Biology subjects

Nikitin, M. A.

Publications and source records attributed to Nikitin, M. A..

6 recordsLinked to original sources

bioRxiv2023

Lack of early animal fossils: insights from taphonomic experiments on placozoans and their traces

Placozoa are simple two-layered multicellular animals. Can such a simple animal preserve in the fossil record? The results of our taphonomic experiments showed that it is chemically possible; however, the chances of the presevation are negligible for two reasons. Firstly, the resistance of living Trichoplax to lethal factors turned out to be quite high. Secondly, post-mortem changes in the body of Trichoplax preclude preservation. We observed these changes for the first time: their bodies immediately disintegrated into a cloud of individual dead cells, apparently unrecognizable in the fossil record. We hypothesized that the absence of a basement membrane, which unites each cell layer into the epithelia, is a key factor in this dramatic disintegration. This hypothesis adds an option to the explainations of the gap between the time of emergence of metazoan body fossils and the reconstructed time of metazoan divergence from a single-celled ancestor. However, it is theoretically possible to find ichnofossils of Trichoplax-like locomotion. For the first time, we observed traces of Trichoplax on a soft substrate and identified a previously unknown type of traces. They were 3D chains of compacted round lumps of the substrate, similar in some features to the Precambrian meniscus ichnofossils.

paleontology↗

bioRxiv2022

Demise of Marimermithida refines primary routes of transition to parasitism in roundworms

Nematodes (roundworms) are ubiquitous animals commonly dominating in ecological communities and networks, with many parasites and pathogen vectors of great economic and medical significance. Nematode parasites are remarkably diverse in life strategies and adaptations at a great range of hosts and dimension scales, from whales to protozoan cells. Their life history is intricate and requires understanding to study the genomic, structural and ecological bases of successful transitions to parasitism. Based on analyses of rDNA for a representative sampling of host-associated and free-living groups, we dismiss the last higher-rank nematode taxon uniting solely parasitic forms (Marimermithida) and show that primarily marine parasitism emerged independently and repeatedly within only few free-living lineages. We re-evaluate the significance of some traditionally important phenotypic characters and report the phenomenon of dramatic adaptation to parasitism on very short evolutionary timescales. A cross-phylum character interpretation vindicates that non-intestinal (in-tissue or cavitary) host capture was likely a primary route of transition to truly exploitive parasitism (vs. intestinal commensalism) in roundworms, and extant nematode parasitoids (larval parasites) infesting the host body cavity or internal organs realise this primary lifestyle. Parasitism may have evolved in nematodes as part of innate pre-adaptations to crossing environmental boarders, and such transitions have been accomplished multiple times successfully in the phylum history.

zoology↗

bioRxiv2021

Life Strategies in Placozoa

Placozoans are essential reference species for understanding the origins and evolution of the animal organization. However, little is known about their life strategies in natural habitats. Here, by establishing long-term culturing for four species of Trichoplax and Hoilungia, we extend our knowledge about feeding and reproductive adaptations relevant to their ecology and immune mechanisms. Three modes of population growth depended upon feeding sources, including induction of social behaviors and different reproductive strategies. In addition to fission, representatives of all haplotypes produced swarmers, which could be formed from the lower epithelium (with greater cell- type diversity) as a separate asexual reproduction stage. In aging culture, we reported the formation of specialized structures ( spheres) from the upper cell layer as a part of the innate immune defense response with the involvement of fiber cells. Finally, we showed that regeneration could be a part of the adaptive reproductive strategies in placozoans and a unique model for regenerative biology in general.

developmental biology↗

bioRxiv2020

Hidden cell diversity in Placozoa: Ultrastructural Insights from Hoilungia hongkongensis

From a morphological point of view, placozoans are among the most simple free-living animals. This enigmatic phylum is critical for our understanding of the evolution of animals and their cell types. Their millimeter-sized, disc-like bodies consist of only three cell layers that are shaped by roughly six major cell types. Placozoans lack muscle cells and neurons but are able to move using their ciliated lower surface and take up food in a highly coordinated manner. Intriguingly, the genome of Trichoplax adhaerens, the founding member of the enigmatic phylum, has disclosed a surprising level of genetic complexity. Moreover, recent molecular and functional investigations have uncovered a much larger, so-far hidden cell-type diversity. Here, we have extended the microanatomical characterization of a recently described placozoan species - Hoilungia hongkongensis. In H. hongkongensis, we recognized the established canonical three-layered placozoan body plan but also came across several morphologically distinct and potentially novel cell types, among them novel gland cells and "shiny spheres"-bearing cells at the upper epithelium. Thus, the diversity of cell types in placozoans is indeed higher than anticipated.

