ISSN:1464-7931    

DOI:10.1111/j.1469-185X.1969.tb00819.x

出版商:Blackwell Publishing Ltd    

年代:1969

数据来源: WILEY

ISSN:1464-7931    

DOI:10.1111/j.1469-185X.1969.tb00820.x

出版商:Blackwell Publishing Ltd    

年代:1969

数据来源: WILEY

摘要:

Summary1. The bulk of the fixed carbon which moves from autotroph to heterotroph in most symbiotic associations is in a single compound, a carbohydrate. Techniques employing14C have been most valuable for investigating this movement.2. Most ‘zoochlorellae’ belong to the Chlorococcales, and they release carbohydrate to the animal tissue as either glucose or maltose. In some molluscs, the ‘zoochlorellae’ are actually chloroplasts, possibly derived from siphonaceous algae. Although it is known that these chloroplasts supply photosynthetically fixed carbon to the animal tissue, the form of the carbon compounds which move is not known. InConvoluta roscoffensisthe ‘zoochlorellae’ belong to the Pyramimonadales, but carbohydrate movement has not yet been directly studied in this association.3. Most ‘zooxanthellae’ belong to the Dinophyceae. In associations involving co‐elenterates and molluscs, glycerol is the main carbohydrate moving to the animal. Homogenates of the host animal tissue stimulate excretion by isolated zooxanthellae.4. In lichens, symbiotic blue‐green algae release glucose to the fungus, but the various genera of green algae that have been studied all release polyols (either ery‐thritol, ribitol or sorbitol). Lichen fungi rapidly synthesize mannitol from all these compounds. When lichen algae are isolated into pure culture, they soon lose the ability to excrete carbohydrate, and intracellular production of the carbohydrate that is excreted either becomes much reduced, or ceases altogether.5. Mostly indirect evidence indicates that sucrose is the main carbohydrate moving from flowering plants to their associated symbiotic fungi. Diversion of the translocation stream towards the site of the association occurs. The fungi convert host sugars to their own carbohydrates, principally trehalose and polyols.6. ‘Saprophytic’ higher plants are all obligately mycotrophic and receive carbohydrate from their associated fungi. In at least some associations, the fungus is simultaneously associated with an autotrophic higher plant, which is the ultimate source of carbohydrate for the association.7. Some parasitic higher plants possess chlorophyll, but the extent to which they depend on their host for carbohydrate varies with different species. Green mistletoes evidently derive negligible carbon from their hosts, but other green parasites derive at least some. There is no evidence that any of the chlorophyll‐containing parasites export carbohydrate back to their hosts. Parasitic higher plants which lack chlorophyll presumably derive all their carbohydrates from their hosts, but experimental investigations of this are scarce.8. Comparison between different types of symbiotic association show that a number of common features emerge.9. The algal symbionts of both invertebrates and lichens have, in comparison to free‐living forms, reduced growth rates and greater incorporation of fixed carbon into soluble carbohydrates. They excrete a much greater proportion of their fixed carbon than free‐living forms, and most of it is usually as a single carbohydrate. Particularly striking is the fact that the excreted carbohydrate is one which is either not the major intracellular carbohydrate, or one which ceases or nearly ceases to be produced in culture.10. The translocation stream of autotrophic higher plants is diverted towards the site of association with either fungi or parasitic higher plants, but it is not known how this is achieved.11. In all associations, the cell walls of the autotroph become reduced or modified at the site of contact with the heterotroph, but it seems likely that this is not directly connected with the mechanism of carbohydrate transfer between the symbionts.12. In many associations, the heterotroph rapidly converts host sugars into other compounds (frequently into its own carbohydrates which are usually different from those of the host). This may serve to maintain a concentration gradient and so ensure a continued flow from the host.13. Polyols feature prominently in symbiotic and parasitic associations, not only as the carbohydrates of many plant heterotrophs, but also as the form of carbohydrate released by both zooxanthellae and the green algae of lichens

ISSN:1464-7931    

DOI:10.1111/j.1469-185X.1969.tb00821.x

出版商:Blackwell Publishing Ltd    

年代:1969

数据来源: WILEY

ISSN:1464-7931    

DOI:10.1111/j.1469-185X.1969.tb00822.x

出版商:Blackwell Publishing Ltd    

年代:1969

数据来源: WILEY

摘要:

Summary1. Interpretation of structural evolution in a group such as the Sarcopterygii requires consideration of a combination of all possible functions, rather than single functions.2. The Dipnoi are probably more closely related to the Crossopterygii than to other groups of fishes. The Sarcopterygii are a ‘natural’ group. Certain characters in common between the elasmobranchs and the Dipnoi or Coelacanthini seem to be the result of convergent evolution.3. Evolution of the skull, in connexion with both respiratory and feeding mechanisms, has resulted in extreme specialization in all Sarcopterygii. The crossopterygian intracranial kinesis has evolved from an earlier mobility between the skull and neck and is adapted for increasing the power of the bite and for enclosing the prey from both above and below, in addition to other factors. Adaptive radiation is seen in the feeding mechanisms of all forms. The evolution of the Amphibia proceeded through elongation of the anterior division of the skull (which is not correlated with any changes in brain morphology) and loss of the kinetic mechanism in this sequence is at least partially associated with improved buccal pumping mechanisms for lung ventilation.4. Adaptive radiation of the respiratory system in Dipnoi shows a progressive increase in the use of aerial respiration. The aquatic condition seen inNeoceratodusis probably secondary. Comparison of the three living genera shows a striking correlation between respiratory physiology and habit. There is little indication of reduction of the branchial respiratory system in known Rhipidistia, in which respiration was probably primarily aquatic. In Dipnoi and Rhipidistia, evolution of the lung allowed a partial control of the hydrostatic properties of the body. In coelacanths, aerial respiration was abandoned, except in certain secondarily freshwater forms, and the single lung is modified as an organ of hydrostatic balance. These changes are reflected in the over‐all body proportions.5. Locomotion in Sarcopterygii (except the coelacanthsLaugiaandPiveteauia) is adapted for contact with the substrate in relatively shallow water in most cases. Adaptive radiation of the locomotor apparatus is seen with respect to the relative roles and functions of the paired and unpaired fins, over‐all body shape, caudal fin shape, and absolute size. An important function of the pectoral fins in advanced Rhipidistia was in supporting the body in shallow water and thus aiding lung ventilation.6. Aestivation is an early feature of dipnoan biology, but was not evolved in Rhipidistia. The common faculty of urea production via the ornithine cycle and urea retention in coelacanths and dipnoans are adaptations to conditions in which the body tissues may become dehydrated (salt water and desiccation, respectively). The common pattern of nitrogen metabolism seems to have evolved during a marine phase in sarcopterygian evolution.7. There is evidence that the earliest members of all sarcopterygian lines included marine forms. However, the subsequent major radiations of Dipnoi and Rhipidistia occurred in fresh waters. The distribution of Sarcopterygii was entirely tropical. The late Palaeozoic distribution of the freshwater forms seems to offer evidence for the occurrence of Continental Drift. Coelacanths were primarily coastal fishes.8. The evolution of a major group of organisms requires a different pattern of evolutionary change than that by which adaptive radiations are produced. It evolves the structural and temporal correlation of modification in a number of different functional systems rather than the separate modification of each system without reference to other systems.9. The first tetrapods evolved in a highly seasonal swampy environment on the shores of inland lakes or rivers, in permanently moist con

ISSN:1464-7931    

DOI:10.1111/j.1469-185X.1969.tb00823.x

出版商:Blackwell Publishing Ltd    

年代:1969

数据来源: WILEY