


卷 7, 编号 5 (2017)
- 年: 2017
- 文章: 7
- URL: https://bakhtiniada.ru/2079-0864/issue/view/12820
Article
Evolution of nitrogen-fixing symbioses on the basis of bacterial migration from mycorrhizal fungi and soil into plant tissues
摘要
A hypothesis is proposed on the emergence of N2-fixing plant symbionts from soil diazotrophs and satellites of Glomeromycota fungi that form arbuscular mycorrhizae (AM). This universal form of plant-microbe symbiosis possibly appeared due to the integration of ancestral land plants (rhyniophytes, psylophytes) and microbial consortia composed of AM-fungi assimilating soil phosphates and bacteria fixing atmospheric CO2 and/or N2. The release of these bacteria from AM-fungal hyphae into plant tissues elicited the selection of genotypes capable of fungi-independent multiplication in plants, as well as the fixation in bacterial genomes of the genes for the synthesis of chitin-like signal factors stimulating the development of symbiotic structures. An early stage of this evolution might been represented by the formation of N2-fixing syncyanoses, and the late stage might have been realized by the formation of nodular symbioses of dicots from the Eurosid I clade with rhizobia (α- and β-proteobacteria) and with the actinobacteria Frankia. The emergence of these symbioses was possibly based on the migration of soil and endophytic bacteria into the storage organs (modified stems or lateral roots), where the optimal conditions were established not only for N2 fixation but also for the evolution of bacteria towards increased symbiotic activity. This evolution resulted in the emergence of primary rhizobia (Bradyrhizobium, Burkholderia), which acted as donors of sym genes for a broad spectrum of microbes transformed into secondary rhizobia (Rhizobium, Sinorhizobium). The subsequent evolution of nodular symbioses was directed at an increased efficiency of symbiotrophic nitrogen nutrition in host plants following two scenarios: (a) “expensive,” which is based on an increase in N2-fixing activity via the transformation of bacteria into nonreproducible bacteroids; (b) “economical,” based on the acquisition of the determinate nodule structure and ureide nitrogen assimilation.



Adaptation of Drosophila melanogaster to unfavorable feed substrate is accompanied by expansion of trophic niche
摘要
Adaptation to stress factors is frequently accompanied by negative side effects that are manifested in a decreased adaptation to an environment in which this stress factor is absent. This can cause ecological specialization of forms adapted to the stress conditions to the point of their separation into a new species (ecological speciation). On the other hand, the existence of eurybiont species with a wide spectrum of ecological tolerance indicates that adaptation to marginal conditions can presumably occur without negative side effects or even be accompanied by positive effects (which leads to expansion of the ecological niche). There are few experimental confirmations of the reality of such an evolutionary scenario. During an evolutionary experiment on the Drosophila melanogaster, experimental fly lines successfully adapted to life on stressful feed substrates with an increased NaCl content for 20 generations. This was manifested in an increase in the total amount of descendants produced by parents from adapted lines on stressful substrate for a fixed time as compared with the control (unadapted) line, as well as in a reduction of the larval development delay caused by high NaCl concentrations. Not only did the adaptation to a standard (favorable) feed medium in lines adapted to salt not decrease, but it even increased. Thus, adaptation to unfavorable medium actually resulted in trophic niche expansion. Along with similar results previously obtained during a study of the D. melanogaster adaptation to a depleted feed medium based on starch, these data indicate that an adaptation to marginal conditions that is accompanied by positive side effects and does not lead to constriction but to expansion of the trophic niche can be a common phenomenon in such eurybiont species as D. melanogaster, which probably largely explains their eurybiontity. The small number of experimentally confirmed examples of such a scenario can be partly explained be the small number of attempted searches for them. Adaptive changes of symbiotic microbiota, which are efficiently transmitted in the Drosophila from parents to descendants when the descendants eat the same substrate as the parents, can be one possible mechanism of “wide-profile” adaptation output during the development of the stressful substrate. Thus, a growth of the number of symbiotic Lactobacilli in the Drosophila intestine can promote acceleration of larvae growth, extension of imago life, and an increased efficiency of the used feed substrate. Further studies are required to detect the specific mechanisms responsible for the changes in adaptability found during the evolutionary experiment.



