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.     

Gametogenesis and auxospore development in actinocyclus (bacillariophyta)

cGametogenesis and auxospore development have been studied in detail in surprisingly few centric diatoms. We studied the development of sperm, eggs and auxospores in Actinocyclus sp., a radially symmetrical freshwater diatom collected from Japan, using LM and electron microscopy of living cultures and thin sections. Actinocyclus represents a deep branch of the 'radial centric' diatoms and should therefore contribute useful insights into the evolution of sexual reproduction in diatoms. Spermatogenesis was examined by LM and SEM and involved the formation of two spermatogonia (sperm mother-cells) in each spermatogonangium through an equal mitotic division. The spermatogonia produced a reduced 'lid' valve, resembling a large flat scale with irregular radial thickenings. Sperm formation was merogenous, producing four sperm per spermatogonium, which were released by dehiscence of the 'lid' valve. The sperm were spindle-shaped with numerous surface globules and, as usual for diatoms, the single anterior flagellum bore mastigonemes. One egg cell was produced per oogonium. Immature eggs produced a thin layer of circular silica scales before fertilization, while the eggs were still contained within the oogonium. Sperm were attracted in large numbers to each egg and were apparently able to contact the egg surface via a gap formed between the long hypotheca and shorter epitheca of the oogonium and a small underlying hole in the scale-case. Auxospores expanded isodiametrically and many new scales were added to its envelope during expansion. Finally, new slightly-domed initial valves were produced at right angles to the oogonium axis, after a strong contraction of the cell away from the auxospore wall. At different stages, Golgi bodies were associated with chloroplasts or mitochondria, contrasting with the constancy of Golgi-ER-mitochondrion (G-ER-M) units in some other centric diatoms, which has been suggested to have phylogenetic significance. Electron-dense bodies in the vacuole of Actinocyclus are probably acidocalcisomes containing polyphosphate.     

滇芎属与瘤果芹属果实表面微形态特征及其分类学意义

对中国伞形科滇芎属(Physosperm opsisW olff)和瘤果芹属(TrachydiumL ind l.)12个种的果实表面微形态特征进行了扫描电镜比较观察。根据果实棱槽、外果皮细胞轮廓和蜡条纹饰的特征,这2个属的果实表面微形态可划分为3种类型:1)棱槽平滑-外果皮细胞轮廓不可见-无或少蜡质纹饰型;2)棱槽具瘤-外果皮细胞轮廓不可见-蜡质纹饰近平直或线纹型;3)棱槽粗糙-外果皮细胞呈4~6边形-蜡质纹饰波纹或网纹型。在此基础上,结合形态学特征探讨了滇芎属与瘤果芹属之间的关系和属下种间关系。     

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.     

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.     

Regeneration of plantlets fromEnteromorpha (Ulvales,Chlorophyta) protoplasts in axenic culture

High yields of protoplasts have been obtained from vegetative thalli of three species ofEnteromorpha by enzymatic degradation of the cell wall. Several commercial and crude enzymes prepared from the digestive system and hepatopancrease of abalone and top-shell were tested at different concentrations and combinations to evaluate the yield. Commercial enzymes in combination with either abalone or top-shell crude enzymes, consistently produced a high yield of protoplasts from all three species. High regeneration rate (85–95%) occurred in the protoplasts cultured at a density greater than 1.72 × 10³ cells cm⁻² at 20 and 25°C. Light intensities tested in the present study did not affect protoplast wall formation and regeneration. Protoplasts, after regenerating the cell wall, followed different types of developmental patterns under identical culture conditions. In one type some cells underwent repeated cell divisions and formed a round and oval shaped hollow thallus with a single layer of cells. In the second type many cells underwent one or two cell divisions (occasionally no division) and soon matured and discharged many motile spores, which on germination grew into normal plantlets. In the third type some cells divided irregularly to form a mass of callus-like cells (exceptE. prolifera). Culture medium supplemented with either mannitol, sorbitol, dextrose, saccharose or NaCl at higher concentrations (> 0.4 M) inhibited cell division and further differentiation in all species. author for correspondence     

红松胚胎发育及其淀粉的动态

本文观察了红松胚胎发育及其淀粉的分布全过程。红松合子位于颈卵器中上部,分裂时形成的纺锤体与颈卵器长轴呈一定角度,第一次有丝分裂产生2个游离核,它们边下沉边进行第二次有丝分裂,形成4个游离核排列成一层.接着进行第三次有丝分裂产生8个游离核,排成两层并形成细胞壁。第四次分裂产生16个细胞排成四层,组成了原胚,经一个半月的生长发育及胚胎选择阶段,7月中旬进入幼胚的发育时期。8月上旬幼胚已经进行组织和器官分化,此时淀粉逐渐由珠孔端向合点端方向推移,9月末种子成熟。     

