The evolution of the female gonad in Platyhelminthes-Turbellaria: ultrastructural investigations

Summary

The structure of the female gonad has undergone important evolutionary modifications in Platyhelminthes-Turbellaria. The primitive condition consists of single oocytes freely distributed in the parenchyma autosynthetically producing both yolk and eggshell forming granules (e.f.g.) (archoophoran level of organization). A derived condition is the presence of a compact, hetero-cellular gonad enveloped by a cellular tunica and/or an extracellular lamina-like layer and composed of germaria with alecithal oocytes and vitellaria with vitelline or yolk cells producing and accumulating both yolk and e.f.g. (neoophoran level of organization). In the last three decades the female gonad of a number of turbellarian taxa has been investigated by means of electron microscopy and cytochemistry, and several characters of oocytes and vitellocytes have been hypothesized to have a potential phylogenetic value. Some of these characters and their possible phylogenetic implications are briefly reviewed.     

Embryology of the Turbellaria and its phylogenetic significance

Developmental characters — including oocyte and yolk cell structure, patterns of cleavage, and modes of gastrulation — are presented and examined in relation to the phylogeny of the Turbellaria. Eggshell granules, which have been demonstrated to occur in the oocytes of entolecithal eggs and the yolk cells of ectolecithal eggs, are compared among species, and their potential value as a taxonomic character is discussed. The quartet 4d spiral cleavage of the entolecithal egg of polyclads is described as reminiscent of the primitive pattern of early development for the Turbellaria. This is compared to duet spiral cleavage of acoels, and possible phylogenetic schemes involving the two types of spiral cleavage are reviewed. The link between the precise spiral cleavage, which characterizes development of most archoophorans, and blastomere separation (Blastomeren-Anarchie), which occurs in several neoophoran orders, is established by the occurrence of quartet 4d spiral cleavage in one neoophoran order, and of both quartet spiral cleavage and Blastomeren-Anarchie in different species of a second neoophoran order. The epibolic gastrulation of polyclads is described as primitive for the Turbellaria because of its similarity to that of other members of the Spiralia. Although no identical process occurs in neoophoran development, the earlier event of formation of the hull membrane in some neoophorans, and the later event of formation of the definitive epidermis in all neoophorans studied are presented as processes of possible homology to the epibolic gastrulation of polyclads. The lack of correspondence between polyclads and neoophorans in the relationship of the definitive body axes to the egg axis is discussed, and an hypothesis is advanced to account for the differences. The phylogenetic relationships indicated by known developmental phenomena differ only slightly from the scheme presented by Karling in 1974.     

Notes on the biology of some psammophile Turbellaria of the Black Sea

The biology of two marine turbellarians, Cercyra hastata (Tricladida) and Pseudomonocelis ophiocephala (Proseriata) has been studied over a period of five years. They are the main components of the biocoenosis of the saccocirrus sand of Sevastopol Bay. These species have a significant role in the processes of secondary production and transformation of the organic matter in the coastal zone.     

Ultrastructure of the nerve cells and sensilla of Geocentrophora baltica (Platyhelminthes,Lecithoepitheliata) and the surface sensilla in the Geocentrophora group

Two types of nerve cell could be distinguished ultrastructurally in the central nervous system of Geocentrophora baltica (Prorhynchida, Lecithoepitheliata). Both show invaginations in the plasma membrane, but they differ in the character of the cytoplasm (light or densely stained) and the distribution of the neuronal vesicles (evenly or in groups). Different kinds of vesicles and neuronal release sites are observed. Special features of the synapses are pronounced local thickenings of the presynaptic membrane connected to paramembranous densities. In G. baltica and five endemic Geocentrophora spp. from Lake Baikal six types of surface sensillum were observed at the epidermal surface: 1. those with a long thin rootlet; 2. a short, balloon-shaped cilium with an aberrant axoneme and a reduced rootlet; 3. a rootlet branching into many striated bundles; 4. a thick rootlet; 5. a reduced rootlet and numerous neurotubules;and 6. collared sensilla each with one cilium in a deep pit surrounded by a collar of 11 to 12 microvilli. The variable number of microvilli in the collared sensillum is considered plesiomorphic relative to the stable number of eight microvilli known in sensilla of the Prolecithophora, Proseriata, and Rhabdocoela. The ultrastructure of the collar sensillum indicates that the Lecithoepitheliata is only distantly related to the Prolecithophora and higher turbellarians.     

