Population genetics of reef coral endosymbionts (Symbiodinium,Dinophyceae)

Symbiodinium is a diverse genus of unicellular dinoflagellate symbionts associating with various marine protists and invertebrates. Although the broadscale diversity and phylogenetics of the Symbiodinium complex is well established, there have been surprisingly few data on fine‐scale population structure and biogeography of these dinoflagellates. Yet population‐level processes contribute strongly to the biology of Symbiodinium, including how anthropogenic‐driven global climate change impacts these symbionts and their host associations. Here, we present a synthesis of population‐level characteristics for Symbiodinium, with an emphasis on how phylogenetic affinities, dynamics within and among host individuals, and a propensity towards clonality shape patterns on and across reefs. Major inferences include the following: (i) Symbiodinium populations within individual hosts are comprised mainly of cells belonging to a single or few genetic clones. (ii) Symbiont populations exhibit a mixed mode of reproduction, wherein at least one sexual recombination event occurs in the genealogy between most genotypes, but clonal propagation predominates overall. (iii) Mutualistic Symbiodinium do not perpetually persist outside their hosts, instead undergoing turnover and replacement via the continuous shedding of viable clonal cells from host individuals. (iv) Symbiont populations living in the same host, but on different reefs, are often genetically subdivided, suggesting low connectivity, adaptation to local conditions, or prolific asexual reproduction and low effective population sizes leading to disproportionate success within and among hosts. Overall, this synthesis forms a basis for future investigations of coral symbiosis ecology and evolution as well as delimitation of species boundaries in Symbiodinium and other eukaryotic microorganisms.     

Sedimentation and overfishing drive changes in early succession and coral recruitment

Sedimentation and overfishing are important local stressors on coral reefs that can independently result in declines in coral recruitment and shifts to algal-dominated states. However, the role of herbivory in driving recovery across environmental gradients is often unclear. Here we investigate early successional benthic communities and coral recruitment across a sediment gradient in Palau, Micronesia over a 12-month period. Total sedimentation rates measured by ‘TurfPods’ varied from 0.03 ± 0.1 SE mg cm⁻² d⁻¹ at offshore sites to 1.32 ± 0.2 mg cm⁻² d⁻¹ at inshore sites. To assess benthic succession, three-dimensional settlement tiles were deployed at sites with experimental cages used to exclude tile access to larger herbivorous fish. Benthic assemblages exhibited rapid transitions across the sediment gradient within three months of deployment. At low levels of sedimentation (less than 0.6 mg cm⁻² d⁻¹), herbivory resulted in communities dominated by coral recruitment inducers (short turf algae and crustose coralline algae), whereas exclusion of herbivores resulted in the overgrowth of coral inhibitors (encrusting and upright foliose macroalgae). An ‘inducer threshold’ was found under increasing levels of sedimentation (greater than 0.6 mg cm⁻² d⁻¹), with coral inducers having limited to no presence in communities, and herbivore access to tiles resulted in sediment-laden turf algal assemblages, while exclusion of herbivores resulted in invertebrates (sponges, ascidians) and terrestrial sediment accumulation. A ‘coral recruitment threshold’ was found at 0.8 mg cm⁻² d⁻¹, below which net coral recruitment was reduced by 50% in the absence of herbivores, while recruitment was minimal above the threshold. Our results highlight nonlinear trajectories of benthic succession across sediment gradients and identify strong interactions between sediment and herbivory that have cascading effects on coral recruitment. Local management strategies that aim to reduce sedimentation and turbidity and manage herbivore fisheries can have measurable effects on benthic community succession and coral recruitment, enhancing reef resilience and driving coral recovery.     

Microbiome structure of the fungid coral Ctenactis echinata aligns with environmental differences

The significance of bacteria for eukaryotic functioning is increasingly recognized. Coral reef ecosystems critically rely on the relationship between coral hosts and their intracellular photosynthetic dinoflagellates, but the role of the associated bacteria remains largely theoretical. Here, we set out to relate coral‐associated bacterial communities of the fungid host species Ctenactis echinata to environmental settings (geographic location, substrate cover, summer/winter, nutrient and suspended matter concentrations) and coral host abundance. We show that bacterial diversity of C. echinata aligns with ecological differences between sites and that coral colonies sampled at the species’ preferred habitats are primarily structured by one bacterial taxon (genus Endozoicomonas) representing more than 60% of all bacteria. In contrast, host microbiomes from lower populated coral habitats are less structured and more diverse. Our study demonstrates that the content and structure of the coral microbiome aligns with environmental differences and denotes habitat adequacy. Availability of a range of coral host habitats might be important for the conservation of distinct microbiome structures and diversity.     

