Volume 7, Nº 5 (2017)

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.

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.

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 ~ H^μ). 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.

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.