摘要:

Summary1. For a number of years considerable attention has been given both in the scientific and lay press to dramatic advances in the rapid clonal multiplication of plants by means of so‐called ‘modern’ tissue culture techniques. Because the words clone, cloning and tissue culture mean different things to different people, there is considerable confusion even among biologists as to what has actually been achieved.2. An historical approach has been used to put the accomplishments into perspective.3. Organ culture, tissue culture, cell or suspension culture and protoplast culture all offer varying degrees of potential for clonal multiplication but the most substantial progress has been made with explanted stem tips or lateral buds which can be stimulated to produce numerous precocious axillary branches. These, in turn, can be separated or subdivided and induced to root and thus yield populations of genetically and phenotypically uniform plantlets.4. Similarly, undifferentiated calluses can sometimes be induced to form shoots and/or roots adventitiously. This method of stimulating organ development on callus is based on the now classic work of Skoog and Miller done in the I950s with theWisconsin 38strain of a ‘Havana’ cultivar of tobacco. Because of the growing criticism that when callus cultures are maintained for more than a few subcultures genetic or chromosomal aberrations are induced and these nullify the ultimate goal ‐ namely to get large numbers of genetically identical plants ‐ callus procedures are generally being relinquished in favour of precocious axillary branching methods.5. The number of plantlets obtainable through the procedures with precocious axillary shoots or callus culture are fewer, in theory, than the number of plantlets obtainable from a somatic embryo system starting with cells grown in suspension culture. But the fact is that cell culture techniques required to produce somatic embryos are also demanding and very much in their infancy as far as their applicability to clonal multiplication is concerned.6. Even so, slowly but steadily, advances are being made in learning how to stimulate formation of somatic or adventive embryos from totipotent cells grown in suspension culture. In cases where it has proven feasible, not only may somatic embryos be generated in great quantities, but their discrete bipolar nature often obviates the need for time‐consuming, laborious procedures to assure their success in a conventional soil or greenhouse setting.7. While protoplast cultures are of very limited direct use for clonal propagation of higher plants, the procedures hold some hope as a vehicle whereby select useful genes can be introduced into plants. Parasexual hybridization, that is the production of hybrids by fusing totipotent protoplasts of different origin, offers no imminent prospects for the development of useful and novel plant types but may be applicable in the long term in very specific situations.8. It is concluded that many problems exist in the producing and growing of totipotent or morphogenetically competent cell suspensions, but the potential benefits that could accrue when investigators learn to overcome these problems far outweigh any disadvantages that they may have. In short, cloning via cultured free cells should be seen in the context of potentialities rather t

ISSN:1464-7931    

DOI:10.1111/j.1469-185X.1982.tb00368.x

出版商:Blackwell Publishing Ltd    

年代:1982

数据来源: WILEY

摘要:

Summary1. A new classification system for chemo‐orientation mechanisms is suggested, based on delineating (a) internal and external orientation information available to an organism, (b) mode of information processing, (c) type of motor output pattern, and (d) guidance systems that modulate or direct motor output.2. Cross‐phyletic comparison of chemo‐orientation indicates that morphology, to a greater extent than phylogeny, determines the kind of chemo‐orientation mechanisms available to an organism. Analogies are drawn between single‐celled organisms with surface chemical receptor sites and higher organisms with elaborate chemosensory organs and nervou

ISSN:1464-7931    

DOI:10.1111/j.1469-185X.1982.tb00369.x

出版商:Blackwell Publishing Ltd    

年代:1982

数据来源: WILEY

摘要:

