喜树与南方红豆杉混交群落生活史型谱特征

研究了不同生境条件下喜树和南方红豆杉混交群落中喜树和南方红豆杉的生活史型谱特征。结果表明,喜树和南方红豆杉在S生活史型得分和比例均为0。喜树和南方红豆杉生活史型V主成分得分均以山地2最高,而纯林的生活史型V主成分得分最低,C也表现为山地2最高,而纯林最低。喜树种群的生活史型比例按照从山地、坡地、平地、纯林排序,V型比例逐渐上升而C型比例逐渐下降的趋势,这就构成了混交群落中喜树种群的生活史型谱。南方红豆杉种群的生活史型谱特征,主要表现为山地2和坡地V型比例最小,而C型比例最大,纯林种植中V型比例较高,而以纯林1的V型比例最高,C型比例最小。     

The evolution of life cycle complexity in aphids: Ecological optimization or historical constraint?

For decades, biologists have debated why many parasites have obligate multihost life cycles. Here, we use comparative phylogenetic analyses of aphids to evaluate the roles of ecological optimization and historical constraint in the evolution of life cycle complexity. If life cycle complexity is adaptive, it should be evolutionarily labile, that is, change in response to selection. We provide evidence that this is true in some aphids (aphidines), but not others (nonaphidines)—groups that differ in the intensity of their relationships with primary hosts. Next, we test specific mechanisms by which life cycle complexity could be adaptive or a constraint. We find that among aphidines there is a strong association between complex life cycles and polyphagy but only a weak correlation between life cycle complexity and reproductive mode. In contrast, among nonaphidines the relationship between life cycle complexity and host breadth is weak but the association between complex life cycles and sexual reproduction is strong. Thus, although the adaptiveness of life cycle complexity appears to be lineage specific, across aphids, life cycle evolution appears to be tightly linked with the evolution of other important natural history traits.     

Trematode life cycles: short is sweet?

Complex life cycles are a hallmark of parasitic trematodes. In several trematode taxa, however, the life cycle is truncated: fewer hosts are used than in a typical three-host cycle, with fewer transmission events. Eliminating one host from the life cycle can be achieved in at least three different ways. Some trematodes show even more extreme forms of life cycle abbreviations, using only a mollusc to complete their cycle, with or without sexual reproduction. The occurrence of these phenomena among trematode families are reviewed here and show that life cycle truncation has evolved independently many times in the phylogeny of trematodes. The hypotheses proposed to account for life-cycle truncation, in addition to the factors preventing the adoption of shorter cycles by all trematodes are also discussed. The study of shorter life cycles offers an opportunity to understand the forces shaping the evolution of life cycles in general.     

REPLY TO COMMENT ON THE LIFE CYCLE AND TOXICITY OF PFIESTERIA PISCICIDA REVISITED1

Free‐living, marine dinoflagellates are typified by a well‐defined, haplontic life cycle with relatively few stages. The most unusual departure from this life cycle is one reported for the heterotrophic dinoflagellate Pfiesteria piscicida Steidinger et Burkholder. This species is alleged to have at least 24 life cycle stages including amoebae and a chrysophyte‐like cyst form ( Burkholder et al. 1992 , Burkholder and Glasgow 1997a ) not previously known in free‐living marine dinoflagellates. Litaker et al. (2002) redescribed the life cycle of P. piscicida from single‐cell isolates and found only life cycle stages typical of free‐living marine dinoflagellates. The discrepancy between these observations and the life cycle reported in the literature prompted a rigorous study to resolve the life cycle of P. piscicida. Burkholder and Glasgow (2002) took exception to this study, arguing that Litaker et al. (2002) misunderstood the life cycle of P. piscicida and ignored recent publications. We present a rebuttal of their criticisms and suggest a simple way to resolve the discrepancies in the P. piscicida life cycle.     