The ultrastructure of oogenesis and yolk formation in Labidocera aestiva (Copepoda: Calanoida)

Yolk formation in the oocytes of the free-living, marine copepod, Labidocera aestiva (order Calanoida) involves both autosynthetic and heterosynthetic processes. Three morphologically distinct forms of endogenous yolk are produced in the early vitellogenic stages. Type 1 yolk spheres are formed by the accumulation and fusion of dense granules within vesicular and lamellar cisternae of endoplasmic reticulum. A granular form of type 1 yolk, in which the dense granules within the cisternae of endoplasmic reticulum do not fuse, appears to be synthesized by the combined activity of endoplasmic reticulum and Golgi complexes. Type 2 yolk bodies subsequently appear in the ooplasm but their formation could not be attributed to any particular oocytic organelle. In the advanced stages of vitellogenesis, a single narrow layer of follicle cells becomes more developed and forms extensive interdigitations with the oocytes. Extra-oocytic yolk precursors appear to pass from the hemolymph into the follicle cells and subsequently into the oocytes via micropinocytosis. Pinocytotic vesicles fuse in the cortical ooplasm to form heterosynthetically derived type 3 yolk bodies.     

Ultrastructural and cytochemical aspects of the female gonad of Geoplana burmeisteri (Platyhelminthes, Tricladida, Terricola)

The ultrastructure of the female gonad of the land planarian Geoplana burmeisteri was investigated by means of electron microscopy and cytochemical techniques. It consists of two small germaria located ventral to the intestine and of two irregular, lateral rows of vitelline follicles, both enveloped by a tunica composed of an extracellular lamina and an inner sheath of accessory cells. Accessory cell projections completely surround developing oocytes and vitellocytes. The main feature of oocyte maturation is the appearance of chromatoid bodies and the development of the rough endoplasmic reticulum (RER) and Golgi complexes. These organelles appear to be correlated with the production of egg inclusions of medium electron density, about 1.5-1.8 microm in diameter, which remain scattered in the ooplasm of mature oocytes. On the basis of cytochemical tests demonstrating their glycoprotein composition, these inclusions were interpreted as residual yolk globules. Vitellocytes are typical secretory cells with well-developed RER and Golgi complexes that are mainly involved in the production of yolk globules and eggshell globules, respectively. Eggshell globules appear to arise from repeated coalescence of small Golgi-derived vesicles and, at an intermediate stage of maturation, show a multigranular pattern. Later, after vesicle fusion, they reach a diameter of 1.3-1.6 microm when completely mature and show a meandering/concentric pattern, as is typical of the situation seen in most Proseriata and Tricladida. The content of yolk globules is completely digested by pronase, while the content of eggshell globules is unaffected. Mature vitellocytes contain, in addition, a large quantity of glycogen and lipid droplets as further reserve material. On the basis of the ultrastructural characteristics of the female gonad described above and in relation to the current literature, we conclude that G. burmeisteri appears to be more closely related to the freshwater triclads, in particular to members of the Dugesiidae, than to the marine triclads.     