Post-fire ecological succession: A theoretical modeling framework

Fire is considered as an extreme disturbance in Mediterranean grasslands or shrublands as it often brings about many sudden changes in the vegetation structure, composition, and diversity patterns. In addition, it creates opportunities for exotic plant species to establish successfully in foreign habitat, and to outperform dominating native species. Monitoring and simulating post-fire successional changes, therefore, are essential tasks to efficiently restore native grasslands or shrublands. In this paper, we develop a theoretical framework for simulating fire-induced successional changes, mainly for Mediterranean vegetation, based on a three-level hierarchy of successional causes. Within this proposed framework, fire effects are considered by associating it with the number of burned sites open-up and specific changes at the burned sites relative to unburned sites. Three distinct site-specific neighborhoods are constructed; changes within each neighborhood allow sequential replacement of plant species by another plant species with greater maximum size, age and lower maximum growth rates and dispersal abilities. The proposed framework can be used to develop a spatially explicit individual-based model which will be useful for monitoring and predicting successional changes and hence for restoring native grasslands or shrublands.     

The trace metal economy of the coral holobiont: supplies,demands and exchanges

The juxtaposition of highly productive coral reef ecosystems in oligotrophic waters has spurred substantial interest and progress in our understanding of macronutrient uptake, exchange, and recycling among coral holobiont partners (host coral, dinoflagellate endosymbiont, endolithic algae, fungi, viruses, bacterial communities). By contrast, the contribution of trace metals to the physiological performance of the coral holobiont and, in turn, the functional ecology of reef-building corals remains unclear. The coral holobiont's trace metal economy is a network of supply, demand, and exchanges upheld by cross-kingdom symbiotic partnerships. Each partner has unique trace metal requirements that are central to their biochemical functions and the metabolic stability of the holobiont. Organismal homeostasis and the exchanges among partners determine the ability of the coral holobiont to adjust to fluctuating trace metal supplies in heterogeneous reef environments. This review details the requirements for trace metals in core biological processes and describes how metal exchanges among holobiont partners are key to sustaining complex nutritional symbioses in oligotrophic environments. Specifically, we discuss how trace metals contribute to partner compatibility, ability to cope with stress, and thereby to organismal fitness and distribution. Beyond holobiont trace metal cycling, we outline how the dynamic nature of the availability of environmental trace metal supplies can be influenced by a variability of abiotic factors (e.g. temperature, light, pH, etc.). Climate change will have profound consequences on the availability of trace metals and further intensify the myriad stressors that influence coral survival. Lastly, we suggest future research directions necessary for understanding the impacts of trace metals on the coral holobiont symbioses spanning subcellular to organismal levels, which will inform nutrient cycling in coral ecosystems more broadly. Collectively, this cross-scale elucidation of the role of trace metals for the coral holobiont will allow us to improve forecasts of future coral reef function.     

Origin and succession of phytoplankton in a river-lake system (Spree,Germany)

The River Spree (Germany) flows through an impoundment and several shallow lakes in its middle and lower course. In this river-lake system, the seasonal and longitudinal dynamics of dominant phytoplankton populations were studied in relation to retention time of water, mixing conditions and nutrient supply from 1988–92. Some phytoplankton species populated the same river section for weeks or months each year at their season. Such stable populations have to origin from river zones functioning like mixed reactors. In the Spree system, centric diatoms originated from an impoundment and filamentous cyanobacteria from a flushed lake with longer retention time of water. Downstream, biomass and composition of phytoplankton altered nearly simultaneously along the system.The fate of planktonic organisms washed from mixed reactors into the flow depended on the conditions at the zones of origin. During spring, populations dominating phytoplankton communities of the well-mixed lakes grew further under river conditions. However the biomass of summer species, adapted to intermittent stratification, was halved along the river course. These seasonal differences were probably caused by lower maximum growth rates of summer species and enhanced losses (photorespiration, sedimentation or grazing of benthic filter feeders, but not of zooplankton) of algal populations under river conditions in summer.Phytoplankton assimilation, settlement of diatoms, or denitrification caused declining (probably growth limiting) concentrations of dissolved inorganic phosphorus (spring), silicon (early summer) or nitrogen (summer) along the river course, respectively. The minimum content of DRP was often followed by a clear-water phase. Reduced DSi supply selected against diatoms and additional DIN shortage favoured N₂-fixing cyanobacteria in the last lake of the system.R-strategists (sensu Reynolds) were selected in both the flushed, shallow lakes and the lowland river. In general, the biomass of cyanobacteria increased within the lakes and declined along the river course. Some diatom populations grew in the river, but were grazed or settled down in the lakes. Beside this general picture, different populations from the same phylogenetic group did not necessarily perform in similar ways.     

