摘要:

SummaryI. An arbitrary classification of avian eggshells is proposed.2. The role of the eggshell in conserving the water in eggs at oviposition is discussed. There is as yet no correlation between this property and the pore systems in avian eggshells.3. The pore systems may act as diffusion pathways and the hypothesis has been advanced that in many eggs the shells are adapted so that restriction of gaseous diffusion by mud, preening oils and nest debris is prevented.4. The mechanical properties of the shell are considered in the novel context of defence against (a) attrition that could lead to the pores being blocked with dust, and (b) cracking that would destroy the diffusion pathways noted in 3.5. The overall objective of the review was to discuss the concept that avian eggshell are adapted to fit an egg to the nest environment.

ISSN:1464-7931    

DOI:10.1111/j.1469-185X.1982.tb00362.x

出版商:Blackwell Publishing Ltd    

年代:1982

数据来源: WILEY

摘要:

Summary1. Many arthropods are predators of vertebrates: four orders of the class arachnida, six orders of insecta, five orders of crustacea and one order of chilopoda include species that have been reported to eat vertebrates. At the population level, some arthropods are responsible for significant mortality among some vertebrates.2. Arthropods are well equipped for this type of predation; many are larger than vertebrates (approximately 20% of the vertebrate fauna of eastern North America is less than 10 cm in length), they may hunt in social groups and many have toxins or other adaptations that increase predatory efficiency.3. Several arthropod predators and vertebrates may be involved in cross predation, the species eating each other. The switch in the role of predator and prey occurs during ‘ontogenetic reversal’ as the vertebrate grows from small and vulnerable to large and predaceous. Cross predation decreases the future risk for one's self or offspring.4. The opportunity for arthropod predation on vertebrates exists in many communities, but a review of some food webs catalogued by Cohen (1978) indicates that this particular link may be easily overlooked. Some arthropods should be investigated as potential predators of vertebrates.5. The information available from the analysis of feeding interactions in a community should be an important link between field and theoretical ecology; however, most food webs are probably underestimates of the complexity that is commonpl

ISSN:1464-7931    

DOI:10.1111/j.1469-185X.1982.tb00363.x

出版商:Blackwell Publishing Ltd    

年代:1982

数据来源: WILEY

摘要:

Summary1. Different plant species show considerable variation in their selenium content. Primary indicators, also termed selenium accumulators, many of which are members of the genusAstragalus, are highly tolerant of selenium; they are known to contain tissue levels of several thousand µg selenium/g. Secondary indicators, tolerant to low concentrations of the element, may absorb up to 1000 µg selenium/g. Non‐accumulators are poisoned by selenium.2. The toxicity of selenate (SeO4‐) and selenite (SeO3‐) to most plants can be attributed to a combination of three factors. Firstly, selenate and selenite are readily absorbed from the soil by roots and translocated to other parts of the plant. Secondly, metabolic reactions convert these anions into organic forms of selenium. Thirdly, the organic selenium metabolites, which act as analogues of essential sulphur compounds, interfere with cellular biochemical reactions.3. Incorporation into proteins of the amino acid analogues selenocysteine and selenomethionine, in place of the equivalent sulphur amino acids, is considered to be the underlying cause of selenium toxicity. The physical and chemical differences between selenium and sulphur will result in small, but significant, changes in the biological properties of a selenium‐substituted protein.4. Selenium‐tolerant accumulator plants differ in at least two respects from sensitive species. Large quantities of Se‐methylselenocysteine and selenocystathionine, two non‐protein selenoamino acids rarely detected in non‐accumulators, have been isolated from the tissues of selenium accumulators. In addition, selenium is kept from entering proteins so that the selenium levels in proteins of accumulator plants is significantly lower than the levels in selenium‐sensitive plants.5. Exclusion of selenium from the proteins of accumulators is thought to be the basis of selenium tolerance. Discrimination against selenocysteine during protein synthesis seems to prevent incorporation of this selenoamino acid into proteins of accumulators. Furthermore, synthesis of Se‐methylselenocysteine and selenocystathionine, which results in diversion of selenium away from the synthesis of selenomethionine, will restrict the amount of this compound available for protein synthesis.6. Selenium accumulation among unrelated plant genera is a striking example of convergent evolution. The possibility that accumulation of this element is associated with a nutritional requirement for selenium, although explored in the past, is still in need of

ISSN:1464-7931    

DOI:10.1111/j.1469-185X.1982.tb00364.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.tb00365.x

出版商:Blackwell Publishing Ltd    

年代:1982

数据来源: WILEY

摘要:

Summary1. An understanding of the adaptive strategies of colonizing animal species depends upon an integration of population genetics and ecology, but behavioural components should not be ignored especially higher in the phylogenetic series than insects.2. An ecologically marginal habitat from which colonists are derived can be regarded as one in which physical stresses of climatic origin tend to be variable and extreme, so that resources are unpredictable and short lived. Prerequisites for genetic analysis are therefore phenotypes relatable to ther Kcontinuum of adaptive strategies. These can be called ‘ecological phenotypes’.3. Ecological phenotypes include tolerances to environmental stresses, development time, and resource utilization variability. Such phenotypes enable distinctions to be made between colonizing species and non‐colonists. For example, colonizing species have ecological phenotypes incorporating high resistance to physical stress, rapid development time, and the exploitation of an array of food resources. They are ecologically versatile generalists. This includes the use of ethanol as a resource inDrosophila.4. There is a substantial literature on variation in central and marginal populations based upon gene and chromosome polymorphisms. Most data show a reduction of chromosome polymorphisms and of lethals and semilethals towards the margins, but no equivalent reduction in enzyme polymorphisms. Widespread species tend to have low levels of chromosome polymorphisms as in marginal populations, but enzyme polymorphism levels vary too much among species for meaningful interpretations. Since these are genotypic assessments not directly relatable to the field situation, the somewhat unsatisfactory nature of these data from the interpretative point of view is understandable. In addition, the fundamental issue is not the variability of the genome, but the nature and role of loci controlling ecological phenotypes.5. Ecological phenotypes can be analyzed at the population level with isofemale strains as the starting material. In theory, genetic activity can then be localized to the chromosomal and even genic levels in a species such asD. melanogaster.Isofemale strain studies inD. melanogasterare interpretable in terms of ther Kcontinuum, and so reflect adaptive strategies in nature.6. It is highly likely that the genetic architecture of ecological phenotypes of marginal populations mainly comprises a few additive genes of relatively large effect. This is an architecture permissive of rapid adaptation to new habitats, provided that the appropriate genes are present. Discussions of speciation via the founder principle, a colonization event in itself, have invoked a similar explan

ISSN:1464-7931    

DOI:10.1111/j.1469-185X.1982.tb00366.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.tb00367.x

出版商:Blackwell Publishing Ltd    

年代:1982

数据来源: WILEY