An ultrastructural study of oogenesis inStreblospio benedicti (spionidae), with remarks on diversity of vitellogenic mechanisms in polychaeta

Summary The ultrastructural features of oogenesis were examined in the spionid polychaeteStreblospio benedicti. Paired ovaries are attached to the genital blood vessels extending into the coelomic space from the circumintestinal sinus. The genital blood vessel wall is composed of flattened, peritoneal cells, large follicle cells and developing oocytes. Vitellogenesis occurs while the oocytes are attached to the blood vessel wall. Two morphologically distinguishable types of yolk are synthesized. Type I is synthesized first by an autosynthetic process apparently involving pinocytosis and the conjoined efforts of the Golgi complex and rough endoplasmic reticulum. Type II yolk appears later through a heterosynthetic process involving the infolding of the oolemma and the sequestering of materials from the blood vessel lumen by endocytosis. During this process, blood pigment molecules appear to be incorporated into endocytotic pits, vesicles and eventually the forming yolk body. The significance of heterosynthetic yolk formation to the general reproductive strategies of polychaetous annelids is discussed.The author is grateful for the very capable technical assistance of Ms. P.A. Linley and the many stimulating discussions with Dr. Stan Rice. Contribution No. 156, Harbor Branch Foundation, Inc.     

Chromosomal evolution in marine triclads and polyclads (Turbellaria)

From an analysis of chromosomal mutations in seven species among five genera of marine triclads and polyclads, three different types of mechanisms of such mutations are identified: 1) rearrangement involving the centromere such that its position is changed, 2) rearrangement of whole chromosome arms, and 3) Robertsonian mechanisms. These mechanisms are the same as those reported for freshwater turbellarians, but aneuploidy and polyploidy, which are common in freshwater species, were not detected in these marine turbellarians.     

Two New Species of Interstitial Marine Triclads from the North American Pacific Coast, with Comments on Evolutionary Trends and Systematics in Tricladida (Turbellaria)

Holmquist, Ch. and Karling, T. G. (Section of Invertebrate Zoology, Swedish Museum of Natural History, Stockholm, Sweden.) Two new species of marine triclads from the North American Pacific coast, with comments on evolutionary trends and systematics in Tricladida (Turbellaria). Zool. Scripta 1(3–4): 175–184, 1972.–Two marine triclads, Oregoniplana opisthopora gen. et sp.n. and Pacificides psammophilus gen. et sp.n. are described from the Pacific coast of USA. They are inhabitants of sand in the surf zone. They are provisionally included in the family Procerodidae but their anatomy is aberrant in several respects; they elucidate some evolutionary trends as well as some unsatisfactory points in the systematics of the aquatic triclads.     

An ultrastructural study of oogenesis in a marine triclad

The ultrastructural features of oocyte differentiation were studied in the marine triclad Cercyra hastata. Oocytes at several stages of maturation, each surrounded by follicle cell projections, are present within each of the two ovaries. A pre-vitellogenic and a vitellogenic stage have been detected in the oogenesis of C. hastata. The pre-vitellogenic stage is mainly characterized by an increase in the nuclear and nucleolar volume and activity, and the appearance and development of cortical granule precursors which are elaborated by the Golgi complex. In early phases of the vitellogenic stage, intense delamination and blebbing of the nuclear envelope occurs which probably contributes to an increase in number of cytoplasmic membranes and to transfer of nuclear material to the cytoplasm. The rough endoplasmic reticulum is extensively developed and often assumes a ‘whorl’ array. Several areas of yolk precursor formation appear in the whorls. Numerous 2–5 μm protein yolk globules are subsequently formed which appear surrounded by a double membrane (cisternae of the smooth endoplasmic reticulum) and become randomly distributed throughout the cytoplasm of mature oocytes. The peripheral ooplasm is occupied by a monolayer of electron-dense cortical granules. Finally, the evolutionary significance of the autosynthetic mechanism of yolk production is discussed.     