A dominant function of CCaMK in intracellular accommodation of bacterial and fungal endosymbionts

In legumes, Ca²⁺/calmodulin‐dependent protein kinase (CCaMK) is a component of the common symbiosis genes that are required for both root nodule (RN) and arbuscular mycorrhiza (AM) symbioses and is thought to be a decoder of Ca²⁺ spiking, one of the earliest cellular responses to microbial signals. A gain‐of‐function mutation of CCaMK has been shown to induce spontaneous nodulation without rhizobia, but the significance of CCaMK activation in bacterial and/or fungal infection processes is not fully understood. Here we show that a gain‐of‐function CCaMK^T265D suppresses loss‐of‐function mutations of common symbiosis genes required for the generation of Ca²⁺ spiking, not only for nodule organogenesis but also for successful infection of rhizobia and AM fungi, demonstrating that the common symbiosis genes upstream of Ca²⁺ spiking are required solely to activate CCaMK. In RN symbiosis, however, CCaMK^T265D induced nodule organogenesis, but not rhizobial infection, on Nod factor receptor (NFRs) mutants. We propose a model of symbiotic signaling in host legume plants, in which CCaMK plays a key role in the coordinated induction of infection thread formation and nodule organogenesis.     

State ethnicization and the crisis of leadership succession among Israel's Druze

States often ethnicize ethnic groups for their own purposes. In doing so, however, they unleash contradictory processes. While the state facilitates the integration of individuals into modern state structures on the basis of personal achievement, it will also traditionalize collective identity by promoting a recognized leader or headman. Over time ethnic reformers will challenge such state ethnicization and attempt to organize the community to achieve more autonomy, using state legislation to achieve these reforms. Paradoxically, this attempt, if successful, will not only encroach upon state power but also curtail the choice of the individual in the ethnic group to define his or her own ethnicity. The following article analyses the Israeli Druze's relationship to the State of Israel and the controversy among the Druze over the group's international organization. It analyses at what point state ethnicization gives way to ethnic autonomy at the expense of state power, and explores its implications on the development of civil society within the ethnic community. The diminution of state ethnicization in a democracy need not necessarily entail the empowerment of the ethnic group but rather the individual who belongs to it.     

Testing the assumptions of chronosequences in succession

Many introductory ecology textbooks illustrate succession, at least in part, by using certain classic studies (e.g. sand dunes, ponds/bogs, glacial till, and old fields) that substituted space for time (chronosequence) in determining the sequences of the succession. Despite past criticisms of this method, there is continued, often uncritical, use of chronosequences in current research on topics besides succession, including temporal changes in biodiversity, productivity, nutrient cycling, etc. To show the problem with chronosequence-based studies in general, we review evidence from studies that used non-chronosequence methods (such as long-term study of permanent plots, palynology, and stand reconstruction) to test the space-for-time substitution in four classic succession studies. In several cases, the tests have used the same locations and, in one case, the same plots as those in the original studies. We show that empirical evidence invalidates the chronosequence-based sequences inferred in these classic studies.     

Dominance of Endozoicomonas bacteria throughout coral bleaching and mortality suggests structural inflexibility of the Pocillopora verrucosa microbiome

