A "chimera" theory on the origin of dicyemid mesozoans: evolution driven by frequent lateral gene transfer from host to parasite

The phylogenetic status of the enigmatic dicyemid mesozoans is still uncertain. Are they primitive multicellular organisms or degenerate triploblastic animals? Presently, the latter view is accepted. A phylogenetic analysis of 18S rDNA sequences placed dicyemids within the animal clade, and this was supported by the discovery of a Hox-type gene with a lophotrochozoan signature sequence. This molecular information suggests that dicyemid mesozoans evolved from an ancestral animal degenerately. Considering their extreme simplicity, which is probably due to parasitism, they might have come from an early embryo via a radical transformation, i.e. neoteny. Irrespective of this molecular information, dicyemid mesozoans retain many protistan-like or extremely primitive features, such as tubular mitochondrial cristae, endocytic ability from the outer surface, and the absence of collagenous tissue, while they do not share noticeable synapomorphy with animals. In addition, the 5S rRNA phylogeny suggests a somewhat closer kinship with protozoan ciliates than with animals. If we accept this clear contradiction, dicyemids should be regarded as a chimera of animals and protistans. Here, we discuss the traditional theory of extreme degeneration via parasitism, and then propose a new "chimera" theory in which dicyemid mesozoans are exposed to a continual flow of genetic information via eating host tissues from the outer surface by endocytosis. Consequently, many of their intrinsic genes have been replaced by host-derived genes through lateral gene transfer (LGT), implying that LGT is a key driving force in the evolution of dicyemid mesozoans.     

Cell number and cellular composition in infusoriform larvae of dicyemid mesozoans (Phylum Dicyemida)

Cell numbers and cellular composition were examined in infusoriform larvae of 44 species of dicyemid mesozoans belonging to 6 genera; Conocyema, Dicyema, Dicyemennea, Dicyemodeca, Microcyema, and Pseudicyema. In addition, literature on infusoriform larvae of another 20 species was reviewed. Infusoriform larvae consist of a constant cell number which is species-specific. Small interspecific variations are found in total cell numbers, 35, 37, 39, 41 and 42. The most frequent cell number encountered in infusoriform larvae studied is either 37 or 39. Infusoriform larvae with 35 cells are found in three genera. Infusoriform larvae with 37 cells are found in four genera. Infusoriform larvae with 39 cells are found in four genera. Most differences in total cell numbers are due to the absence or presence of particular ventral cells. In all infusoriform larvae, the lateral, dorsal and caudal areas are cell constant, whereas in the apical and ventral areas a distinct and variable configuration of cells are present. In cellular composition, a total of 29 cells (15 cell types) were recognized in all infusoriform larvae examined. Additional cell types are characteristic of a relatively few species. Even in infusoriform larvae with the same total cell numbers, cellular composition varies by species. Thus, there are 7 variations of cellular composition in infusoriform larvae with 37 cells. Differences in larval cell numbers and types do not warrant traditional generic separation of dicyemids.     

Fine structure of dicyemid mesozoans,with special reference to cell junctions

The fine structure of the dicyemid mesozoan, Dicyema acuticephalum, from Octopus vulgaris, was studied with special attention to intercellular junctional complexes between various kinds of cells. Two types of intercellular junction, namely, adherens junctions and gap junctions, were found in both vermiform stages and in infusoriform embryos. Adherens junctions were classified into two types. Zonulae adherentes-like junctions were observed between adjacent peripheral cells at vermiform stages, between adjacent external cells of infusoriform embryos, and between members of groups of internal cells that covered the urn in infusoriform embryos. Maculae adherentes-like junctions were seen between a peripheral cell and an axial cell at vermiform stages. In infusoriform embryos, these junctions were observed between various types of cells, excluding urn cells. Gap junctions were found between adjacent peripheral cells at vermiform stages, whereas in infusoriform embryos these junctions were located between various types of cells excluding urn cells. Dicyemids might be the most primitive multicellular animals to possess these basic types of cell junctions. Ciliary rootlet systems at vermiform stages and in infusoriform embryos were unique in structure compared with those of other primitive multicellular animals. J Morphol 231:297–305, 1997. © 1997 Wiley-Liss, Inc.     