Evolution of the concepts of vitellogenesis in Polychaete Annelids

Summary

In polychaete annelids, two types of vitellogenesis have been described: a heterosynthetic mechanism (in a number of different of worms) and an autosynthetic process (other including Nereis). Recent biochemical results suggest that the heterosynthetic mechanism is more general than previously thought. In Nereis, the vitellogenin (the precursor) is synthesized in coelomocytes and after transfer through coelomic fluid incorporated into oocytes. In germinal cells, a conversion process, involving proteolytic cleavages of vitellogenin, produces mature vitellins which are accumulated in yolk granules.

The neurohormones identified so far are not essential for vitellogenin synthesis. It is possible that these neurohormones regutate enzymatic activities in the oocytes.     

Ultrastructure of the ovary and oogenesis in the polychaete Capitella jonesi (Hartman, 1959)

Ultrastructural features of the ovary and oogenesis in the polychaete Capitella jonesi (Hartman, '59) have been described. The ovaries are paired, sac-like follicles suspended by mesenteries in the ventral coelom throughout the midbody region of the mature worm. Oogenesis is unsynchronized and occurs entirely within the ovary, where developing gametogenic stages are segregated spatially within a germinal and a growth zone. Multiplication of oogonia and differentiation of oocytes into the late stages of vitellogenesis occur in the germinal region of the ovary, whereas late-stage vitellogenic oocytes and mature eggs are located in a growth zone. Follicle cells envelop the oocytes in the germinal zone of the ovary and undergo hypertrophy and ultrastructural changes that correlate with the onset of vitellogenesis. These changes include the development of extensive arrays of rough ER and numerous Golgi complexes, formation of microvilli along the surface of the ovary, and the initiation of extensive endocytotic activity. Oocytes undergo similar, concomitant changes such as the differentiation of surface microvilli, the formation of abundant endocytotic pits and vesicles along the oolemma, and the appearance of numerous Golgi complexes, cisternae of rough ER, and yolk bodies. Yolk synthesis appears to occur by both autosynthetic and heterosynthetic processes involving the conjoined efforts of the Golgi complex and rough ER of the oocyte and the probable addition of extraovarian (heterosynthetic) yolk precursors. Evidence is presented that implicates the follicle cells in the synthesis of yolk precursors for transport to the oocytes. At ovulation, mature oocytes are released from the overy after the overlying follicle cells apparently withdraw. Bundles of microfilaments within the follicle cells may play a role in this withdrawal process.     

Electron microscopical observations on vitellocytes and germocytes inNematoplana coelogynoporoides (Plathelminthes,Proseriata)

Summary Fine-structural features of vitellaria and germaria inNematoplana coelogynoporoides are documented and compared with those of other free-living plathelminths with ectolecithal eggs. Emphasis is mainly put on the pattern of eggshell material, yolk bodies deposited in vitellocytes, and marginal granules of the female germ cells. In this species, encompassed in the taxon Proseriata Unguiphora, the eggshell granules show a meandering pattern also known from species of the taxon Proseriata Lithophora. In contrast, the yolk globules exhibit crystalline components unknown from the Lithophora. The marginal granules in the ooplasm have an extremely large diameter. They consist of a flocculent core and a crescent-shaped cortex. Marginal granules of this appearance have not been found in any other taxon of free-living Neoophora.Abbreviations cc crystalline component - co cortex - gER granular endoplasmic reticulum - go Golgi complex - gl glycogen - lp lipid droplet - mg marginal granule - n nucleus - nl nucleolus - sg eggshell granule - sp spermatozoa - yg yolk globule     

Ultrastructure of oogenesis of two oviparous demosponges: Axinella damicornis and Raspaciona aculeata (Porifera)