Gimme shelter – the relative sensitivity of parasitic nematodes with direct and indirect life cycles to climate change

Climate change is expected to alter the dynamics of host–parasite systems globally. One key element in developing predictive models for these impacts is the life cycle of the parasite. It is, for example, commonly assumed that parasites with an indirect life cycle would be more sensitive to changing environmental conditions than parasites with a direct life cycle due to the greater chance that at least one of their obligate host species will go extinct. Here, we challenge this notion by contrasting parasitic nematodes with a direct life cycle against those with an indirect life cycle. Specifically, we suggest that behavioral thermoregulation by the intermediate host may buffer the larvae of indirectly transmitted parasites against temperature extremes, and hence climate warming. We term this the ‘shelter effect’. Formalizing each life cycle in a comprehensive model reveals a fitness advantage for the direct life cycle over the indirect life cycle at low temperatures, but the shelter effect reverses this advantage at high temperatures. When examined for seasonal environments, the models suggest that climate warming may in some regions create a temporal niche in mid‐summer that excludes parasites with a direct life cycle, but allows parasites with an indirect life cycle to persist. These patterns are amplified if parasite larvae are able to manipulate their intermediate host to increase ingestion probability by definite hosts. Furthermore, our results suggest that exploiting the benefits of host sheltering may have aided the evolution of indirect life cycles. Our modeling framework utilizes the Metabolic Theory of Ecology to synthesize the complexities of host behavioral thermoregulation and its impacts on various temperature‐dependent parasite life history components in a single measure of fitness, R₀. It allows quantitative predictions of climate change impacts, and is easily generalized to many host–parasite systems.     

Review of criteria for evaluating LCA weighting methods

Purpose

In the process of selecting where effective environmental measures should be directed, the weighting step of life cycle assessment (LCA) is an optional, controversial, but nevertheless important tool. A set of criteria for evaluating weighting methods has relevance in the process of acquiring meta-knowledge, and thus competence, in assigning relative weights to environmental impact categories. This competence is helpful when choosing between presently available weighting methods, and in creating new weighting methods.

Methods

Criteria in LCA-related literature are reviewed. The authors have focused on identifying lists of criteria rather than extracting criteria from bulks of text. An important starting point has been the actual use of the term “criterion”, while at the same time disqualifying certain definitions of the term which are too far removed from the two definitions provided in this article.

Results and discussion

Criteria for evaluating weighting methods are shown to fall into two general categories. The first being general criteria for weighting methods, demanding that weighting methods have a broad scope, are practical for users and scientists, are scientific and have ethical goals. The second being criteria proposing characteristics of concrete environmental damage which should be taken into account by a weighting method. A noteworthy example is reversibility.

Conclusions

While the comprehensive tables of criteria speak for themselves, it can be observed that the need for transparency is particularly highlighted in literature. Furthermore, ISO 14044’s statement that the weighting step is “not scientifically based” would appear to defy a significant proportion of the other criteria reviewed; this, however, depends on its interpretation.     

Life cycle replacement by gene introduction under an allee effect in periodical cicadas

Periodical cicadas (Magicicada spp.) in the USA are divided into three species groups (-decim, -cassini, -decula) of similar but distinct morphology and behavior. Each group contains at least one species with a 17-year life cycle and one with a 13-year cycle; each species is most closely related to one with the other cycle. One explanation for the apparent polyphyly of 13- and 17-year life cycles is that populations switch between the two cycles. Using a numerical model, we test the general feasibility of life cycle switching by the introduction of alleles for one cycle into populations of the other cycle. Our results suggest that fitness reductions at low population densities of mating individuals (the Allee effect) could play a role in life cycle switching. In our model, if the 13-year cycle is genetically dominant, a 17-year cycle population will switch to a 13-year cycle given the introduction of a few 13-year cycle alleles under a moderate Allee effect. We also show that under a weak Allee effect, different year-classes ("broods") with 17-year life cycles can be generated. Remarkably, the outcomes of our models depend only on the dominance relationships of the cycle alleles, irrespective of any fitness advantages.     

