Hidetaka Furuya

PHYLOGENETIC POSITION OF THE DICYEMID MESOZOA INFERRED FROM 18S RDNA SEQUENCES

The dicyemid mesozoa, obligate symbionts in the cephalopod kidney, are simply organized multicellular animals. They have long been the subject of phylogenetic debates. Some authors have suggested that dicyemids represent an offshoot from an early metazoan ancestor. Other workers considered them to be degenerated progeny of higher metazoa, possibly parasitic trematodes. We determined the almost complete nucleotide sequences of 18S rDNA in two species of dicyemid, Dicyema orientale and Dicyema acuticephalum, isolated purely from cephalopod urine. We compared these sequences with sequences determined in the present study from three flatworm species, as well as with a variety of eukaryote sequences obtained from databases. The phylogenetic trees reconstructed with the use of the neighbor-joining, maximum-parsimony, and maximum-likelihood methods indicated that the dicyemids belong among the triploblastic animals (Bilateria). However, we cannot firmly establish the position of the dicyemids within the Bilateria because we cannot ignore the problem of long branch attraction between the myxozoans, dicyemids, nematodes, and acoel flatworms. The present results favor the hypothesis that the dicyemids do not represent an early divergent metazoan group, but rather a group degenerated from a triploblastic ancestor.

Dicyemids are higher animals

Dicyemids, which are microscopic parasites of squids and octopuses, have among the simplest body plans of all multicellular animals. They lack body cavities and almost all the organs that characterize animals, such as a gut or nervous system, and their development proceeds without germ layers and gastrulation. The adult body consists of a solitary axial cell surrounded by a single layer of 10–40 ciliated outer cells. Here we use information from Hox gene sequences to investigate the phylogenetic affinities of dicyemids, and conclude that dicyemids are lophotrochozoans that have secondarily lost many morphological characters, so the simplicity of their body plan is not a primitive condition.

Biology of Dicyemid Mesozoans

Abstract 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.

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.

Phylogenetic Relationships among Major Species of Japanese Coleoid Cephalopods (Mollusca: Cephalopoda) Using Three Mitochondrial DNA Sequences

Abstract Phylogenetic relationships among 36 species of major coleoid cephalopods from Japanese waters were studied using partial sequences of three mitochondrial genes, 16S rDNA, 12S rDNA, and cytochrome c oxidase subunit I gene. Octopoda and Decapoda were monophylic groups. Within Sepioidea, Sepiadariidae and Sepiolidae were not closely related to Sepiidae, but rather related to Teuthoidea. Sepiidae with a distinct calcareous shell formed a single cluster. Myopsida was closely related to Oegopsida. Within Octopoda, Opisthoteuthis depressa and Argonauta argo diverged earlier than Octopodiidae. The common octopuses in Japanese waters were separated into three clusters. The first cluster occupied a basal position, and includes large-sized octopuses, such as Enteroctopus dofleini and Octopus (Paroctopus) conispadiceus from the continental shelf and upper slope. The second cluster consisted of long-armed octopuses, such as O. ornatus, O. minor, and O. sasakii. The third cluster contained small- to medium-sized octopus, such as Amphioctopus fangsiao, A. areolatus, O. cyaneus, and O. vulgaris, in which several species possess ocelli on the web. The second cluster formed the sister group to the third cluster.

Development of the Infusoriform Embryo of Dicyema japonicum (Mesozoa: Dicyemidae)

The cleavage pattern and cell lineage of the infusoriform embryo of the dicyemid mesozoan Dicyema japonicum were studied in fixed material with the aid of a light microscope. The early cleavages are holoblastic and spiral. At the 16-cell stage, the animal pole consists of four mesomeres, the equatorial region consists of four macromeres with four alternating sub-macromeres, and the vegetal pole is composed of four micromeres. At around the 20- to 24-cell stage, cleavage becomes asynchronous and its pattern changes from spiral to bilateral. The four micromeres, namely, the presumptive germinal cells, do not divide further and are finally incorporated into the cytoplasm of four urn cells, which are generated after divisions of the sub-macromeres. The blastomeres situated in the animal hemisphere give rise to ciliated cells that cover the posterior part of the embryo. Two blastomeres (2a2 and 2d2) undergo extremely unequal divisions and the much smaller sister blastomeres degenerate and ultimately disappear during embryogenesis. The fully formed embryo consists of 37 cells. These cells are produced after only four to eight rounds of cell division. The cell lineage appears to be invariant among embryos, apart from the derivation of the lateral cells.