ABSORBING TRICHOMES IN THE PLEUROTHALLIDINAE (ORCHIDACEAE)

Glandular trichomes occur on both surfaces of leaves of all examined genera and species of the subtribe Pleurothallidinae (Orchidaceae). Trichome initiation is effected by one periclinal division of a protodermal cell, producing a thin-walled, globose apical cell with a relatively large nucleus and a subapical stalk cell with heavily cutinized lateral walls. In some species a second periclinal division produces a third small basal cell also having thick lateral walls but thin transverse walls. As leaf development proceeds, the trichome apparatus assumes a sunken position due to continued anticlinal divisions of protoderm. Prior to laminar expansion and guard-mother-cell division on the abaxial surface, the wall of the apical cell ruptures and is replaced by a brown opaque residue. Finally, after vascular tissue differentiation and the cessation of meristematic activity, two or more pitted foot cells develop at the base of the trichome and adjacent to the water-storing hypodermal layers. Preliminary investigations indicate that the trichome apparatus is absorptive throughout its development and similar in function to tillandsioid scales in Bromeliaceae.     

SPERMATOGENETIC CLONES DEVELOPING FROM REPOPULATING STEM CELLS SURVIVING A HIGH DOSE OF AN ALKYLATING AGENT

We investigated stem cell renewal and differentiation in 10- and 15-days-old spermatogonial clones developing in mouse seminiferous epithelium after an extremely large cell loss, inflicted by high doses of the alkylating agent Myleran. The spermatogonial clones arise from cells that resemble the A_is spermatogonia but have a larger nuclear diameter. In spite of their mitotic activity these ‘re-populating stem cells’ lie mainly isolated or in pairs. This is explained by migration and differentiation. Migration appeared to occur at random in all directions along the basement membrane of the seminiferous tubule. After one or more divisions of the stem cells, a second type of cell appears, which is called the ‘differentiating spermatogonium’. The time elapsing before this type of cell appears, depends on the dose of Myleran: the larger the dose the later differentiation starts. A relation could be demonstrated between the stage of the cycle of the seminiferous epithelium and the start of differentiation. Differentiating cells were found isolated or in groups of two, four, eight or sixteen cells. Hence we concluded that at least up to their fourth division differentiating cells divide synchronously without degenerations. Three types of division of repopulating stem cells were distinguished, producing (1) two repopulating stem cells, (2) one repopulating stem cell and one cell starting spermatogonial differentiation, or (3) two differentiating cells. Type 1 divisions were found most frequently.     

Reproduction,Chromosome Number,and Postembryonic Development of Panagrellus redivivus (Nematoda: Cephalobidae)

Panagrellus redivivus (L.) T. Goodey reproduced amphimictically; the sexual primordia of males had nine chromosomes, those of females had ten. Eggs contained five chromosomes, sperm four or five. There were four molts, all after hatching. The sexes could be separated at the second molt by development of a lobe of somatic cells in the gonad, anteriorly in males, posteriorly in females. The lobe in males reflexed posteriorly at the third molt and joined the rectum at the fourth molt. Third molt females had a thickened vaginal primordium and at the fourth molt the spermathecal and uterine primordia were evident. The uterus elongated enormously in the adult. The 15 ventral chord nuclei between esophagus and rectum in the first stage increased to approximately 63 during the first molt; specialized nuclei, not evident until the third molt, participate in vaginal lining formation in fourth molt females. Sperm were first produced at the late fourth molt. Eggs, not produced until after copulation, hatched within the uterus.     

荚果蕨胚胎发育的研究

利用石蜡切片法研究了荚果蕨（Matteuccia struthiopteris(L.) Todaro）胚胎发育过程。合子第一次分裂,分裂面垂直于原叶体纵轴且平行于颈卵器颈部;第二次分裂面平行于原叶体纵轴且垂直于颈卵器颈部;第三次分裂面同时平行于原叶体纵轴和颈卵器颈部。经多次分裂的球形胚胎,胚胎的外上和外下区域几乎同时分别发育出第一叶顶端细胞和第一根顶端细胞。随着发育的进行,它们分别斜向分裂产生第一叶原基和第一根原基。随后,第一叶原基迅速分裂,突破帽状体形成第一幼叶;而第一根原基的分裂速度稍慢,第一根发育速度稍慢于第一叶。     

