Showing papers in "Biological Reviews in 1982"

Selenium: toxicity and tolerance in higher plants

Abstract: Summary 1. Different plant species show considerable variation in their selenium content. Primary indicators, also termed selenium accumulators, many of which are members of the genus Astragalus, 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 further clarification.

Coefficients of association and similarity, based on binary (presence‐absence) data: an evaluation

Abstract: Summary Forty-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); A17 yields 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 genus Chaetomium, 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 among Chaetomium, and A34, A14, possibly also A36 and A40 seemed 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 (Kulczynski), 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 out whenever possible

The form and function of metal-containing ‘granules’ in invertebrate tissues

Abstract: 646 . . . . . . VI . Variation in composition of metal-containing granules . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . VI I. Formation of metal-containing granules ( I ) Intracellular metal-containing granules . . . . . . . . 650 (2) Extracellular metal-containing granules . . . . . . . . 655 . . . . . . . . . . . . . VIII. Granule function 656 659 . . . . . . . . . . IX. General discussion and summary X. Acknowledgements . . . . . . . . . . . . 661 XI. References . . . . . . . . . . . . . . 661

Evolution of horns in ungulates: ecology and paleoecology

Abstract: 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 of existing communities.

Environmental effects on animals used in biomedical research.

Abstract: Summary I. Reliability of the results of bio-medical research clearly depends upon the animals used showing as standard responses as is possible. 2. The majority of animals used in this field are small, homoiothermic mammals which have sensitive and strong homeostatic mechanisms. If a change in ambient conditions is of sufficient magnitude to unbalance homeostasis, then the neuroendocrine system is stimulated so as to restore it, and this can interfere with the response to test conditions or agents. 3. The homeostatic effectors involved are diverse and can include both physiological and behavioural changes in the animal. These can affect metabolic rate, body temperature, activity, food consumption, hormone concentration, wake/sleep patterns, maturation, posture, lactation and many other bodily functions. Any of these changes is potentially capable of influencing experimental results. 4. The evidence presented shows how environmental factors may affect the outcome of experiments in the fields of animal behaviour, cancer research, immunology, pathology, pharmacology, psychology, reproduction, teratology and toxicology; particular attention is paid to the effects of ambient temperature, relative humidity, air movement and quality, light and sound. 5. While a constant, reproducible environment would be ideal, there is little possibility of controlling all the variables; nevertheless all investigators should minimize those environmental variables that have been shown to be important. 6. To enable other investigators to repeat experiments or carry out comparative studies, environmental conditions pertaining during an experiment should be adequately described in any publications.

Patterns of parental care and parental investment in marsupials

Abstract: I. Information on growth, development and care of young has been assembled for 62 species of marsupial.

Arthropods that prey on vertebrates

Abstract: Summary 1. 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 commonplace.

Form and function in marine phytoplankton

Abstract: Summary What is presented here is a tentative synthesis of morphological, cytological, physiological and ecological data on planktonic algae, which I hope will help in the understanding of mutual relationships. Emphasis is put on the marine phytoplankton although effort has been made to include the more significant limnological information. (1) All the algal classes, but two, are present in the marine plankton – which makes 13 classes. Many or most of them possess one or several features that are commonly viewed as animal characters, and so Bacillariophyceae (diatoms) are the more typical ‘algae’ in marine plankton. Coincidently or not, they have received much more attention than any other class. (2) Both structurally and morphodynamically, colonies of cells often appear as something else or something more than sums of cells. (3) There is a profuse variety of flagellar types and flagellar appendages, whose functional significance is open to investigation. In general, swimming velocity (ca. 1 m h-l) exceeds sedimentation rate (ca. 0·7 m day-1) by one order of magnitude or more. (4) Very few phytoplankters can truly be described as “naked”. Cell coverings fall into eight major categories which differ by chemical composition, structure and ontogeny. An additional, external organic coating may be widespread. Mucus and microfibrils may also be more common than previously thought. (5) The variability of chloroplast morphology and ultrastructure has not been explored for functional relationships. (6) Evidence for the presence of intracellular bacteria and viruses is rapidly increasing. (7) The suspension of algal cells in the medium depends on a number of morphological factors whose effects are often opposite. The sinking rate increases with increasing cell size and is maximum for spheroid (not spherical) shapes; colonies sink faster than the individual cells. The incidence of various shapes, appendages, mucilage and cell orientation is, essentially, intricate and/or insufficiently known. Lipidic inclusions are no longer viewed as floating devices but the ancient theory of ionic exchange has been revived. As now understood, the suspension of phytoplankton is no more a matter of floating, but rather exploring different layers and being tossed around by physical entrainment. However, questions remain the same: how do the “morphological adaptations” contribute to this, and how do different forms compare to each other? (8) As far as the absorption of nutrients is concerned, there is an advantage for phytoplankters to be small and either motile or rapidly sinking. The permeability of the various cell coverings has been ignored. The advantage of being small is confirmed by the consideration that growth rates and all the metabolic rates decrease with increasing size. However, the balance of photosynthesis against respiration for varying sizes is a complex problem. (9) After an extensive review of the literature, the existence of several “shade species” is confirmed (without ensuring that light is the responsible factor). These taxa exhibit the full range of shapes, sizes, structures and behaviour, so that the relevant morphological adaptations, if any, are at least polymorphic. (10) Although grazing certainly moulds the size spectrum and the algological spectrum of phytoplankton to a large extent, the effect of a given morphological feature can hardly be generalized, except that long appendages, mucilages, and probably colonies discourage grazers. The role of bioluminescence and trichocyst expulsion may also be considered. (11) The hope of correlating cell size with a single factor of the environment, whether temperature or something else, should be abandoned since many more factors are involved. On the other hand, multi-parametric models can justify or predict which cell size predominates under a given set of conditions (note the works of H. J. Semina and co-workers, T. R. Parsons and M. Takahashi, and E. A. Laws). (12) The ratio of surface area to volume of the cell is a meaningful physiological index. Its relative conservation among the vagaries of sizes and shapes, and its ecological regulation need further investigation. (13) Considering the profusion of data that has accumulated on the structure and functioning of planktonic algae, and realizing that sophisticated techiques are available for both kinds of studies, now is the time, it seems, for fruitful research into the relationships between form and function. Such research will certainly increase our understanding of specific variability, adaptation and diversity.