Three new species of dicyemid mesozoans (phylum Dicyemida) from Amphioctopus fangsiao (Mollusca: Cephalopoda), with comments on the occurrence patterns of dicyemids

Three new species of dicyemid mesozoans are described from the renal appendages of Amphioctopus fangsiao, collected off Akashi, in Harima Nada, and from Osaka Bay. Dicyema akashiense n. sp. is a small species that reaches about 900 microm in length. The vermiform stages are characterized as having 15-17 peripheral cells, a conical calotte, and an axial cell that extends to the base of the metapolar cells. Infusoriform embryos consist of 37 cells; two nuclei are present in each urn cell, and the refringent bodies are solid. Dicyema helocephalum n. sp. is a small species that reaches about 800 microm in length. The vermiform stages are characterized as having 22 peripheral cells, a disc-shaped calotte, and an axial cell that extends to the base of the propolar cells. Infusoriform embryos consist of 37 cells; a single nucleus is present in each urn cell, and the refringent bodies are solid. Dicyema awajiense n. sp. is a small species that reaches about 300 microm in length. The vermiform stages are characterized as having 22 peripheral cells, a conical calotte, and an axial cell that extends to the middle of the propolar cells. Infusoriform embryos consist of 37 cells; a single nucleus is present in each urn cell, and the refringent bodies are solid. In A. fangsiao various occurrence patterns of dicyemid species were observed, including instances where different dicyemid species were found in the renal appendage on each side. This suggests that dicyemids infect each renal appendage independently. The prevalence, reproductive traits, calotte shapes, and co-occurrence patterns of dicyemids are briefly discussed.     

Distinction of cell types in Dicyema japonicum (phylum Dicyemida) by expression patterns of 16 genes

Dicyemids (phylum Dicyemida) are endoparasites, or endosymbionts, typically found in the renal sac of benthic cephalopod molluscs. The body organization of dicyemids is very simple, consisting of only 9 to 41 somatic cells. Dicyemids appear to have no differentiated tissues. Although categorization of somatic cells, to some types, is based on differences in the pattern of cilia and their position in the body, whether or not these cells are functionally different remains to be revealed. To provide insight into the functional differentiation, we performed whole mount in situ hybridization (WISH) to detect expression patterns of 16 genes, i.e., aquaglyceroporin, F-actin capping protein, aspartate aminotransferase, cathepsin-L-like cysteine peptidase, Ets domain-containing protein, glucose transporter, glucose-6-phosphate 1-dehydrogenase, glycine transporter, Hsp 70, Hsp 90, isocitrate dehydrogenase subunit alpha, Rad18, serine hydroxymethyltransferase, succinate-CoA ligase, valosin-containing protein, and 14-3-3 protein. In certain genes, regional specific expression patterns were observed among somatic cells of vermiform stages and infusoriform larvae of dicyemids. The WISH analyses also revealed that the Ets domain-containing protein and Rad18 are molecular markers for agametes.     

The expression of tubulin and tektin genes in dicyemid mesozoans (Phylum: Dicyemida)

Dicyemid mesozoans (Phylum Dicyemida) are endoparasites (or endosymbionts) that typically are found in the renal sac of benthic cephalopod mollusks such as octopuses and cuttlefishes. Adult dicyemids likely adhere to the renal appendage of hosts via cilia of calotte peripheral cells. These cilia seem to be continuously worn away in the interaction between the dicyemids and the epidermal cells of host renal appendages. We cloned 4 cDNAs and genes, alpha-tubulin, beta-tubulin, tektin B, and tektin C, which are thought to play a key role in ciliogenesis, from Dicyema japonicum, and studied expression patterns of these genes by whole-mount in situ hybridization. We detected coexpression of these genes in the calotte peripheral cells, but not in the trunk peripheral cells. This suggests that regeneration and turnover of cilia continuously occur in the calotte. In vermiform and infusoriform embryos, we also detected coexpression patterns of these genes, which might correlate with ciliogenesis during the embryogenesis. We also predicted the secondary structure and the coiled-coil regions of dicyemid tektins.     

Renal organs of cephalopods: a habitat for dicyemids and chromidinids

The renal organs of 32 species of cephalopods (renal appendage of all cephalopods, and renal and pancreatic appendages in decapods) were examined for parasite fauna and for histological comparison. Two phylogenetically distant organisms, dicyemid mesozoans and chromidinid ciliates, were found in 20 cephalopod species. Most benthic cephalopods (octopus and cuttlefish) were infected with dicyemids. Two pelagic cephalopod species, Sepioteuthis lessoniana and Todarodes pacificus, also harbored dicyemids. Chromidinid ciliates were found only in decapods (squid and cuttlefish). One dicyemid species was found in branchial heart appendages of Rossia pacifica. Dicyemids and chromidinids occasionally occurred simultaneously in Euprymna morsei, Sepia kobiensis, S. peterseni, and T. pacificus. The small-sized cephalopod species, Idiosepius paradoxus and Octopus parvus, harbored no parasites. Comparative histology revealed that the external surface of renal organs varies morphologically in various cephalopod species. The small-sized cephalopod species have a simple external surface. In contrast, the medium- to large-sized cephalopod species have a complex external surface. In the medium- to large-sized cephalopod species, their juveniles have a simple external surface of the renal organs. The external surface subsequently becomes complicated as they grow. Dicyemids and chromidinids attach their heads to epithelia or insert their heads into folds of renal appendages, pancreatic appendages, and branchial heart appendages. The rugged and convoluted external surface provides a foothold for dicyemids and chromidinids with a conical head. They apparently do not harm these tissues of their host cephalopods.     