Dynamic intracellular localization of Dazl protein during <Emphasis Type="Italic">Xenopus</Emphasis> germline development

Xenopus dazl encoding an RNA-binding protein has been identified as a component of the germ plasm and is involved in the migration and differentiation of the primordial germ cells (PGCs). Here, we investigated the intracellular localization of Dazl in germline cells throughout the lifetime of Xenopus. In early embryogenesis, Dazl was detected initially in the germ plasm and then translocated to a perinuclear region. Then, it was detected within the nucleus in PGCs. Dazl was observed only in the cytoplasm in PGCs when sex differentiation began in the gonads. Dazl was distributed in both the nucleus and cytoplasm of the primary oogonium and spermatogonium, but only in the cytoplasm of the secondary oogonium and spermatogonium. In spermatocytes, Dazl was distributed throughout cytoplasm and localized at the spindles and cytoplasm during meiosis. Then, it was detected as speckles in the nucleus in the round spermatid. The dynamic intracellular localization suggests that Dazl is a multifunctional protein regulating RNA metabolism required for Xenopus germline development.     

Phylogenetic analysis of the genus Aquaspirillum based on 16S rRNA gene sequences

Phylogenetic analysis of 15 species of the genus Aquaspirillum based on 16S rRNA gene (rDNA) sequences indicated that the genus Aquaspirillum is phylogenetically heterogeneous and the species could be divided into four groups as follows: Aquaspirillum serpens, the type species of this genus, A. dispar and A. putridiconchylium are situated in the family Neisseriaceae; members of the second group, A. gracile, A. delicatum, A. anulus, A. giesbergeri, A. sinuosum, A. metamorphum and A. psychrophilum, are included in the family Comamonadaceae; the two members of the third group, A. arcticum and A. autotrophicum, are included in the family Oxalobacteriaceae; and members of the fourth group, A. polymorphum, A. peregrinum, and A. itersonii, are included in the alpha-subdivision of Proteobacteria. Thus, phylogenetic studies indicated that all the species excepting A. serpens, the type species, should be transferred to distinct genera.     

Change of shoot architecture during juvenile-to-adult phase transition in soybean

Juvenile-to-adult phase change is an indispensable event which guarantees a successful life cycle. Phase change has been studied in maize, Arabidopsis and rice, but is mostly unknown in other species. Soybean/Fabaceae plants undergo drastic changes of shoot architecture at the early vegetative stage including phyllotactic change and leaf type alteration from simple to compound. These characteristics make soybean/Fabaceae plants an interesting taxon for investigating vegetative phase change. Following the expansion of two cotyledons, two simple leaves simultaneously emerge in opposite phyllotaxy. The phyllotaxy of the third and fourth leaves is not fixed; both opposite and distichous phyllotaxis are observed within the same population. Leaves were compound from the third leaf. But the third leaf was rarely simple. Morphological and quantitative changes in early vegetative phase were recognized in leaf size, leaf shape, number of trichomes, stipule size and shape, and shoot meristem shape. Two microRNA genes, miR156 and miR172, are known to be associated with vegetative phase change. Examination of the expression level revealed that miR156 expression was high in the first two leaves and subsequently down-regulated, and that of miR172 showed the inverse expression pattern. These expression patterns coincided with the case of other species. Taken all data together, the first and second leaves represent juvenile phase, the fifth and upper leaves adult phase, and the third and fourth leaves intermediate stage. Further investigation of soybean phase change would give fruitful understandings on plant development.     

Diverse programs of ascus development in pseudohomothallic species of Neurospora,Gelasinospora, and Podospora