Intergenerational epigenetic effects: Influence of increased temperature on lifespan of Drosophila melanogaster
摘要
Morphophysiological changes not associated with DNA sequence violation belong to the field of epigenetic phenomena. The results of this study demonstrate that impulse, not mutagenic, heat stress on the emerging egg cells (proembryo) causes intergenerational epigenetic positive changes of the lifespan in the offspring. This effect depends on heterozygosity for the lgl mutation, which is obtained from mothers. Determination of the property of increased survival of F1 descendants occurs during early oogenesis. The largest effect is observed from the cell heating of the earliest proembryo stages. The present study is the first experimental work modeling the intergenerational aftereffect of prezygotic (proembryo) stress on the survival rate and lifespan of the progeny with the involvement of the lgl tumor suppressor. The significance of the obtained data was discussed in connection with human epigenetics.



From uncertainty to an exact number: Developing a method to estimate the fitness of a clonal species with polyvariant ontogeny
摘要
The task to estimate the fitness of a clonal plant with polyvariant ontogeny reduces to compiling a life cycle graph, constructing and calibrating the corresponding matrix model of the discrete-structured population, and calculating the dominant eigenvalue (λ1) of the model matrix. We demonstrate a solution to this task with a sample of the woodreed (Calamagrostis epigeios), a perennial long-rhizome grass propagating vegetatively, and data on the age-stage structure of its local population. A traditional technique of successive censuses fixing the age-stage status of all individuals (ramets) on a permanent sample plot (SP) provides for calculating the frequencies of ontogenetic transitions directly from the data, but leaves uncertain the status-specific reproduction rates as the recruit parents are unknown (“reproductive uncertainty”). Uncertainty in data was leading to that in the estimation and dictating a need to modify the method of field study. Thus, the description of above-ground parts of plants within SPs has been completed with the analyses of rhizome parent- daughter links revealed after digging the ramets out. However, the traditional fixed area of SPs (1 m2) forced cutting the links off along the perimeter, while those within the SP turned out quite entangled already in a 4-year-old colony. As a result, the reproductive uncertainties were not eliminated completely, and the next step in the method development has become to determine the contour of the entire woodreed colony and to carefully dig the colony out. Analyzing both the above- and below-ground spheres of the colony has enabled us to calculate uniquely all the elements of the model matrix, hence the value of λ1, while accounting for the actual area of the contour in the current and previous years amends the value of λ1 needed for comparison with the results of previous studies.



Evolutionary trajectories in the parameter space of the sectional model of spruce (Picea abies) crown: Emergence of the “protoplant”
摘要
A sectional model of the dynamics of the system of regular spruce (Picea abies) branches, as well as submodels of the inhibition of initial growth and inter-verticillate branches, were extended to the range of (0, 3) of the fractаl parameter μ which reflects the association of green biomass with tree size (B ~ Hμ). It is shown that the proto-spruce branches of the first three orders appear in the subrange of (0, 1) of the μ parameter. The branches of the first order appear at μ ≈ 0.25, while inter-verticillate branches appear at μ ≈ 1.4, which may be an element of spruce adaptation to unstable illumination conditions. The presence of green biomass at μ < 1.0 indicates that it can be represented as a set of spatial photosynthesizing points (hypothetical cyanobacteria). Therefore, the fractal properties of the set of these points located on a line segment are considered as the model. The condition of μ < 1.0 is shown to be true if the points are arranged in groups. In this case, μ is practically independent of groups allocation at the segment and depends only on the number and the type of distribution of points within the groups. For a fixed number of points in a group (for example, ng = 2) distributed randomly and uniformly, with increasing number of Ng groups, the parameter μ decreases from 1 to ≈0.25. Conversely, for a fixed large numberNg of groups with increasing from 2 number of points ng in the group, the parameter μ increases, tending to 1. Based on these fractal properties of placing the point groups, as well as on the hypothesis of the trophic nature of the organelle symbiogenesis in the eukaryotic cell, a two-stage mechanism of the emergence of protoplants was suggested. This mechanism is manifested in motion along the endosymbiosis trajectory, which is characterized by a constant number of points in a group and by increasing number of groups until, in the course of evolution, the host organism creates an infrastructure that allows the supplying of and interaction between cyanobacterial groups. At this stage, μ decreases from 1 to ≈0.25. When the infrastructure is created, and it becomes possible to increase the number of points in the group, the second stage begins. This stage is characterized by motion along the trajectory, which runs by doubling of cyanobacteria in the group (μ tends to 1). At the first stage, motion along such a composite trajectory results in a slow increase in the size of the photosynthetic system, even if the number of cyanobacterial groups grows exponentially. However, at the second stage, this size grows rapidly. The similar inhibition of initial growth is observed in today trees and manifests in the initial deceleration of the increase of branch orders.