evolutionary biology↗

bioRxiv2020

Action Potentials and Na+ voltage-gated ion channels in Placozoa

Placozoa are small disc-shaped animals, representing the simplest known, possibly ancestral, organization of free-living animals. With only six morphological distinct cell types, without any recognized neurons or muscle, placozoans exhibit fast effector reactions and complex behaviors. However, little is known about electrogenic mechanisms in these animals. Here, we showed the presence of rapid action potentials in four species of placozoans (Trichoplax adhaerens [H1 haplotype], Trichoplax sp.[H2], Hoilungia hongkongensis [H13], and Hoilungia sp. [H4]). These action potentials are sodium-dependent and can be inducible. The molecular analysis suggests the presence of 5-7 different types of voltage-gated sodium channels, which showed substantial evolutionary radiation compared to many other metazoans. Such unexpected diversity of sodium channels in early-branched animal lineages reflect both duplication events and parallel evolution of unique behavioral integration in these nerveless animals. HighlightsO_LIPlacozoans are the simplest known animals without recognized neurons and muscles C_LIO_LIWith only six morphological cell types, placozoans showed complex & rapid behaviors C_LIO_LISodium-dependent action potentials have been discovered in intact animals C_LIO_LIVoltage-gated sodium channels (Nav) in Placozoa support a rapid behavioral integration C_LIO_LIPlacozoans have more Nav channels that any studied invertebrate animal so far C_LIO_LIDiversification of Nav-channels highlight the unique evolution of these nerveless animals C_LI O_FIG O_LINKSMALLFIG WIDTH=200 HEIGHT=157 SRC="FIGDIR/small/243113v2_ufig1.gif" ALT="Figure 1"> View larger version (52K): org.highwire.dtl.DTLVardef@4d9c5forg.highwire.dtl.DTLVardef@15539bforg.highwire.dtl.DTLVardef@4246faorg.highwire.dtl.DTLVardef@141e5fe_HPS_FORMAT_FIGEXP M_FIG C_FIG

evolutionary biology↗

bioRxiv2020

Nitric oxide (NO) signaling in Trichoplax and related species: Microchemical characterization and the lineage-specific diversification

Nitric oxide (NO) is a free radical gaseous messenger with a broad distribution across the animal kingdom. However, the early evolution of nitric oxide-mediated signaling in animals is unclear due to limited information about prebilaterian metazoans such as placozoans. Here, we analyzed NO synthases (NOS) in four different species of placozoans (haplotypes H1, H2, H4, H13). In contrast to all other invertebrates studied, Hoilungia and Trichoplax have three distinct NOS genes, including PDZ domain-containing NOS. To characterize NOS activity in Trichoplax adhaerens, we used capillary electrophoresis for microchemical assays of NO-related metabolites. Specifically, we quantified nitrites (products of NO oxidation) and L-citrulline (co-product of NO synthesis from L-arginine), which were affected by NOS inhibitors confirming the presence of functional NOS. Next, using fluorescent single-molecule in situ hybridization, we showed that distinct NOSs are expressed in different subpopulations of cells, with a noticeable distribution close to the edge regions of Trichoplax. These data suggest the compartmentalized release of this messenger and a greater diversity of cell types in placozoans than anticipated. We also revealed a dramatic diversification of NO receptor machinery, including identification of both canonical and novel NIT-domain containing soluble guanylate cyclases as putative NO/nitrite/nitrate sensors. Thus, although Trichoplax is considered to be one of the morphologically simplest free-living animals, the complexity of NO-cGMP-mediated signaling is greater to those in vertebrates. This situation illuminates multiple lineage-specific diversifications of NOSs and NO/nitrite/nitrate sensors from the common ancestor of Metazoa. Short AbstractNitric oxide (NO) is a ubiquitous gaseous messenger, but we know little about its early evolution. Here, we analyzed NO synthases (NOS) in four different species of placozoans - one of the early-branching animal lineages. In contrast to other invertebrates studied, Trichoplax and Hoilungia have three distinct NOS genes, including PDZ domain-containing NOS. Using ultra-sensitive capillary electrophoresis assays, we quantified nitrites (products of NO oxidation) and L-citrulline (co-product of NO synthesis from L-arginine), which were affected by NOS inhibitors confirming the presence of functional enzymes in Trichoplax. Using fluorescent single-molecule in situ hybridization, we showed that distinct NOSs are expressed in different subpopulations of cells, with a noticeable distribution close to the edge regions of Trichoplax. These data suggest both the compartmentalized release of NO and a greater diversity of cell types in placozoans than anticipated. NO receptor machinery includes both canonical and novel NIT-domain containing soluble guanylate cyclases as putative NO/nitrite/nitrate sensors. Thus, although Trichoplax and Hoilungia exemplify the morphologically simplest free-living animals, the complexity of NO-cGMP-mediated signaling in Placozoa is greater to those in vertebrates. This situation illuminates multiple lineage-specific diversifications of NOSs and NO/nitrite/nitrate sensors from the common ancestor of Metazoa.

evolutionary biology↗