A new species of Dicyemennea Whitman, 1883 (Phylum Dicyemida) from Sepia latimanus (Mollusca: Cephalopoda: Decapodidae) off Okinawa, Japan

A new species of dicyemid mesozoan is described from a cuttlefish Sepia latimanus Quoy and Gaimard collected off Nago, Naha and Onnason, Okinawa Islands, Ryukyu Islands, Japan. Dicyemennea ryukyuense n. sp. is a large species that reaches about 5 mm in length. The vermiform stage is characterised by 23 peripheral cells, a conical calotte and an axial cell that extends to the base of the propolar cells. The infusoriform embryo consists of 37 cells; two nuclei are present in each urn cell and the refringent bodies are solid. In the type of reproductive strategy, rhombogens of D. ryukyuense form a small number of infusorigens and produce a relatively large number of gametes per infusorigen. In the family Dicyemidae, Dicyemennea Whitman, 1883 is the largest group after Dicyema von Kölliker, 1849. Other dicyemid species, including those belonging to Dicyema, were not detected. Dicyemids have never previously been detected in cephalopods living on corals and rocks off the Ryukyu Islands, even though they are benthonic in habitat. This is the first report of a dicyemid mesozoan from S. latimanus and also from off the Ryukyu Islands. S. latimanus inhabits coral reefs, often swimming over the coral but usually lying on the sandy bottom. Dicyemids cannot possibly infect cephalopods which live exclusively over coral and rock.     

Biology of dicyemid mesozoans

We reviewed recent advances of some aspects on the biology of dicyemid mesozoans. To date 42 species of dicyemids have been found in 19 species of cephalopod molluscs from Japanese waters. The body of dicyemids consists of 10-40 cells and is organized in a very simple fashion. There are three basic types of cell junction, septate junction, adherens junction, and gap junction. The presence of these junctions suggests not only cell-to-cell attachment, but also cell-to-cell communication. In the development of dicyemids, early stages and cell lineages are identical in vermiform embryos of four genera, Conocyema, Dicyema, Microcyema, and Pseudicyema. Species-specific differences appear during later stages of embryogenesis. In the process of postembryonic growth in some species, the shape of the calotte changes from conical to cap-shaped and discoidal. This calotte morphology appears to result from adaptation to the structure of host renal tissues and help to facilitate niche separation of coexisting species. In most dicyemids distinctly small numbers of sperms are produced in a hermaphroditic gonad (infusorigen). The number of eggs and sperms are roughly equal. An inverse proportional relationship exists between the number of infusorigens and that of gametes, suggesting a trade-off between them. Recent phylogenetic studies suggest dicyemids are a member of the Lophotrochozoa.     

Cloning of chitinase-like protein1 cDNA from dicyemid mesozoans (Phylum: Dicyemida)

Dicyemid mesozoans are endoparasites found in the renal sacs of benthic cephalopods. Adult dicyemids insert the distinct anterior region, termed a "calotte," into renal tubules of the host. We cloned cDNA encoding chitinase-like protein from the dicyemid Dicyema japonicum (Dicyema-clp 1), and also cloned the gene fragment corresponding to the cDNA. Dicyema-clp1 has the hydrophobic amino acid-rich region, but not the chitin-binding domains at the C terminus. Analyses using the SignalP prediction program suggest this hydrophobic amino acid-rich region is the anchor sequence to plasma membranes. The putative catalytic site in glyco18 domain exhibited 1 substitution from aspartic acid to asparagine. The gene fragment had short 9 introns (22-26 bp), and the coding sequence consisted of 10 exons (30-233 bp). Specific and strong expression of Dicyema-clpl was detected in the calotte of vermiform stages by whole mount in situ hybridization. N-acetyl-D-glucosamine was detected on the outer surface of both peripheral cells of dicyemids and epidermal cells of host renal appendages. Dicyema-clp appears to be associated with N-acetyl-D-glucosamine in the interface between dicyemid peripheral cells and epidermal cells of the host renal appendage, and possibly aids in adhering the calotte to host epidermal cells.     

