Catastrophes after crossing species barriers

Probably the most tragic examples of virus infections that have caused the deaths of many millions of people in the past century were the influenza and AIDS pandemics. These events occurred as a direct result of the introduction of animal viruses into the human population. Similarly, mass mortalities among aquatic and terrestrial mammals were caused by the introduction of viruses into species in which they had not previously been present. It seems paradoxical that at a time when we have managed to control or even eradicate major human virus infections like polio and smallpox we are increasingly confronted with new or newly emerging virus infections of humans and animals. A complex mix of social, technological and ecological changes, and the ability of certain viruses to adapt rapidly to a changing environment, seems to be at the basis of this phenomenon. Extensive diagnostic and surveillance networks, as well as novel vaccine- and antiviral development strategies should provide us with the safeguards to limit its impact.     

Incompatibility in angiosperms

 Since Darwinian times considerable knowledge has accumulated on the distribution, physiology and genetics of self-incompatibility (SI) in higher plants. In the second half of this century the first attempts were made to identify the biochemical bases of SI. These included thediscovery that cutinase enables pollen tube penetration at the surface* of the stigma in Cruciferae, sorting of segregation pollen S-phenotypes by serological techniques, a lock-and-key model of the SI reaction, the first detection and characterisation of SI proteins and the discovery of the role of the tapetum in the determination of pollen phenotypes in homomorphic sporophytic SI. This pioneering work was followed by a worldwide effort to identify and understand the cellular and molecular processes which lead to the recognition and rejection of SI pollen. The present review article summarizes briefly the current state of knowledge in areas essential for the understanding and exploitation of SI and outlines new information that has become available during recent years. Received: 14 March 1997 / Revision accepted: 10 June 1997     

Dichogamy in angiosperms

We obtained information on dichogamy and other aspects of the biology of over 4200 species of angiosperms from several hundred published and unpublished sources. We used this information to describe patterns of occurrence of dichogamy and to test specific hypotheses relating dichogamy to other characteristics of plants or their environments. Protandry was more common than protogyny at the intrafloral level, but the reverse was true at the interfloral level. Patterns of dichogamy varied significantly among major taxa, with protogyny more common among monocotyledons and primitive dicotyledons, and protandry expecially common in the Asteridae. Arctic species tended to be less dichogamous and more protogynous than temperate and tropical species. Aquatic and alpine species were especially protogynous. Patterns of dichogamy varied among sexual systems, with gynomonoecious and gynodioecious species especially protandrous, and monoecious species highly protogynous. Autogamous and self-compatible species were disproportionately protogynous. Flowers of intraflorally dichogamous species were slightly larger than those of adichogamous species, owing to the presence of many autogamous species in the latter group. Species with interfloral protogyny bore much smaller flowers than did species with interfloral protandry. Early-blooming species in north-temperate and polar regions were disproportionately protogynous. Sexual structures that abscised, shriveled or moved after completion of their function tended to be presented first, and those that facilitated the other sexual function were presented second. A negative association existed between type of intrafloral and interfloral dichogamy in diclinous species. Most animal-pollinated flowers were protandrous, except beetle-pollinated and refuge and trap blossoms. Wind pollination was markedly associated with protogyny. Vertical inflorescences visited by upwardly-moving vectors were protandrous.     

Anther plastids in angiosperms

In the anther of angiosperms, all types of plastids are found in the course of pollen development. They are located in the different cell layers of the microsporangium and have various functions that contribute to the formation of the functional male gametophyte. This includes photosynthesis, stomata opening, sugar storage and/or mobilization, lipid synthesis and secretion for pollenkitt formation, as well as serving as a physiological buffer under stress conditions. They are also involved in plastid inheritance, but to different extents, according to the species. The plastid is a semi-autonomous organelle. Plastid division in the anther is synchronous with cell division, except in the vegetative cell during pollen maturation. Furthermore, recent data seem to show that plastids are affected by programmed cell death and DNA degradation, which occur in the whole anther throughout pollen development. However, the timing of plastid disappearance fluctuates in the different cell layers and also depending on species. In vitro, following androgenesis, plastids that originate in the microspore are responsible for the occurrence of albino plantlets in Poaceae. This trait reflects the relative independence of the plastid genome when compared with that of the nucleus. In this family, microspore plastids may become so involved in programmed cell death that they are unable to follow the alternative sporopohytic program. The different pathways of plastid differentiation in neighboring anther cell layers require an accurate regulation of cell development that remains widely unknown in the anther.     

