Size and fecundity variation in populations of Poecilia vivipara Block & Schneider (Teleostei; Poeciliidae) inhabiting an environmental gradient

The reproductive variables [female size (geometric centroid size), fecundity and offspring size (longest diameter of the embryo)] of the livebearer Poecilia vivipara in the quaternary lagoons of northern Rio de Janeiro State varied significantly among sites and were partially associated within and among sites. Mean female size was directly related to salinity. Fecundity (number of offspring) was directly related to female size both within and among sites. The female size × fecundity relationship, however, was not constant among sites. Offspring size was partially and directly associated with female size and salinity. The relationship between fecundity and female size was positively related with fecundity, suggesting a non-linear relationship of these variables when the total sample was considered. The results suggest that life-history differences among sites were more related to different predation regimes than to simple effects of plasticity caused by increased growth in saltwater environments.     

Feeding and energy budget estimations inDaphnia obtusa

The reproductive and growth strategies ofDaphnia obtusa are discussed in terms of energy budget. The results obtained through feeding and life table experiments allow us to understand the role of biotic factors, such as competition and predation, in the evolution of life history traits.     

Seasonal plasticity in growth and development of the speckled wood butterfly, Pararge aegeria (Satyrinae)

Regulation of growth and development by photoperiod was studied in a population of the speckled wood butterfly, Purarge aegeria L. (Lepidoptera: Satyrinae), from southern Sweden. Individuals were reared in a range of photoperiodic regimes (9L. to 22L) and temperatures (13°C to 21° C). Plasticity was found for important life-history traits- generation time, growth rate and final weight and seasonal regulation of development in response to photoperiod was found to occur at two levels. Purarge aegeria hibernates as a third instar larva or in the pupal stage, cantering one of four major developmental pathways in response to photoperiod: (1) direct development in both the larval and pupal stages, (2) pupal winter diapause with or (3) without a preceding larval summer diapause, or (4) larval winter diapause. In addition to this high-level regulation of individual development, larval growth rate and pupal development rate also appear to be finally regulated by photoperiod within each major pathway. As photoperiods decreased from 22 h to 17 h at 17° C, growth rate among directly developing larvae increased progressively, as was the case for larva? developing according to a univoltine life cycle from 17 h to 14 h. At two photoperiods, 13 h and 16 h (corresponding to shifts between major pathways), both larval and pupal development were extremely variable with the fastest individuals developing directly and the slowest developing with a diapause. This indicates a gradual nature of diapause itself, suggesting that the two level may not he fundamentally different.     

A search for trade-offs among life history traits inDrosophila melanogaster

Summary Are there underlying developmental and physiological properties of organisms that can be used to build a general theory of life history evolution? Much of the theoretical work on the evolution of life histories is based on the premise of negative developmental and genetic correlations among life history traits. If negative correlations do not exist as a general rule then no general theory taking them into account is possible. Negative genetic correlations among life history traits can come about by antagonistic pleiotropy. One cause of antagonistic pleiotropy is cost allocation trade-offs. Since cost allocation trade-offs are due to underlying physiological constraints they are expected to be common to closely related groups. A second form of antagonistic pleiotropy is specialization of genotypes to different niches. This type of antagonistic pleiotropy is expected to be specific to each population. We looked for trade-offs in life history traits of longevity and fecundity inDrosophila melanogaster. We used a half-sib mating design and raised the offspring at two temperatures, 19°C and 25°C. Correlations between longevity and fecundity showed some evidence of antagonistic pleiotropy at high temperature with no evidence of any trade-offs at low temperature. Correlations of early and late fecundity traits did show evidence of cost allocation trade-offs at both temperatures. Antagonistic pleiotropy was also found for cross-environmental correlations of fecundity traits. We conclude that, although life history trade-offs can not be generally assumed, they are frequently found among functionally related traits. Thus, we provide guidelines for the development of general theories of life history evolution.     