Meiosis and ascospore development in the four-spored pseudohomothallic ascomycetes Neurospora tetrasperma, Gelasinospora tetrasperma, Podospora anserina, and P. fefraspora have been reexamined, highlighting differences that reflect independent origins of the four-spored condition in the different genera. In these species, as in the heterothallic eight-spored N. crassa, fusion of haploid nuclei is followed directly by meiosis and a postmeiotic mitosis. These divisions take place within a single unpartitioned giant cell, the ascus, which attains a length of >0.1 mm before nuclei are enclosed by ascospore walls. Two basically different modes underlie the delivery of opposite mating type nuclei into each of the four ascospores in the different genera. In N. tefrasperma on the one hand, the mating type locus is closely centromere-linked. Mating types therefore segregate at the first meiotic division. The second division spindles of N. tefrasperma overlap and are usually parallel to one another, in contrast to the their tandem arrangement in N. crassa. As a result, nonsister nuclei of opposite mating type are placed close together in each half-ascus and a pair is enclosed in each ascospore. In the Podospora and Gelasinospora species on the other hand, the second-division spindles are in tandem, with sister nuclei of opposite mating type associated as a pair in each half-ascus. It is established for P. anserina and inferred for P. fetraspora and G. fefrasperma that a single reciprocal crossing over almost always occurs in the mating type-centromere interval, ensuring that mating types segregate at the second meiotic division and that nuclei of opposite mating type are enclosed in each ascospore. Other differences are also seen that are less fundamental. Neurospora tetrasperma differs from the other species in the orientation of chromosomes and spindle pole body plaques at interphase (I.) Third-division spindles are oriented parallel to the ascus wall in Gelasinospora but across the ascus in Podospora and Neurospora. The two Podospora species differ from one another in nuclear behavior following mitosis in the young ascospores. In P. tefraspora, two of the four nuclei migrate into the tail cell, which degenerates, leaving one functional nucleus of each mating type. In P. anserina, by contrast, only one of the four nuclei moves into the tail cell, leaving the germinating ascospore with two functional nuclei of one mating type and one of the other. The pseudohomothallic condition with its heterokaryotic vegetative phase has significant consequences for both the individual organism and the breeding system. Genetic controls of development and recombination are complex. Inbreeding is not obligatory. © 1994 WiIey-Liss, Inc.     

Patterns in stage duration and development among marine and freshwater calanoid and cyclopoid copepods: a review of rules,physiological constraints,and evolutionary significance

Studies of development time of marine and freshwater copepods have taken separate tracks. Most studies on marine copepods report development time of each individual development stage, whereas studies on freshwater copepods report only development time, from egg to nauplius and nauplius to adult. This bias allows comparison of total development time but prevents detailed comparisons of patterns in stage-specific developmental schedules. With respect to egg to adult development time, three general relationships are known: developmental rates are dependent upon temperature and food concentration but independent of terminal body size; freshwater calanoids develop significantly slower than marine calanoids; freshwater cyclopoids develop at the same rate as marine calanoids. Two rules describe stage-specific developmental rates: the equiproportional rule and the isochronal rule. The first rule states that the duration of a given life history stage is a constant proportion of the embryonic development time; the second rule states that the time spent in each stage is the same for all stages. This review focuses on the second rule. From the 80+ published studies of copepod stage-specific developmental times, no species follows the isochronal rule strictly: Acartia spp. come closest with isochronal development from third nauplius (N3) to fourth copepodite (C4). The only pattern followed by all species is rapid development of the first and/or second naupliar stages, slow development of the second and/or third nauplius and prolonged development of the final copepodite stage. Once adulthood is reached, males are usually short-lived, but females can live for weeks to months in the laboratory. Adult longevity in the sea is, however, on the order of only a few days. The evolution of developmental patterns is discussed in the context of physiological constraints, along with consideration of possible relationships between stage-specific mortality rates and life history strategies. Physiological constraints may operate at critical bottlenecks in development (e.g. at the first feeding nauplius, N6, and the fifth copepodite stage). High mortality of eggs may explain why broadcast eggs hatch 2–3 times faster than eggs carried by females in a sac; high mortality of adults may explain why adults do not grow rather they maximize their reproductive effort by partitioning all energy for growth into egg production.     

Renewal and proliferation of spermatogonia during spermatogenesis in the Japanese quail,Coturnix coturnix japonica

Summary Four different types of spermatogonia were identified in the seminiferous tubules of the Japanese quail: a dark type A (A_d), 2 pale A type (A_p1 and A_p2), and a type B. A model is proposed describing the process of spermatogonial development in the quail. The A_d spermatogonia are considered to be the stem cells. Each divides to produce a new A_d spermatogonium and a A_p1 spermatogonium during Stage IX of the cycle of the seminiferous epithelium. An A_p1 spermatogonium produces two A_p2 spermatogonia during Stage II of the cycle, A_p2 spermatogonia produce four type B spermatogonia during Stage VI of the cycle, and type B spermatogonia produce eight primary spermatocytes during Stage III of the cycle. Consequently, 32 spermatids can result from each division of an A_d spermatogonium. Spermatogonial development in the quail differs from the process described in mammals in that there are fewer mitotic divisions and they are all synchronized with the cycle of the seminiferous epithelium. It is suggested that the fewer mitotic divisions explain why a smaller area of the seminiferous tubule is occupied by a cellular association in the quail than in mammals like the rat, ram and bull. The duration of spermatogenesis from the division of the A_d spermatogonia to sperm release from the seminiferous epithelium was estimated to be 12.77 days.