Graph decompositions for demographic loop analysis

A new approach to loop analysis is presented in which decompositions of the total elasticity of a population projection matrix over a set of life history pathways are obtained as solutions of a constrained system of linear equations. In loop analysis, life history pathways are represented by loops in the life cycle graph, and the elasticity of the loop is interpreted as a measure of the contribution of the life history pathway to the population growth rate. Associated with the life cycle graph is a vector space -- the cycle space of the graph -- which is spanned by the loops. The elasticities of the transitions in the life cycle graph can be represented by a vector in the cycle space, and a loop decomposition of the life cycle graph is then defined to be any nonnegative linear combination of the loops which sum to the vector of elasticities. In contrast to previously published algorithms for carrying out loop analysis, we show that a given life cycle graph admits of either a unique loop decomposition or an infinite set of loop decompositions which can be characterized as a bounded convex set of nonnegative vectors. Using this approach, loop decompositions which minimize or maximize a linear objective function can be obtained as solutions of a linear programming problem, allowing us to place lower and upper bounds on the contributions of life history pathways to the population growth rate. Another consequence of our approach to loop analysis is that it allows us to identify the exact tradeoffs in contributions to the population growth rate that must exist between life history pathways.     

Cycles within cycles: the interplay between differentiation and cell division in Trypanosoma brucei

The life cycle o f the African trypanosome is divided between the mammal and the tsetse. Those life cycle stages which traverse between these two hosts appear to be pre-adopted for survival in their new habitat They are also non-dividing. Here, Keith Matthews and Keith Gull discuss how and why trypanosomes might enmesh the control o f their cell cycle with their regulation o f the transition between different life cycle forms.     

The life cycle of Laodicea indica (Laodiceidae,Leptomedusae, Cnidaria)

The life cycle of Laodicea indica is described. In the Bismarck Sea this species shows a normal alternation of generations in the wet season; but in the dry season the cycle is contracted, and planulae give rise to gonothecae without formation of a hydroid colony. Medusae are liberated about 3 d after planula settlement. This life cycle pattern is previously unreported in hydromedusae. The possible adaptive value of such a life cycle and the evolution of polyp reduction in hydromedusan life cycles are discussed.     

A classification system for mosquito life cycles: life cycle types for mosquitoes of the northeastern United States.

A system for the classification of mosquito life cycle types is presented for mosquito species found in the northeastern United States. Primary subdivisions include Univoltine Aedine, Multivoltine Aedine, Multivoltine Culex/Anopheles, and Unique Life Cycle Types. A montotypic subdivision groups life cycle types restricted to single species. The classification system recognizes 11 shared life cycle types and three that are limited to single species. Criteria for assignments include: 1) where the eggs are laid, 2) typical larval habitat, 3) number of generations per year, and 4) stage of the life cycle that overwinters. The 14 types in the northeast have been named for common model species. A list of species for each life cycle type is provided to serve as a teaching aid for students of mosquito biology.     

Defining the baseline in social life cycle assessment

Background, aim and scope  

A relatively broad consensus has formed that the purpose of developing and using the social life cycle assessment (SLCA) is to improve the social conditions for the stakeholders affected by the assessed product’s life cycle. To create this effect, the SLCA, among other things, needs to provide valid assessments of the consequence of the decision that it is to support. The consequence of a decision to implement a life cycle of a product can be seen as the difference between the decision being implemented and ‘non-implemented’ product life cycle. This difference can to some extent be found using the consequential environmental life cycle assessment (ELCA) methodology to identify the processes that change as a consequence of the decision. However, if social impacts are understood as certain changes in the lives of the stakeholders, then social impacts are not only related to product life cycles, meaning that by only assessing impacts related to the processes that change as a consequence of a decision, not all changes in the life situations of the stakeholders will be captured by an assessment following the consequential ELCA methodology. This article seeks to identify these impacts relating to the non-implemented product life cycle and establish indicators for their assessment.     

