well with modern, behavioural definitions of migration, as a phase of the depression of growth‐promoting functions, during which the phoretic mite is transported while it shows a special readiness for being moved.III. Thus, modern definitions of migration encompass ‘passive’ movements which, none the less, involve active phases of seeking out of the host.IV. An examination of mite loads suggests that phoresy is most effective where the host gathers within the range of the mite; hence the association of phoresy with micro‐habitats between which the mite requires to be carried. Monocultural plant‐stands and ecological climaxes are not characterized by phoretic associations.V. The role of phoretic mites in colonization is clarified, in that phoretic migration is associated with sub‐climactic communities. Thus, phoresy is invited where habitats are discrete and temporary, in which case it is manifest, as typical of migration, as a means of colonizing and exploiting irregularly changing habitats byr‐selected, pre‐reproductive individuals.VI. Waiting‐stages, marked by the depression of growth‐promoting functions, occur within the life‐cycle of the phoretic mite. The ‘hypopus facies' characterize the typical phoretic stage, whose association with the host is an adaptation for survival in extreme environments.VII. Attachment pattern is a function of specificity towards the host. Structural and behavioural adaptations for attachment are developed and some sensory mechanisms have been shown, as have some physiological relationships with changes in the substrate; these changes also affect detachment.VIII. That phoresy is not caused by unfavourable conditions but is related to those that allow optimum dispersal is supported by sound evidence.IX. Physiologically, waiting stages have analogies with diapause which, together with migration, have been characterized by a temporary failure of the migrant to respond, by further growth and development, to the conditions that will eventually promote these processes.X. With phoresy are contrasted relations, between mite and insect, where the association is assured and more or less permanent.XI. The study of phoresy is very fragmented. However, a case has now been made for putting the dispersive role of phoresy beyond presumption, so that phoretic associations can clearly be fitted into modern

ISSN:1464-7931    

DOI:10.1111/j.1469-185X.1982.tb00374.x

出版商:Blackwell Publishing Ltd    

年代:1982

数据来源: WILEY

ISSN:1464-7931    

DOI:10.1111/j.1469-185X.1982.tb00375.x

出版商:Blackwell Publishing Ltd    

年代:1982

数据来源: WILEY

摘要:

SummaryForty‐three association (similarity) coefficients were collected and evaluated in this survey. Some of them are synonyms or direct correlates with earlier described indices (A8, A9, A12, A31, A33), others are mere transforms from one range of values to another (A10, A24, A33). Several coefficients are incompatible with suggested admissibility conditions of the minimum‐maximum value (A13, A16, A27, A28, A29, A31), symmetry (A1, A2, A13, A16, A26), discrimination between positive and negative association (A27, A28, A31) or monotonicity with (χ2) (A19, to A24); A17yields very low and erratic values.As a result, 23 coefficients were excluded and the remaining 20 measures were subjected to an empirical trial on interspecific association data among fungi of the genusChaetomium, with the use of a cluster analysis. The classification produced five main clusters of related coefficients, with several subgroups. It was then demonstrated that representative indices from different clusters yield different dendrograms of interspecific association amongChaetomium, and A34, A14, possibly also A36and A40seemed to be less sensible. A set of measures that generally work well (at least in the interspecific association) comprises A4(Jaccard), A4(Dice‐Sφrensen), A7(Kulczyński), A11(Driver‐Kroeber‐Ochiai) and, with some reservation A30(Pearson tetrachoric) and A32(Baroni‐Urbani‐Buser). For some purposes, however, other ‘admissible’ coefficients would be more optimal, and the choice of a measure should be related to the nature of the data. It is tentatively suggested that three or so alternative coefficients be used and the results compared on the same data basis; moreover, significance tests on association should be carried o

ISSN:1464-7931    

DOI:10.1111/j.1469-185X.1982.tb00376.x

出版商:Blackwell Publishing Ltd    

年代:1982

数据来源: WILEY

ISSN:1464-7931    

DOI:10.1111/j.1469-185X.1982.tb00377.x

出版商:Blackwell Publishing Ltd    

年代:1982

数据来源: WILEY

ISSN:1464-7931    

DOI:10.1111/j.1469-185X.1982.tb00378.x

出版商:Blackwell Publishing Ltd    

年代:1982

数据来源: WILEY