Late-acting self-incompatibility in angiosperms

In most self-incompatible (SI) plants, pollen tube growth in self-pollinated flowers is inhibited on the stigma or in the style. SI systems that operate in the ovary have been assumed to be extremely rare. Evidence from many plant species is presented to show that the SI barriers in the ovary, described here as late-acting SI systems, are quite common. The late-acting SI systems are divided into four categories: (1) ovarian inhibition of incompatible pollen tubes before the ovule is reached; (2) prefertilization inhibition in the ovule; (3) post-zygotic rejection of the embryo, and (4) ovular inhibition for which the cytological details have not been established. Whether or not post-zygotic incompatibility systems can be distinguished from inbreeding depression depends upon the assumptions underlying the genetic models of self-incompatibility. However, four approaches are outlined that could distinguish between active uniform rejections that are presumably evolved responses to inbreeding depression and the passive, variable failures that are commonly understood to be expressions of typical inbreeding depression. Possible advantages of late-acting SI include an extended period of time over which pollen genotypes may be evaluated by the maternal parent and greater flexibility in the choice of male parents. Due to a paucity of data regarding the genetics and physiology of lateacting SI systems, little can be said at this time about the possible diversity of such systems of their evolutionary relationships with classical gametophytic and sporophytic SI. An hypothesis for the operation of post-zygotic SI is described whereby maternal resources to developing embryos are terminated if the embryo (and/or endosperm) fall below a threshold level of heterosis. This hypothesis is a modification of one first proposed by Westoby and Rice in 1982 to explain variable maternal resource allocation to developing embryos.     

Half tetrad analysis in angiosperms

A biochemical technique involving analysis of endosperm is proposed for detecting meiotic crossovers and for gene mapping in angiosperms with bisporic embryo sacs. In bisporic embryo sac development two spores resulting from meiosis-II division of the same meiosis-I daughter cell contribute two thirds of the total genetic information to the triploid endosperm nucleus, the other third coming from a sperm in the fertilizing pollen grain. In controlled crosses where the marker gene codes for allozymes with phenotypes sensitive to gene dosage, the maternal meiotic contribution to the endosperm nucleus may be determined, thereby allowing crossovers between the marker locus and the centromere to be detected.     

Cold resistance in Antarctic angiosperms

Deschampsia antarctica Desv. (Poaceae) and Colobanthus quitensis (Kunth) Bartl. (Cariophyllaceae) are the only two vascular plants that have colonized the Maritime Antarctic. The primary purpose of the present work was to determine cold resistance mechanisms in these two Antarctic plants. This was achieved by comparing thermal properties of leaves and the lethal freezing temperature to 50% of the tissue (LT50). The grass D. antarctica was able to tolerate freezing to a lower temperature than C. quitensis. The main freezing resistance mechanism for C. quitensis is supercooling. Thus, the grass is mainly a freezing‐tolerant species, while C. quitensis avoids freezing. D. antarctica cold acclimated; thus, reducing its LT50. C. quitensis showed little cold‐acclimation capacity. Because day length is highly variable in the Antarctic, the effect of day length on freezing tolerance, growth, various soluble carbohydrates, starch, and proline contents in leaves of D. antarctica growing in the laboratory under cold‐acclimation conditions was studied. During the cold‐acclimation treatment, the LT50 was lowered more effectively under long day (21/3 h light/dark) and medium day (16/8) light periods than under a short day period (8/16). The longer the day length treatment, the faster the growth rate for both acclimated and non‐acclimated plants. Similarly, the longer the day treatment during cold acclimation, the higher the sucrose content (up to 7‐fold with respect to non‐acclimated control values). Oligo and polyfructans accumulated significantly during cold acclimation only with the medium day length treatment. Oligofructans accounted for more than 80% of total fructans. The degrees of polymerization were mostly between 3 and 10. C. quitensis under cold acclimation accumulated a similar amount of sucrose than D. antarctica, but no fructans were detected. The suggestion that survival of Antarctic plants in the Antarctic could be at least partially explained by accumulation of these substances is discussed.     

Root-nodule formation in non-leguminous angiosperms

Summary The world-wide survey under the IBP of root-nodule formation in non-leguminous Angiosperms is progressing reasonably satisfactorily, and it is anticipated that when all the results have been collated a useful body of new data will be yielded. In recent studies, also forming part of the IBP, in the author's laboratory, the nodules of further species in the genera Alnus, Myrica, Ceanothus, Coriaria and Dryas have been examined for nitrogen-fixing properties, with positive results. Also the extent to which the nodule endophytes from species of Alnus and Myrica respectively are able to symbiose satisfactorily with other host species in the same genus has been investigated, and the conclusion reached that especially in Myrica there is very considerable specialisation among the endophytes. A marked diurnal variation in the rate of fixation of nitrogen in the nodules of non-legumes growing in a glasshouse lit by daylight has been found, with maximal rates being attained around midday. The implication is that this is the period of the maximal availability of carbohydrates in the nodules, but actual analyses have so far failed to reveal this. Analyses of the amino acid composition of the nodules in several genera have shown that except in Alnus, where citrulline is prominent, asparagine is in most cases the dominant amino acid.     

Aperture pattern ontogeny in angiosperms

Pollen grains display a wide range of variation in aperture number and arrangement (pattern) in angiosperms. Apertures are well-defined areas of the pollen wall surface that permit pollen tube germination. For low aperture numbers, aperture patterns are characteristic of the major taxonomic divisions of angiosperms. This paper presents a developmental model that explains most of the aperture patterns that are recorded in angiosperms. It is based on the analysis of the different events that occur during meiosis and lead to microspore differentiation. It demonstrates that variation occurring during meiosis in angiosperms is sufficient to produce the core morphological set of the most commonly observed pollen morphologies.