Genetic regulation of life cycle transitions in the brown alga Ectocarpus

The life cycle of an organism is one of its most elemental features, underpinning a broad range of phenomena including developmental processes, reproductive fitness, mode of dispersal and adaptation to the local environment. Life cycle modification may have played an important role during the evolution of several eukaryotic groups, including the terrestrial plants. Brown algae are potentially interesting models to study life cycle evolution because this group exhibits a broad range of different life cycles. Currently, life cycle studies are focused on the emerging brown algal model Ectocarpus. Two life cycle mutants have been described in this species, both of which cause the sporophyte generation to exhibit gametophyte characteristics. The ouroboros mutation is particularly interesting because it induces complete conversion of the sporophyte generation into a functional, gamete-producing gametophyte, a class of mutation that has not been described so far in other systems. Analysis of Ectocarpus life cycle mutants is providing insights into several life-cycle-related processes including parthenogenesis, symmetric/asymmetric initial cell divisions and sex determination.     

植物生活史型的多样性及动态分析

主要阐述了植物生活史型的基本定义和基本模式。根据植物的生态幅（Ｅｃｏｌｏｇｉｃａｌ　ａｍｐｌｉｔｕｄｅ）、适合度（Ｆｉｔｎｅｓｓ）和能量分配格局将植物生活史型划分出Ｖ生活史型、Ｓ生活史型和ｃ生活史型３个基本类型以及ＶＳ生活史型、ＳＶ生活史型、ｃＳ生活史型、Ｓｃ生活史型等６个具有混合特征的过渡类型。文中分析了权衡（丁ｒａｄｅ—ｏｆｆ）植物生活史各阶段的能量需求,使之合理地进行能量分配,进而使植物生活史型获得最佳的繁殖和存活效益以及最大的适合度的重要性,指出韧生代谢和次生代谢增值物生活史型及其生活史型之间相互转换的密切关系。韧生代谢物质主要用于营养生长,次生代谢物质主要用于促进繁育和拮抗环境胁迫。植物生活史型在特定时空中依生境的连续变化而发生相互转换,呈现出具动态特征的植物生活史型诺。提出了植物生活史型的形成机制,即生境中的资源状况和干扰程度构成了环境筛的径度,进而形成选择压力,以使植物按需分配能量,合成初级代谢产物或次级代谢产物来应对选择压力,形成自身的生态幅和适应对策,最终与生境相互作用过程中表现出的适合度来表征相应的生活史型。还提出了植物生活史型之间相互转化的机制,即每一种植物生活史型均有与该生活史型相对应的生境类型、选择压力、代谢物质和生活史对策,由于时空的连续变化,生境类型也发生过渡性变化,形成过渡类型（ＥＤ、ＤＥ、ＤＦ、ＦＤ）,因而导致选择压力、代谢物质、生活史对策也发生过渡性变化,形成过渡类型ＬＭ、ＭＬ、ＭＨ、ＨＭ、ＫＲ、ＲＫ、ＲＴ、ＴＲ、ＢＰ、ＰＢ、ＰＡ、ＡＰ,最终通过ＶＳ、ＳＶ、ＳＣ、ＣＳ等过渡类型的形成而实现植物生活史型之间的相互转换。文中以高山红景天（Ｒｈｏｄｉｏｌａ　ｓａｃｈａｌｉｎｅｎｓｉｓ）等５种植物生活史型谱为例,分析了各植物生活史型谱的动态特征并指出：Ｖ生活史型的植物因营养体较为发达、寿命较长,且能通过正常的有性生殖繁衍后代,通常都能产生稳定种群;以Ｓ生活史型为主的植物,因台子中含有来自双亲的两套基因,故有性生殖过程能产生较多遗传性不同的后代,使种群的适应环境变化的能力加强,因而容易形成爆发种群;以ｃ生活史型为主的植物,其遗传物质与母体完全相同,故种群适应环境变化的能力较弱,因而容易导致种群濒危。     

