Different phylogenomic methods support monophyly of enigmatic 'Mesozoa' (Dicyemida + Orthonectida, Lophotrochozoa)

TAXES OF DICYEMIDS (PHYLUM DICYEMIDA)

Dicyemids (Phylum Dicyemida) are endosymbionts present in the kidneys of benthic cephalopods. They usually consist of 10 to 40 cells and are characterized by 2 distinct body types: vermiform individuals and infusoriform larvae. Vermiform individuals remain attached to the internal surface of the host's renal appendages, while infusoriform larvae leave the renal sac to search for a new host. To investigate how dicyemids respond to various host and environmental cues, we evaluated phototaxis, chemotaxis, thigmotaxis, and rheotaxis responses of vermiform individuals and infusoriform larvae of 2 dicyemid species in a laboratory setting. Vermiform individuals did not exhibit phototaxis and chemotaxis to the major components of the host: urine, tissue fluids, or extracts of the host gills. However, they showed positive thigmotaxis and positive rheotaxis to slow water flow, probably contributing to enabling attachment to the renal appendages and remaining in the renal sac, respectively. The infusoriform larvae exhibited negative chemotaxis to host blood and negative thigmotaxis, but there was no evidence of phototaxis and rheotaxis. Negative thigmotaxis may facilitate the release of infusoriform embryos from the renal appendages. Negative chemotaxis to the host blood suggests that the infusoriform larvae do not enter through the vascular system to gain access to the renal sac, so the process by which infusoriform larvae enter the cephalopod host is yet to be determined.

Evolution of Orthonectida body plan

Orthonectida is an enigmatic group of animals with still uncertain phylogenetic position. Orthonectids parasitize various marine invertebrates. Their life cycle comprises a parasitic plasmodium and free-living males and females. Sexual individuals develop inside the plasmodium; after egress from the host they copulate in the external environment, and the larva, which has developed inside the female infects a new host. In a series of studied orthonectid species simplification of free-living sexual individuals can be clearly traced. The number of longitudinal and transverse muscle fibers is gradually reduced. In the nervous system, simplification is even more pronounced. The number of neurons constituting the ganglion is dramatically reduced from 200 in Rhopalura ophiocomae to 4-6 in Intoshia variabili. The peripheral nervous system undergoes gradual simplification as well. The morphological simplification is accompanied with genome reduction. However, not only genes are lost from the genome, it also undergoes compactization ensured by extreme reduction of intergenic distances, short intron sizes, and elimination of repetitive elements. The main trend in orthonectid evolution is simplification and miniaturization of free-living sexual individuals coupled with reduction and compactization of the genome.

RNA-seq analysis of parasitism by Intoshia linei (Orthonectida) reveals protein effectors of defence, communication, feeding and growth

Orthonectida is a group of multicellular endoparasites of a wide range of marine invertebrates. Their parasitic stage is a multinuclear shapeless plasmodium infiltrating host tissues. The development of the following worm-like sexual generation takes place within the cytoplasm of the plasmodium. The existence of the plasmodial stage and the development of a sexual stage within the plasmodium are unique features to Bilateria. However, the molecular mechanisms that maintain this peculiar organism, and hence enable parasitism in orthonectids, are unknown. Here, we present the first-ever RNA-seq analysis of the plasmodium, aimed at the identification and characterization of the plasmodium-specific protein-coding genes and corresponding hypothetical proteins that distinguish the parasitic plasmodium stage from the sexual stage of the orthonectid Intoshia linei Giard, 1877, parasite of nemertean Lineus ruber Müller, 1774. We discovered 119 plasmodium-specific proteins, 82 of which have inferred functions based on known domains. Thirty-five of the detected proteins are orphans, at least part of which may reflect the unique evolutionary adaptations of orthonectids to parasitism. Some of the identified proteins are known effector molecules of other endoparasites suggesting convergence. Our data indicate that the plasmodium-specific proteins might be involved in the plasmodium defense against the host, host-parasite communication, feeding and nutrient uptake, growth within the host, and support of the sexual stage development. These molecular processes in orthonectids have not been described before, and the particular protein effectors remained unknown until now.