Summary(1) The savanna ungulate faunas of the North American Miocene were broadly similar to those of present‐day East Africa in terms of overall morphological and taxonomic diversity. However, the predominant ungulates of the African faunas are bovids, which possess bony horns that are primitively sexually dimorphic in their occurrence. The predominant ungulates of the North American Tertiary were equids, camelids and oreodonts, which all lacked horns. A limited number of horned ruminants were present, but these were largely Miocene immigrants from Eurasia. Horns were also absent from the large‐bodied herbivores in the endemic faunas of South America and Australia.(2) The absence of horns in equids and tylopod artiodactyls is unlikely to be due to genetic insufficiency. Bony horns were present in brontotheres, which were closely related to equids, and in protoceratids, which were closely related to camelids. Nasal horns were present in one oreodont genus.(3) Studies on living ungulates show that a strong correlation exists between habitat type, feeding behaviour, social behaviour and morphology. It is possible to use the morphological remains of extinct ungulates to reconstruct the types of feeding and social behaviour, and to use the distribution of morphologies and body sizes in a community of mammals, in conjunction with geological and paleobotanical evidence, to reconstruct the type of habitat.(4) The importance of the post‐Eocene climatic changes to the history of mammalian evolution is stressed. Continents at higher latitudes have become increasingly seasonal in terms of temperature and rainfall since the equable global conditions of the early Tertiary. Savanna mosaic were the predominant biome in North America by the early Miocene, and in Eurasia by the middle Miocene. Living temperate‐latitude species of ungulates may not be a reliable guide for the assessment of the interrelationship between behaviour and morphology in an evolutionary perspective, as their behaviour may have been recently adapted to a habitat type that has only been in existence since the Pleistocene.(5) The primitive condition in eupecorans and protoceratids is the absence of horns, with the presence of large sabre‐like canines in the males. The first horned members of these divisions had horns in the males only. Small present‐day antelope, where horns may also be present in the females of the species, are probably secondarily small.(6) Horns were acquired independently in ruminant artiodactyls at least three times, and a maximum number of seven times is not unlikely. In each case, horns first appeared at a critical body weight of about 18 kg, and in correlation with a change in habitat from closed to open woodland.(7) Horns in living ruminants are associated with territorial defence by males holding exclusive feeding and reproductive territories in woodland habitats. Such behaviour in present‐day antelope is correlated with a body size of greater than 15 kg and a folivorous diet. It is argued that horns evolved in ruminant artiodactyls on the adoption of this type of territorial behaviour once the critical combination of body size, diet and habitat type had been attained in their evolution from small, essentially frugivorous, forest‐dwelling animals.(8) Perissodactyls never evolved sexually dimorphic bony horns of the type seen in ruminant artiodactyls. This is because their foraging and digestive strategies necessitate a larger daily intake of food. In a woodland habitat they were never able to adopt a feeding area small enough to make exclusive territory maintenance an economical proposition. Territory holding in male perissodactyls is seen, but under the opposite conditions of habitat to territorial behaviour in ruminant artiodactyls.(9) Study of the morphology and paleoecology of oreodonts suggests that they were woodland herd‐forming browsers with exclusively folivorous diets. They probably had some forestomach fermentation, but did not chew the cud. Similar studies of Tertiary camelids suggest that they were predominantly selective browsers eating herbage at a low level in open country and formed mixed‐sex feeding groups. These combinations of feeding and social behaviour suggest a more open structure of the mid‐Tertiary habitat in North America than in Eurasia.(10) Studies of the behaviour and morphology of living members of the Ruminantia, and of the morphology and paleoecology of their fossil ancestors, suggest that they were primitively tree browsers living in closed woodland habitats. Such habitats were abundant in the Old World, but in limited supply in North America during the Oligocene, where the protoceratids were the only ungulates to parallel the eupecoran type of feeding and social behaviour. South America appears to have had an even more open habitat in the Oligocene than North America, and no parallel to the eupecorans was seen amongst the indigenous ungulates. The radiation of the Bovidae into open grassy habitats in the Pliocene may have been dependent on the immigration of grazing equids into the Old World.(11) I conclude that there was a difference in habitat structure between North America and the Old World during the Tertiary. The food resources in North America were more widely dispersed, and this may have been the result of the trees being more widely spaced. A possible causal mechanism for this was the stable land mass of the North American continent during the Tertiary, resulting in a more continental climate, with a more severe effect of the post‐Eocene seasonality on the vegetation. The faunal record of the two continents also implied a greater density of trees in the Old World.(12) Thus most endemic North American ruminants did not evolve horns because, at the critical combination of body size and diet seen in the evolution of horns in the Old World ruminants, the dispersal of the food resources within the vegetation was too great for an effective home range to be maintained as an exclusive territory.(13) Attention is drawn to the dangers of constructing evolutionary stories about living animals without primary reference to the fossil record to see if the hypotheses are upheld, and of assuming that fossil animal communities can be made to fit models

ISSN:1464-7931    

DOI:10.1111/j.1469-185X.1982.tb00370.x

出版商:Blackwell Publishing Ltd    

年代:1982

数据来源: WILEY

摘要:

Summary1. Many insects reproduce by parthenogenesis. In one of the largest orders of the animal kingdom, the Hymenoptera, most of its members reproduce by arrhenotokous parthenogenesis. Egg activation in parthenogenetic animals obviously cannot be caused by fertilization of the egg. The question of what initiates egg development in parthenogenetically reproducing animals has been studied for a few insect species and is discussed in this article.2. The grasshopperMelanoplus differentialisis one of several Orthoptera displaying accidental parthenogenesis. In this species, egg laying provides the stimulus to the completion of meiosis and start of embryonic development in unfertilized and probably also in fertilized eggs. The same holds true for the dipteran insectDrosophila melanogasterwhich exhibits rudimentary parthenogenesis, and forD. mercatorumshowing accidental parthenogenesis. The precise way in which oviposition affects the egg is unknown.3. The stick insectCarausius morosusreproduces by obligatory thelytoky. The triggering factor for removal of the meiotic block and initiation of embryonic development is oxygen from the air which penetrates to the egg through the micropyle immediately after oviposition. The oviposition act itself is not necessary for activation of the egg.4. Comparative studies of the different types of oogenesis in the dipteran insectHeteropeza pygmaeashow that in paedogenetically developing follicles meiotic arrest in prophase is of very short duration and a meiotic block at the end of oogenesis is absent. It is suggested that in this case triggering events for egg development are dispensable. On the other hand, under certain experimental conditions a meiotic block can be established in some of these follicles.5. Investigations on the Ichneumonid waspPimpla turionellaehave shown that unfertilized, male‐determined eggs ‐ and most likely also fertilized, femaledetermined eggs ‐ are activated by mechanical stress exerted on the eggs during natural or imitated oviposition. This mechanical stress, in addition, activates a streaming system which is independent of meiotic completion and nuclear multiplication. Egg activation by egg distortion is also found in the Pteromalid species Nasonia vitripennis and occurs presumably in many other Hymenoptera.6.Carausius morosus, Pimpla turionellae and Nasonia vitripennisare species with parthenogenetic reproduction for which the natural factors responsible for the initiation of egg development have been identified. The cases of Pimpla turionellae and Nasonia vitripennis are of particular interest because of the feasibility of artificially imitating the natural activating mechanism.7. It is concluded that apart from fertilization various events at oviposition may trigger egg development. In addition, the occurrence of rudimentary parthenogenesis in many sexually reproducing animal species suggests that sperm entry and fertilization may frequently be necessary for the continuation of egg development rather than for its initi

ISSN:1464-7931    

DOI:10.1111/j.1469-185X.1982.tb00371.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.tb00372.x

出版商:Blackwell Publishing Ltd    

年代:1982

数据来源: WILEY