Phoresy as migration ‐ some functional aspects of phoresy in mites

Abstract: Summary I. 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 by r-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 treatments of migration.

Properties of avian egg shells and their adaptive value

Abstract: Summary I. 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.

Cloning higher plants from aseptically cultured tissues and cells

Abstract: A review of aseptic culture methods for higher plants is presented, which focuses on the existing problems that limit or prevent the full realization of cloning plants from free cells. It is shown that substantial progress in clonal multiplication has been made with explanted stem tips or lateral buds which can be stimulated to produce numerous precocious axillary branches. These branches can then be separated or subdivided and induced to root in order to yield populations of genetically and phenotypically uniorm plantlets. Similarly, undifferentiated calluses can sometimes be induced to form shoots and/or roots adventitiously. Although the cell culture techniques required to produce somatic embryos are presently rudimentary, steady advances are being made in learning how to stimulate formation of somatic or adventive embryos from totipotent cells grown in suspension cultures. It is concluded that many problems exist in the producing and growing of totipotent or morphogenetically competent cell suspensions, but the potential benefits are great.

Ultrastructure of plastid inheritance: green algae to angiosperms

Abstract: Summary 1. 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 as Chlamydomonas are 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 other evolutionary events.

Egg activation and parthenogenetic reproduction in insects

Abstract: Summary 1. 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 grasshopper Melanoplus differentialis is 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 insect Drosophila melanogaster which exhibits rudimentary parthenogenesis, and for D. mercatorum showing accidental parthenogenesis. The precise way in which oviposition affects the egg is unknown. 3. The stick insect Carausius morosus reproduces 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 insect Heteropeza pygmaea show 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 wasp Pimpla turionellae have 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 vitripennis are 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 initiation.

Environmental instability and community diversity

Abstract: Early attempts to explain community structure were based upon the idea that species distributions were controlled by physicochemical factorsofthe environment. This was an entirely logical and necessary first step in the development of community ecology. In freshwater plankton for example, much effort has been expended trying to relate biota to a variety of water qualities: Noland (1925) and Ward ( I 940) did early work in this area. Some studies found correlations between p H (Carter, 1971; Sprules, 1975), salinity (Moore, 1952; LaBarbera & Kilham, 1974), or temperature (Ward, 1940) and the distribution of certain species. But in general, most species have rather broad tolerance ranges and, therefore, measurements of physicochemical parameters have little predictive value except at extreme ranges (Macan, 1961 ; Yongue et a l . , 1973; Moore, 1978; Bamforth, 1980).

Sound and its significance for laboratory animals

Abstract: Summary 1. Several methods of varying accuracy have been used to assess what sounds small laboratory animals such as rodents are capable of hearing. Most rodents can detect sounds from 1000 Hz (the frequency of the Greenwich Time Signal) up to 100000 Hz, depending on the strain, with usually one or more commonly two peaks of sensitivity within this range. Dogs can detect sound most easily from 500 Hz to 55000 Hz, depending on the breed. 2. Rodents also produce sound signals as a behavioural response and for communication in a variety of situations. Ultrasonic calls in the range 22000–70000 Hz are the main communicating pathway during aggressive encounters, mating, and mothering. Similar calls have also been recorded from isolated animals associated with inactivity, rest and possibly even sleep. 3. Very loud sounds cause seizures in rats and mice, or can make them more susceptible to other sounds later in life. This effect is possible even when animals are fully anaesthetized. Sound tends to startle and reduce activity in several species of animal. Even offspring of mice that have been sound-stressed exhibit abnormal behaviour patterns. Sounds also elicit various responses in rats from increasing aggression to making them more tolerant to electric shocks. 4. Levels of sound above 100 dB are teratogenic in several species of animals and several hormonal, haematological and reproductive parameters are disturbed by sounds above 80 dB. When rats are chemically deafened the disturbance to their fertility disappears. Lipid metabolism is disrupted in rats when exposed to over 95 dB of sounds, leading to increases in plasma triglycerides. Atherosclerosis can be produced in rabbits by similar levels of sound. 5. It has also been shown in guinea pigs and cats that hearing damage is governed by the duration as well as the intensity of the sound and is irreversible. Work on chinchillas hs demonstrated that sounds above 95 dB lead to this injury, but that sounds of 80 dB have no permanent effect on hearing sensitivity.

Adaptive strategies of colonizing animal species

Abstract: Summary 1. 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 the r K continuum 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 in Drosophila. 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 as D. melanogaster. Isofemale strain studies in D. melanogaster are interpretable in terms of the r K continuum, 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 explanation.