Partial life cycle analysis: a model for pre-breeding census data

Matrix population models have become popular tools in research areas as diverse as population dynamics, life history theory, wildlife management, and conservation biology. Two classes of matrix models are commonly used for demographic analysis of age‐structured populations: age‐structured (Leslie) matrix models, which require age‐specific demographic data, and partial life cycle models, which can be parameterized with partial demographic data. Partial life cycle models are easier to parameterize because data needed to estimate parameters for these models are collected much more easily than those needed to estimate age‐specific demographic parameters. Partial life cycle models also allow evaluation of the sensitivity of population growth rate to changes in ages at first and last reproduction, which cannot be done with age‐structured models. Timing of censuses relative to the birth‐pulse is an important consideration in discrete‐time population models but most existing partial life cycle models do not address this issue, nor do they allow fractional values of variables such as ages at first and last reproduction. Here, we fully develop a partial life cycle model appropriate for situations in which demographic data are collected immediately before the birth‐pulse (pre‐breeding census). Our pre‐breeding census partial life cycle model can be fully parameterized with five variables (age at maturity, age at last reproduction, juvenile survival rate, adult survival rate, and fertility), and it has some important applications even when age‐specific demographic data are available (e.g., perturbation analysis involving ages at first and last reproduction). We have extended the model to allow non‐integer values of ages at first and last reproduction, derived formulae for sensitivity analyses, and presented methods for estimating parameters for our pre‐breeding census partial life cycle model. We applied the age‐structured Leslie matrix model and our pre‐breeding census partial life cycle model to demographic data for several species of mammals. Our results suggest that dynamical properties of the age‐structured model are generally retained in our partial life cycle model, and that our pre‐breeding census partial life cycle model is an excellent proxy for the age‐structured Leslie matrix model.     

Ultraviolet Inactivation and Photoreactivation in Synchronized Chlorella

Synchronous cultures of Chlorella pyrenoidosa have been used in studies of the action of UV-irradiation during different stages in the life cycle on cell division capacity. These experiments show that there is a considerably higher sensitivity to UV-irradiation during the first hours of the life cycle. The variations in photoreactivating capacity of UV-damaged cells during a life cycle have been investigated. The results from these investigations show that the photoreactivating capacity varies and that it is higher in young daughter cells (autospores) and 12–15 hour cells. These variations can be due to variations in the activity of the photoreactivating enzyme during the life cycle.     

Geographic variation in life cycle strategies of a progenetic trematode

Numerous parasite species have evolved complex life cycles with multiple, subsequent hosts. In trematodes, each transmission event in multi-host life cycles selects for various adaptations, one of which is facultative life cycle abbreviation. This typically occurs through progenesis, i.e., precocious maturity and reproduction via self-fertilization within the second intermediate host. Progenesis eliminates the need for the definitive host and facilitates life cycle completion. Adopting a progenetic cycle may be a conditional strategy in response to environmental cues related to low probability of transmission to the definitive host. Here, the effects of environmental factors on the reproductive strategy of the progenetic trematode Stegodexamene anguillae were investigated using comparisons among populations. In the 3-host life cycle, S. anguillae sexually reproduces within eel definitive hosts, whereas in the progenetic life cycle, S. anguillae reproduces by selfing within the metacercaria cyst in tissues of small fish intermediate hosts. Geographic variation was found in the frequency of progenesis, independent of eel abundance. Progenesis was affected by abundance and length of the second intermediate fish host as well as encystment site within the host. The present study is the first to compare life cycle strategies among parasite populations, providing insight into the often unrecognized plasticity in parasite developmental strategies and transmission.     

Certain principles of the secondary simplication of the life cycles of helminths]

On the basis of the analysis of regularities accompanying the secondary simplification of the life cycles of helminths on account of the reduction in the number of the animals-hosts 8 rules have been formulated. They are based on the following important regularities. 1. At the secondary simplification of the life cycles of helminths never fall out the first intermediate host in Trematoda and the definitive host in Nematoda. This phenomenon is suggested to be called "the host stability in the life cycle". 2. Mostly often from the life cycles fall secondarily out those hosts which join in the life cycle at its first complication later. 3. The phase of the helminth having transformed into a parasitic form at the first complication of its life cycle remains the same at the secondary simplification of this cycle.