The importance of Symbiodinium algal endosymbionts and a diverse suite of bacteria for coral holobiont health and functioning are widely acknowledged. Yet, we know surprisingly little about microbial community dynamics and the stability of host‐microbe associations under adverse environmental conditions. To gain insight into the stability of coral host‐microbe associations and holobiont structure, we assessed changes in the community structure of Symbiodinium and bacteria associated with the coral Pocillopora verrucosa under excess organic nutrient conditions. Pocillopora‐associated microbial communities were monitored over 14 days in two independent experiments. We assessed the effect of excess dissolved organic nitrogen (DON) and excess dissolved organic carbon (DOC). Exposure to excess nutrients rapidly affected coral health, resulting in two distinct stress phenotypes: coral bleaching under excess DOC and severe tissue sloughing (>90% tissue loss resulting in host mortality) under excess DON. These phenotypes were accompanied by structural changes in the Symbiodinium community. In contrast, the associated bacterial community remained remarkably stable and was dominated by two Endozoicomonas phylotypes, comprising on average 90% of 16S rRNA gene sequences. This dominance of Endozoicomonas even under conditions of coral bleaching and mortality suggests the bacterial community of P. verrucosa may be rather inflexible and thereby unable to respond or acclimatize to rapid changes in the environment, contrary to what was previously observed in other corals. In this light, our results suggest that coral holobionts might occupy structural landscapes ranging from a highly flexible to a rather inflexible composition with consequences for their ability to respond to environmental change.     

The relative importance of population versus community processes in microbial primary succession

Interpretations of successional patterns in ecological communities have traditionally adhered to the dichotomy between the Clementsian view that emphasizes community level processes and the Gleasonian view that stresses individual population responses. The present study evaluates the relative importance of each type of process during protistan primary succession in initially barren aquatic isolates (200-1 plastic pools) over a 170-d period. Species availability to these systems was manipulated by erecting exclosures around individual mesocosms to successively eliminate access to different dispersal vectors responsible for passive protistan dispersal. Increased exclosure significantly reduced access of autotrophs to the pools, but had little effect on heterotroph species availability. The species replacement process was directional through time and occurred at similar rates in all treatments. Both lower and upper temporal boundaries of heterotrophic and autotrophic species were contagious through time, as predicted by the Clementsian hypothesis, although the independence of these two boundary types suggested an individualistic model. Dominant and subdominant species were correlated into four temporal groups: pioneer, early successional, mid-successional, late successional. The dominance of several mid- and late successional species was reduced with increased exclosure. The loss of these species from successional pathways in more exclosed pools had no significant effect on the distribution of other species within the same temporal group. However, the establishment of these other mid- and late successional species may be dependent on initial colonization by pioneer and early successional species. Increased abundances of mid- and late successional species in less exclosed pools coincided with significant attenuations in the distribution of many early successional species. Interactions between successional groups may be related to the supply of inorganic resources as well as allelopathic effects. Patterns of protist succession are the result of both population and community processes; while species-specific characteristics (i.e., dispersal ability) may dominate the process in more isolated systems, increased species availability increases the relative importance of interspecific interactions.     

Competition Coalitions and Conflict Interventions among Captive Female Gorillas

Like mountain gorillas (Gorilla gorilla beringei), western gorillas (Gorilla gorilla gorilla) at Howletts Wild Animal Park in Kent, England, intervene in conflicts on behalf of kin. However, in each of the 3 study groups, the female gorillas also appeared to form political alliances: all members of the group almost exclusively supported familiar adult females, i.e., the ones with the greatest group tenure, and their offspring in conflicts involving adult females, the silverback, and immatures. The long-term resident high-status females (HSFs) appeared to form a supportive clique, providing effective competition against low-status females (LSFs). The former maintained dominance status over younger, less familiar adult females that were more recent to the group. Such a pattern is not typical of mountain gorillas in the wild—the subspecies for which data on female relationships are available— except perhaps when groups are unusually large, possibly because mountain gorillas experience little competition over food resources that are widely distributed and relatively freely available. In contrast, the Howletts gorillas had periodic and irregular access to high-energy/-nutrient food resources, for which dominant individuals were able to monopolize the limited available feeding spots. The pattern of agonistic alliances of Howletts females show some similarities with that of some female-philopatric cercopithecines, which also compete over defendable food resources. In female-transfer species, such as gorillas, long-term resident female cliques may be equivalent to matrilines in cercopithecines when resources are patchily distributed, highly nutritious, and defendable.     