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

Abstract 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.

Developmental Patterns and Cell Lineages of Vermiform Embryos in Dicyemid Mesozoans

Patterns of cell division and cell lineages of the vermiform embryos of dicyemid mesozoans were studied in four species belonging to four genera: Conocyema polymorpha, Dicyema apalachiensis, Microcyema vespa, and Pseudicyema nakaoi. During early development, the following common features were apparent: (1) the first cell division produces prospective cells that generate the anterior peripheral region of the embryo; (2) the second cell division produces prospective cells that generate the posterior peripheral region plus the internal cells of the embryo; (3) in the lineage of prospective internal cells, several divisions ultimately result in cell death of one of the daughter cells. Early developmental processes are almost identical in the vermiform embryos of all four dicyemid genera. The cell lineages appear to be invariant among embryos and are highly conserved among species. Species-specific differences appear during later stages of embryogenesis. The number of terminal divisions determines variations in peripheral cell numbers among genera and species. Thus, the numbers of peripheral cells are fixed and hence species-specific.

Calotte morphology in the phylum Dicyemida: niche separation and convergence

The renal sacs of a diversity of cephalopod molluscs were examined to study the morphology of dicyemid mesozoans. Most of the dicyemid species studied were found to be host specific. Typically, two or more species of dicyemids were present in each host species or each host individual. When dicyemid species co-occurred, their calotte shapes were distinctly different. The following variations in calotte shapes were usually detected within a given host individual: (1) when two species of dicyemids were present, two distinct calotte shapes, conical and discoidal, were observed; (2) when three species of dicyemids were present, three types of calotte configurations were observed, conical (two grades) and discoidal; (3) when more than four species of dicyemids were present, at least one species was characterized by its rare irregularly shaped calotte. As a rule, when more than two dicyemid species were present in a single host individual, calotte shapes were dissimilar. Calotte shapes in dicyemid species from different host species more closely resemble each other than those of dicyemids observed within the same host species. Dicyemids with conical or dome-shaped calottes are found within the convolutions or folds of the renal appendages, whereas those with flat, discoidal calottes attach to the surface of the renal appendages. In the dicyemids, calotte morphology seems to result from adaptation to the structure of host renal tissues and helps to facilitate niche separation of coexisting species.

Phylogenetic Analysis of Dicyemid Mesozoans (Phylum Dicyemida) From Innexin Amino Acid Sequences: Dicyemids Are Not Related to Platyhelminthes

Abstract Dicyemid mesozoans are endoparasites, or endosymbionts, found only in the renal sac of benthic cephalopod molluscs. The body organization of dicyemids is very simple, consisting of usually 10 to 40 cells, with neither body cavities nor differentiated organs. Dicyemids were considered as primitive animals, and the out-group of all metazoans, or as occupying a basal position of lophotrochozoans close to flatworms. We cloned cDNAs encoding for the gap junction component proteins, innexin, from the dicyemids. Its expression pattern was observed by whole-mount in situ hybridization. In adult individuals, the innexin was expressed in calottes, infusorigens, and infusoriform embryos. The unique temporal pattern was observed in the developing infusoriform embryos. Innexin amino acid sequences had taxon-specific indels which enabled identification of the 3 major protostome lineages, i.e., 2 ecdysozoans (arthropods and nematodes) and the lophotrochozoans. The dicyemids show typical, lophotrochozoan-type indels. In addition, the Bayesian and maximum likelihood trees based on the innexin amino acid sequences suggested dicyemids to be more closely related to the higher lophotrochozoans than to the flatworms. Flatworms were the sister group, or consistently basal, to the other lophotrochozoan clade that included dicyemids, annelids, molluscs, and brachiopods.