New species of Dicyemennea (phylum: Dicyemida) in deep-water Graneledone (Mollusca: Cephalopoda: Octopoda) from the Antarctic

Two new species of dicyemids are described from 2 species of deep benthic cephalopods, Graneledone antarctica and G. macrotyla, collected in the Southern Ocean south of the Antarctic Convergence. Dicyemennea bathybenthum n. sp. was found in G. antarctica. It is a medium-sized dicyemid whose length does not exceed 1,000 microm. The calotte is bluntly rounded or conical in small individuals, but the shape becomes discoidal in large individuals. Nematogens and vermiform embryos have 23 peripheral cells. An anterior abortive axial cell is present in vermiform embryos. Other stages in the life cycle were not observed. Dicyemennea dorycephalum n. sp. was found in G. macrotyla. It is a medium to large dicyemid that rarely exceeds 4,000 microm. The calotte is distinctly pointed, similar in shape to a spearhead. Vermiform stages typically have either 25 or 27 peripheral cells. An anterior abortive axial cell is present in vermiform embryos. Infusoriform embryos have 37 cells, the refringent bodies are solid in composition, and 2 nuclei are present in each urn cell.     

Abd-B expression in Porcellio scaber Latreille, 1804 (Isopoda: Crustacea): conserved pattern versus novel roles in development and evolution

The Hox genes are intimately involved in patterning the animal body during development and are considered to have had a pivotal role in the evolution of different body plans among the metazoans. From this perspective, crustaceans, a group that has evolved an extreme diversity of body structures, represent a choice group in which to study the evolution of these genes and their expression. The expression of one of these genes, Abdominal-B (Abd-B), has only been studied in two distantly related crustaceans, Artemia and Sacculina, where it shows dissimilar patterns, highly differentiated from the one described in other arthropods. Moreover, we have no information for the Malacostraca. Thus, we cloned the gene Abd-B and followed its expression through development by in situ hybridization in the isopod Porcellio scaber. We found a highly dynamic expression pattern of PsAbd-B during embryonic development. In early stages, it is expressed in the posterior-most part of the germ band, in a domain common to several arthropods studied to date, and later it is expressed in the developing limb buds of the pleon and still later in the endopodites of the third to fifth pleopodites. This raises the interesting possibility of the involvement of this gene in the later respiratory specialization of these appendages. In association with the above expression domain, Abd-B appears to be expressed in later stages also in the ventral ectoderm, raising the further suggestion of its possible involvement in patterning the developing nervous system. Moreover, we show that the first pleopod and the endopodite of the second pleopod, whereas present as limb buds in early embryonic stages, are later reduced and actually absent in the first postembryonic stage, although they reappear again in adults. These appendages thus represent an example of Lazarus appendages. Our data show strong plasticity in the use of a key developmental gene and point out the necessity of further research that may end with a revision of the current understanding of its role in animal evolution.     

Progenesis in dicyemids

Dicyemids (Phylum Dicyemida) are the most common and characteristic endosymbiont living in the renal sac of benthic cephalopod molluscs. Precocious development of a hermaphroditic gonad occurs in the larvae and smaller juveniles of 40 dicyemid species from 17 cephalopod species so far and is the usual phenomenon in dicyemids. Based on the developmental and morphological features of precocious individuals, progenesis (a form of heterochrony) is the appropriate term for such precocious development. In general, progenetic individuals have much lower fecundity than normal ones because of their smaller body size, and therefore, it appears to be a disadvantageous reproductive trait. Nonetheless, the number of progenetic individuals consists of 30%–50% of the population, a relatively large proportion suggesting that the presence of progenetic individuals probably plays an important role in life history strategy. Precocious development significantly reduces growth time and enables early maturation. Progenetic individuals are common in short-living cephalopod species, in which precocious development seems appropriate for dicyemids, enabling fast larval release before the end of the host's life span.     

Components of both major axial patterning systems of the Bilateria are differentially expressed along the primary axis of a 'radiate' animal, the anthozoan cnidarian Acropora millepora

Cnidarians are animals with a single (oral/aboral) overt body axis and with origins that nominally predate bilaterality. To better understand the evolution of axial patterning mechanisms, we characterized genes from the coral, Acropora millepora (Class Anthozoa) that are considered to be unambiguous markers of the bilaterian anterior/posterior and dorsal/ventral axes. Homologs of Otx/otd and Emx/ems, definitive anterior markers across the Bilateria, are expressed at opposite ends of the Acropora larva; otxA-Am initially around the blastopore and later preferentially toward the oral end in the ectoderm, and emx-Am predominantly in putative neurons in the aboral half of the planula larva, in a domain overlapping that of cnox-2Am, a Gsh/ind gene. The Acropora homologs of Pax-3/7, NKX2.1/vnd and Msx/msh are expressed in axially restricted and largely non-overlapping patterns in larval ectoderm. In Acropora, components of both the D/V and A/P patterning systems of bilateral animals are therefore expressed in regionally restricted patterns along the single overt body axis of the planula larva, and two 'anterior' markers are expressed at opposite ends of the axis. Thus, although some specific gene functions appear to be conserved between cnidarians and higher animals, no simple relationship exists between axial patterning systems in the two groups.