林窗干扰与森林群落演替

林窗干扰是影响森林群落演替的一个重要因素。该文从林窗干扰和森林群落演替理论及林窗对森林群落微环境、植物入侵和定居、群落结构、群落演替的影响等方面简要介绍了当前国内外有关林窗干扰与森林群落演替的研究现状和研究前景。     

弃耕演替与恢复生态学

弃耕演替是一种重要的演替模式 ,可以为生态恢复规划提供指导。弃耕演替为进展演替 ,但要恢复为稳定的生态系统可能会经历很长一段时间。这是由演替本身的规律决定的。适当的人为干预可能会改变这种状况 ,并使演替进程大大缩短。对弃耕演替的研究是恢复生态学兴起的原始策动力 ,恢复生态学是在对弃耕演替的研究中形成并获得发展的。弃耕演替研究隶属于恢复生态学 ,是恢复生态学涵盖的一个重要领域。     

Evolution and diversity of Arsenophonus endosymbionts in aphids

Endosymbiotic bacteria are important drivers of insect evolutionary ecology, acting both as partners that contribute to host adaptation and as subtle parasites that manipulate host reproduction. Among them, the genus Arsenophonus is emerging as one of the most widespread lineages. Its biology is, however, entirely unknown in most cases, and it is therefore unclear how infections spread through insect populations. Here we examine the incidence and evolutionary history of Arsenophonus in aphid populations from 86 species, characterizing the processes that shape their diversity. We identify aphids as harbouring an important diversity of Arsenophonus strains. Present in 7% of the sampled species, incidence was especially high in the Aphis genus with more than 31% of the infected species. Phylogenetic investigations revealed that these Arseno‐phonus strains do not cluster within an aphid‐specific clade but rather exhibit distinct evolutionary origins showing that they undergo repeated horizontal transfers (HT) between distantly related host species. Their diversity pattern strongly suggests that ecological interactions, such as plant mediation and parasitism, are major drivers for Arsenophonus dispersal, dictating global incidence across insect communities. Notably, plants hosting aphids may be important ecological arenas for global exchange of Arsenophonus, serving as reservoirs for HT.     

湿地植物间竞争和促进互作的研究进展

湿地植物间关系主要包括竞争(负效应)和促进(正效应),是湿地生态研究的核心内容之一,对植物分布、群落和生态系统组成具有重要的调节作用。本文阐述了竞争理论及影响湿地植物竞争能力的主要因素,介绍了促进作用及其在湿地生态系统中的形成机理,详细综述了植物间关系转变的主要理论以及影响湿地植物间关系转变的主要环境因子(水位、温度、盐度、生物因子)。随着环境胁迫强度的增加,植物间关系一般会从竞争转变为促进作用,即优越条件下以竞争为主,胁迫条件下以促进作用为主。针对当前湿地植物间关系的研究多集中于滨海盐生湿地,类型较单一的情况,认为将来需加强对淡水湿地、河口湿地等各类系统的研究。同时,加强湿地植物间关系转变的生物学机制、时间效应,对新研究方法的探索等方面的研究也有助于进一步理解湿地植物间竞争和促进作用的发生及转变机理。     

Post-burn succession on Brittany heathlands

Abstract. Vegetation succession after severe burning of a Brittany heathland was recorded in permanent plots for ten years. Mosses are the first colonizers, soon forming a dense layer which locally prevents establishment of new species. Ten years later, the moss layer is still an important component of the regenerating heathland. After ten years the pattern of species distribution is mosaic-like with grassy patches (Agrostis), heath patches (Erica, Flex), and wood (Betula). Processes which may be involved in this successional pattern are discussed. They are found to be pluriform: various strategy types, types of response to disturbance and various succession models operate simultaneously.     

Cockroaches that lack Blattabacterium endosymbionts: the phylogenetically divergent genus Nocticola

Phylogenetic relationships among termites, mantids and the five traditionally recognized cockroach families have been the subject of several studies during the last half-century. One cockroach lineage that has remained notably absent from such studies is the Nocticolidae. This group of small, elusive surface- and cave-dwelling species from the Old World Tropics has been proposed to represent an additional family. Using molecular sequences, we performed an initial phylogenetic examination of Nocticola spp. The hypothesis that they are phylogenetically divergent was confirmed from the analyses of three genes and a combined dataset. To supplement our phylogenetic analyses, we attempted to amplify 16S rRNA from the obligate mutualistic endosymbiont Blattabacterium cuenoti, present in all cockroaches studied to date. Unexpectedly, amplification was unsuccessful in all Nocticola spp. examined. This result was confirmed by microscopic examinations of fat body tissue. These Nocticola spp. are the first cockroaches found to be uninfected by B. cuenoti, which raise questions about when the bacterium first infected cockroaches.