We investigated the cytology of the oogenic cycle in two oviparous demosponges, Axinella damicornis and Raspaciona aculeata, during 2 consecutive years both by light and electron microscopy. Oocytes of both species were similar in their basic morphological features but differences were noticed in time required to complete oocyte maturation and mechanisms of acquisition of nutritional reserves. The oogenic cycle of A. damicornis extended for 7-8 months in autumn-spring, while that of R. aculeata did it for 3-5 months in summer-autumn. Yolk of A. damicornis was predominantly formed by autosynthesis. Oocytes endocytosed bacteria individually and stored them in groups in large vesicles. Bacteria were digested and lipidic material was added to the vesicles to produce a peculiar granular yolk hitherto unknown in sponges. Scarce cells carrying heterogeneous inclusions were observed in the perioocytic space, and were interpreted as putative nurse cells. Such cells were presumably releasing lipid granules to the perioocytic space. In contrast, large numbers of nurse cells were found surrounding the oocytes of R. aculeata. They transported both lipid granules and heterogeneous yolk bodies to the oocytes. R. aculeata also produced some of their yolk by autosynthesis. The involvement of nurse cells in the vitellogenesis of R. aculeata shortened the oocyte maturation, whereas a largely autosynthetic vitellogenesis in A. damicornis prolonged the duration of oogenesis.     

Utility of complete large and small subunit rRNA genes in resolving the phylogeny of the Neodermata (Platyhelminthes): implications and a review of the cercomer theory

We combined nearly complete sequences of large (LSU) and small (SSU) subunit rDNA from 32 flatworm species to estimate the phylogeny of the Platyhelminthes using maximum parsimony, maximum likelihood and Bayesian inference methods. Rooted against the Catenulida, combined evidence trees offered no support for the Revertospermata, which was also rejected by constraint analysis. Generally, nodal support was higher for groupings estimated from the combined data partitions and all methods of analysis provided congruent estimates of phylogeny. The Monogenea and Proseriata were resolved as monophyletic, rejecting previous suggestions of paraphyly based on SSU and partial LSU data sets and thus supporting widely accepted morphological synapomorphies. Monophyly of the Neodermata was supported and its sister group was a clade of neoophoran 'turbellarians' to the exclusion of the Proseriata which in turn was more basal. Taxa with similar spermatology to the Neodermata ( Ichthyophaga , Notentera , Urastoma and Kronborgia ) were the sister group to Tricladida + Prolecithophora, which in turn were sister to the Rhabdocoela. Polycladida + Macrostomida + Lecithoepitheliata was the earliest divergent offshoot of the Rhabditophora. Among the Neodermata, the Cercomeromorphae (Cestoda + Monogenea) was not supported, whereas Cestoda + Trematoda was well supported. Although there is no known synapomorphy for this latter grouping, our data highlight problems associated with the 'cercomer theory' and we reject putative homologies regarding neodermatan 'cercomers' that have been sustained in the literature without careful scrutiny.  © 2003 The Linnean Society of London, Biological Journal of the Linnean Society , 2003, 78, 155–171.     

Ultrastructural observations of the early and late stages of gorgonian coral (Junceella juncea) oocytes

The developmental oogenesis of gorgonian coral was investigated at the histological level. The objective of this study was to examine and improve the understanding of Junceella juncea oogenesis using ultrastructural methods, such as histological sectioning and transmission electron microscopy. At least three types of yolk materials were observed in this study: yolk body, lipid granules and cortical alveoli. Some of the complex yolk materials were encompassed by concentric or arched layers of smooth and rough endoplasmic reticulum and the Golgi complex in early stage oocytes. Different types of vesicles were found in both early and late stage oocytes and some granules could be seen inside the empty vesicles. This may be a possible method for elaborating complex yolk materials. Homogeneous yolks from different types of inclusions were abundant and the autosynthesis of yolk may be a major mechanism in J. juncea oocytes. This is the first report of the ultrastructural observation of oogenesis in gorgonian coral species using transmission electron microscopy. Our study obtained relatively detailed information at the ultrastructural level, and it provides an overview of the oocyte ultrastucture of the gorgonian coral J. juncea.     