Phoronida-A small clade with a big role in understanding the evolution of lophophorates

Phoronids, together with brachiopods and bryozoans, form the animal clade Lophophorata. Modern lophophorates are quite diverse-some can biomineralize while others are soft-bodied, they could be either solitary or colonial, and they develop through various eccentric larval stages that undergo different types of metamorphoses. The diversity of this clade is further enriched by numerous extinct fossil lineages with their own distinct body plans and life histories. In this review, I discuss how data on phoronid development, genetics, and morphology can inform our understanding of lophophorate evolution. The actinotrocha larvae of phoronids is a well documented example of intercalation of the new larval body plan, which can be used to study how new life stages emerge in animals with biphasic life cycle. The genomic and embryonic data from phoronids, in concert with studies of the fossil lophophorates, allow the more precise reconstruction of the evolution of lophophorate biomineralization. Finally, the regenerative and asexual abilities of phoronids can shed new light on the evolution of coloniality in lophophorates. As evident from those examples, Phoronida occupies a central role in the discussion of the evolution of lophophorate body plans and life histories.

Acceleration of genome rearrangement in clitellate annelids

Comparisons of multiple metazoan genomes have revealed the existence of ancestral linkage groups (ALGs), genomic scaffolds sharing sets of orthologous genes that have been inherited from ancestral animals for hundreds of millions of years (Simakov et al. 2022; Schultz et al. 2023) These ALGs have persisted across major animal taxa including Cnidaria, Deuterostomia, Ecdysozoa and Spiralia. Notwithstanding this general trend of chromosome-scale conservation, ALGs have been obliterated by extensive genome rearrangements in certain groups, most notably including Clitellata (oligochaetes and leeches), a group of easily overlooked invertebrates that is of tremendous ecological, agricultural and economic importance (Charles 2019; Barrett 2016). To further investigate these rearrangements, we have undertaken a comparison of 12 clitellate genomes (including four newly sequenced species) and 11 outgroup representatives. We show that these rearrangements began at the base of the Clitellata (rather than progressing gradually throughout polychaete annelids), that the inter-chromosomal rearrangements continue in several clitellate lineages and that these events have substantially shaped the evolution of the otherwise highly conserved Hox cluster.

Plasmodium structure of Intoshia linei (Orthonectida)

Orthonectids are enigmatic parasitic bilaterians whose exact position on the phylogenetic tree is still uncertain. Despite ongoing debate about their phylogenetic position, the parasitic stage of orthonectids known as "plasmodium" remains underexplored. There is still no consensus on the origin of the plasmodium: whether it is an altered host cell or a parasitic organism that develops in the host extracellular environment. To determine the origin of the orthonectid parasitic stage, we studied in detail the fine structure of the Intoshia linei orthonectid plasmodium using a variety of morphological methods. The orthonectid plasmodium is a shapeless multinucleated organism separated from host tissues by a double membrane envelope. Besides numerous nuclei, its cytoplasm contains organelles typical for other bilaterians, reproductive cells, and maturing sexual specimens. Reproductive cells, as well as developing orthonectid males and females, are covered by an additional membrane. The plasmodium forms protrusions directed to the surface of the host body and used by mature individuals for egress from the host. The obtained results indicate that the orthonectid plasmodium is an extracellular parasite. A possible mechanism for its formation might involve spreading parasitic larva cells across the host tissues with subsequent generation of a cell-within-cell complex. The cytoplasm of the plasmodium originates from the outer cell, which undergoes multiple nuclear divisions without cytokinesis, while the inner cell divides, giving rise to reproductive cells and embryos. The term "plasmodium" should be avoided and the term "orthonectid plasmodium" could be temporarily used instead.