Cell and molecular biology of bryophytes: ultimate limits to the resolution of phylogenetic problems

Ultrastructure, biochemistry and 5S rRNA sequences link tracheophytes, bryophytes and charalean green algae, but the precise interrelationships between these groups remain unclear. Further major clarification now awaits primary sequence data. These are also needed to determine directionality in possible evolutionary trends within the bryophytes, but are unlikely to overturn current schemes of classification or phylogeny. Comparative ultrastructural studies of spermatogenesis, sporogenesis, the cytoskeleton and plastids reinforce biochemical and morphogenetic evidence for the wide phyletic discontinuities between mosses, hepatics and hornworts, and also rule out direct lines of descent between them. Direct ancestral lineages from charalean algae to bryophytes and to tracheophytes are also unlikely. EM studies of gametophyte/sporophyte junctions, plus immunological investigations of bryophyte cytoskeletons, are likely to accentuate the differences between mosses, hepatirs and hornworts. Other priorities for systematics include elucidation of oil body ultrastructure, analysis of the changes in nuclear proteins during spermatogenesis and a detailed comparison of bryophyte and charalean plastids. The combined evidence from ultrastrueture, biochemistry, morphology and morphogenesis warrants general acceptance of the polyphyletic origin of the bryophytes. Ultrastructural attributes should be more widely used in bryophyte systematics.     

Life history of the importedScymnus (Nephus) Reunioni [Col.: Coccinellidae] predator of mealybugs

The life history ofNephus reunioni, introduced from the Reunioni island has been studied. Optimum temperature for the development of the entomophagous insect is 24–25°C. In this case the population has the highest survival at preimaginal stages, life duration of adults and reproductive capacity. The rate of population growth was calculated by the formula:

Annual plant life histories and the paradigm of resource allocation

Summary Much of life history theory follows from the idea that natural selection acts on the allocation of resources to competing and independent demographic functions. This paradigm has stimulated much research on the life histories of annual plants. Models of whole-plant resource budgets that use optimal control theory predict periods of 100% vegetative and 100% reproductive growth, sometimes with periods of mixed growth. I show here that this prediction follows from the assumption of independence of the competing vegetative and reproductive compartments. The prediction is qualitatively unchanged even after relaxing important simplifying assumptions used in most models. Although it follows naturally from the assumptions of the models, this kind of allocation pattern is unlikely to occur in many plants, because it requires that (1) leaf and flower buds can never simultaneously be carbon sinks; and (2) organs that accompany flowers, such as internodes and bracts, can never be net sources of photosynthate. Thus while resources are doubtless important for annual plants, an exclusively resource-based perspective may be inadequate to understand the evolution of their life histories. Progress in research may require models that incorporate, or are at least phenomenologically consistent with, the basic developmental reles of angiosperms.     

The effect of time-varying mortality and carbon assimilation on models of carbon allocation in annual plants

Summary Models of optimal carbon allocation schedules have influenced the way plant ecologists think about life history evolution, particularly for annual plants. The present study asks (1) how, within the framework of these models, are their predictions affected by within-season variation in mortality and carbon assimilation rates?; and (2) what are the consequences of these prediction changes for empirical tests of the models? A companion paper examines the basic assumptions of the models themselves. I conducted a series of numerical experiments with a simple carbon allocation model. Results suggest that both qualitative and quantitative predictions can sometimes be sensitive to parameter values for net assimilation rate and mortality: for some parameter values, both the time and size at onset of reproduction, as well as the number of reproductive intervals, vary considerably as a result of small variations in these parameters. For other parameter values, small variations in the parameters result in only small changes in predicted phenotype, but these have very large fitness consequences. Satisfactory empirical tests are thus likely to require much accuracy in parameter estimates. The effort required for parameter estimation imposes a practical constraint on empirical tests, making large multipopulation comparisons impractical. It may be most practical to compare the predicted and observed fitness consequences of variation in the timing of onset of reproduction.     

Environmental networks,compensating life histories and habitat selection by white-footed mice

Summary Analysis of 6 years' data on a population of free-living white-footed mice documents both phenotypic and environmental control of litter size. Litter size was positively correlated with maternal body size. Maternal size depended upon both seasonal and annual variation. Paradoxically, the proportion of small versus large litters varied among habitats independently of the effects of body size. The result is an influence of habitat on life history that yields patterns of reproduction and survival opposite to the predictions of demographic theory. The habitat producing the largest litters had a relatively high ratio of adult/juvenile survival. Litter size was small in the habitat where the adult/juvenile survival ratio was smallest. All of these anomalous patterns can be explained through density-dependent habitat selection by female white-footed mice. Life-history studies that ignore habitat and habitat selection may find spurious correlations among traits that result in serious misinterpretations about life history and its evolution.