Ultrastructural studies on vitellocytes of Parotoplaninae (Plathelminthes,Proseriata) with special reference to the structure of eggshell-forming granules

Summary The general organisation of the vitellocytes inParotoplana capitata, P. papii, Parotoplanella progermaria, andP. heterorhabditica is described. Special emphasis is put on the substructure of eggshell-forming granules. In all parotoplanid species studied the eggshell granules show uniformly shaped substructures typical of all the Proseriata and besides this feature, globules exist which are bipartite, i.e. two different patterns exist within the same granule. This characteristic, which is unknown for other taxa of the Proseriata, is hypothesized as an autapomorphy of the monophylum Parotoplaninae.Abbreviations gER granular endoplasmic reticulum - lp lipid droplets - n nucleus - sg eggshell granule - y yolk platelet     

The female gonad in two species of Microplana (Platyhelminthes,Tricladida, Rhynchodemidae): Ultrastructural and cytochemical investigations

The female gonad of the land planarians Microplana scharffi and Microplana terrestris consists of two small germaria located ventrally in the anterior third of the body and of two ventro‐lateral rows of oblong vitelline follicles distributed between the intestinal pouches. Both these structures are enveloped by a tunica composed of an outer extracellular lamina and an inner sheath of accessory cells. Oocyte maturation is characterized by the appearance of chromatoid bodies and the development of endoplasmic reticulum and Golgi complexes. These organelles appear to be correlated with the production of egg granules with a fenestrated/granular content of medium electron density, about 4–5 μm in diameter, which remain dispersed in the ooplasm of mature oocytes. On the basis of cytochemical tests showing their glycoprotein composition, and their localization in mature oocytes, these egg granules have been interpreted as yolk. In the vitelline follicles, vitellocytes show the typical features of secretory cells with well‐developed rough endoplasmic reticulum and Golgi complexes involved in the production of eggshell globules and yolk. The eggshell globules, which appear to arise from repeated coalescences of two types of Golgi‐derived vesicles, contain polyphenols and, when completely mature, they measure about 1–1,2 μm in diameter and show a meandering/concentric content pattern as is typical of the situation observed in most Proseriata and Tricladida. Mature vitellocytes also contain a large amount of glycogen and lipids as further reserve material. On the basis of the ultrastructural features of the female gonad and in relation to the current literature the two species of rhynchodemids investigated appear to be closely related to the freshwater planarians belonging to the family Dugesiidae. J. Morphol. 2009. © 2009 Wiley‐Liss, Inc.     

Lipid yolk synthesis in a marine teleost,Blennius pholis L.

Summary The site of lipid yolk synthesis in the marine teleost Blennius pholis L. is examined. The relative distributions of cellular organelles and inclusions in the developing oocytes are analysed by morphometric analysis, which indicates that synthesis is probably endogenous occurring in the perinuclear cytoplasm. The role of lipid yolk in this species is discussed.     

Ultrastructural study of oogenesis in Monocelis lineata (Turbellaria,Proseriata)

Summary

Oogenesis in the marine turbellarian proseriat Monocelis lineata was investigated at the ultrastructural level. Oocyte differentiation is not synchronous so that successive stages of germ cell maturation were simultaneously detected in each of the two ovaries. Each developing oocyte is enveloped by follicle cell projections which are presumably involved in a morphologically undetectable support of vitellogenesis. The main features evidenced during oocyte differentiation are: (1) The synthesis of cortical granules by the rough endoplasmic reticulum and Golgi complex, occurring in the earlier stages of oogenesis; (2) The synthesis of yolk globules by the rough endoplasmic reticulum (RER) and Golgi complex, occurring in the later stages of oogenesis, namely late meiotic prophase I. Neither morphologically visible endocytotic activity, nor the presence of intercellular bridges, nor even the development of microvilli were observed at the oolemma or cortical ooplasm, so that the sole mechanism of vitellogenesis appears to be autosynthetic. The significance of these findings is discussed in relation to the taxonomic position of M. lineata and more generally in relation to the phylogenetic history of the class Turbellaria.