Local population of Eritrichium caucasicum as an object of mathematical modelling. I. Life cycle graph and a nonautonomous matrix model
摘要
For the plant species, which is considered a short-lived perennial, we have composed a scale of ontogenetic stages and the life cycle graph (LCG) according to annual observations on permanent sample plots in an Alpine lichen heathland during the 2009–2014 period. The LCG that reflects seed reproduction has been reduced to the one that avoids the stage of soil seed bank, yet preserves the arcs of annual recruitment. The corresponding matrix model of stage-structured population dynamics has four stages: juvenile plants (including seedlings), vegetative, generative, and ‘terminally generative’ (the plants die after seed production). Model calibration reduces to directly calculating the rates of transition between stages and those of delays within stages from the data of only one time step, while keeping the two reproduction rates uncertain, yet confined to the quantitative bounds of observed recruitment. This has enabled us to determine a feasible range for the dominant eigenvalue of the model matrix, i.e., the quantitative bounds for the measure of how the local population adapts to its environment, at each of the five time steps, resulting in a formally nonautonomous model. To obtain “age-specific parameters” from a stage-classified model, we have applied the technique that constructs a virtual absorbing Markov chain and calculates its fundamental matrix. In a nonautonomous model, the estimates of life expectancy also depend on the time of observation (that fixes certain environmental conditions), and vary from two to nearly seven years. The estimates reveal how specifically short lives the short-lived perennial, while their range motivates the task to average the model matrices over the whole period of observation. The model indicates that Eritrichium caucasicum plants spend the most part of their life span in the adult vegetative stage under each of the environment conditions observed, thus revealing the space holder strategy by C. Körner (2003), or the delayed-development strategy by L.A. Zhukova (1995). We discuss the prospects of model experiments with a logically nonautonomous model to forecast the long-term dynamics of E. caucasicum, should a scenario of climate changes be given.



Phylogenesis of reproductive strategies in labyrinth fishes (Anabantoidei) and their sister groups
摘要
Clado- and semogenetic approaches, when used in concert, make it possible to resolve questions concerning the phylogenetic relationships between group representatives, as well as the phylogenesis of those representatives’ characters. Parental care patterns and other forms of reproductive behavior, along with the reproductive strategy as a whole, can be the subject of semogenetic analysis to no less an extent than morphological structures sensu stricto. One highly specialized form of parental care in fish, including representatives of the suborder of labyrinth fishes and their sister groups, appears to be parental food provisioning. In our view, the evolutionary origin of postembryonic brood provisioning in bony fishes is characterized by three main features: (1) in fish, different forms of postembryonic food provisioning are convergent in their origin; (2) any kind of brood provisioning is realized through exploitation of the already existing character and is maintained by selection due to an enhancement of offspring fitness; (3) the main evolutionary path of the emergence and development of this phenomenon consists in function expansion and replacement. The hypothesis does have heuristic power, since it enables prediction of the presence of the reproductive strategy component in question through the identification of adequate basic adaptations. Although parental care occurs in the majority of anabantoid fishes, there are still several species for which such care is not known. On the cladogram, these species by no means take the basal position but are surrounded by fishes that provide care for their eggs, or even their hatchlings. The parsimony principle leads to the suggestion that parental care is a plesiomorphic character in the suborder Anabantoidei (or in the order Anabantiformes). It seems that the ancestors of present day noncarrying species that take various positions within this phylogenetic group were fishes showing parental care. Their reproductive strategy later changed as a result of r selection. If this hypothesis is correct, the absence of parental care should be considered a case of reproductive strategy degradation. It is quite probable that parental food provisioning was a component of the ancestral reproductive strategies. It is also possible that the reproductive strategy of present-day anabantoids that supposedly do not care for their offspring actually includes some optional forms of parental care.


