Different Modes Of Development Research Articles

Mechanism of an Evolutionary Change in Muscle Cell Differentiation in Ascidians with Different Modes of Development

We have investigated the mechanism of an evolutionary change in ascidian muscle cell differentiation. The ascidiansMolgula oculataandMolgula occultaare closely related species with different modes of development.M. oculataembryos develop into conventional tadpole larvae with a tail containing striated muscle cells, whereasM. occultaembryos develop into tailless larvae with undifferentiated vestigial muscle cells. The muscle actin geneMocuMA1was isolated from anM. oculatagenomic library.MocuMA1is a single-copy, larval-type muscle actin gene which appears to lack introns. However, the 5′ upstream region ofMocuMA1is sufficient to drive expression of alacZfusion construct in the larval muscle cells, implying that it is a functional gene.MocuMA1mRNA first appears in the prospective muscle cells ofM. oculataembryos during gastrulation, and transcripts continue to be present throughout embryogenesis. Muscle actin mRNA was not detected duringM. occultaembryogenesis, although the same probe was capable of detecting muscle actin mRNA in more distantly related ascidian species with tail muscle cells. Interspecific hybrids produced by fertilizingM. occultaeggs withM. oculatasperm recover the ability to express muscle actin mRNA in the vestigial muscle cells, suggesting thattrans-acting factors responsible for muscle actin gene expression are conserved inM. occulta.The presence of thesetrans-acting factors was confirmed by showing that theMocuMA1/lacZfusion construct is expressed in the vestigial muscle cells ofM. occultalarvae. The orthologous larval muscle actin genesMoccMA1aandMoccMA1bwere isolated from aM. occultagenomic library. The coding regions of these genes contain deletions, insertions, and codon substitutions that would make their products nonfunctional. Although the 5′ upstream regions of theM. occultamuscle actin genes also contain numerous changes, expression ofMoccMA1a/lacZandMoccMA1b/lacZfusion constructs showed that they both retain specific promoter activity, although it is reduced inMoccMA1b.The results suggest that the regression of muscle cell differentiation is mediated by changes in the structure of muscle actin genes rather than in thetrans-acting regulatory factors required for their expression.

8 Development and Evolution of an Egg Cytoskeletal Domain in Ascidians

The egg is the only metazoan cell that can accumulate yolk, pause in the meiotic cycle, or initiate development of a new individual by fusing with a sperm. Developmental totipotency is also a special quality of the egg. As a totipotent cell, the egg contains instructions for generating the body plan and different cell types of the organism. Considering the unique properties of the egg, it is not surprising that the egg cytoskeleton also performs specialized roles. These roles include maintaining the integrity of a large cell, providing a framework for meiosis and syngamy, and generating a specific cleavage pattern. Perhaps the most unique role of the egg cytoskeleton is to sequester and correctly position regulatory factors in the embryo. Maternal mRNAs localized at the poles of the Drosophila egg encode transcription factors and other regulatory proteins that determine the ante- roposterior axes and germline. These polar mRNAs are maintained in their correct spatial positions by interactions with the cytoskeleton. Maternal mRNAs are also localized in the vegetal hemisphere of Xenopus eggs via cytoskeletal elements. As a gamete, the egg transmits genotypic information and establishes a phenotype that is influenced by mutation and selection in previous generations. Thus, evolutionary changes are transmitted through the egg and its product, the embryo. This chapter reviews an egg cytoskeletal domain in ascidians and its role in the evolution of development. It considers the ascidian life cycle and mode of development. The structure and function of the myoplasmic cytoskeletal domain (MCD) and its developmental roles are reviewed and the ascidian model system consisting of closely related species with different modes of development is described. Finally, the chapter considers how changes in the MCD may be involved in the evolution of development.

Modifications of cell fate specification in equal-cleaving nemertean embryos: alternate patterns of spiralian development

The nemerteans belong to a phylum of coelomate worms that display a highly conserved pattern of cell divisions referred to as spiral cleavage. It has recently been shown that the fates of the four embryonic cell quadrants in two species of nemerteans are not homologous to those in other spiralian embryos, such as the annelids and molluscs (Henry, J. Q. and Martindale, M. Q. (1994a) Develop. Genetics 15, 64-78). Equal-cleaving molluscs utilize inductive interactions to establish quadrant-specific cell fates and embryonic symmetry properties following fifth cleavage. In order to elucidate the manner in which cell fates are established in nemertean embryos, we have conducted cell isolation and deletion experiments to examine the developmental potential of the early cleavage blastomeres of two equal-cleaving nemerteans, Nemertopsis bivittata and Cerebratulus lacteus. These two species display different modes of development: N. bivittata develops directly via a non-feeding larvae, while C. lacteus develops to form a feeding pilidium larva which undergoes a radical metamorphosis to give rise to the juvenile worm. By examining the development of certain structures and cell types characteristic of quadrant-specific fates for each of these species, we have shown that isolated blastomeres of the indirect-developing nemertean, C. lacteus, are capable of generating cell fates that are not a consequence of that cell's normal developmental program. For instance, dorsal blastomeres can form muscle fibers when cultured in isolation. In contrast, isolated blastomeres of the direct-developing species, N. bivittata do not regulate their development to the same extent. Some cell fates are specified in a precocious manner in this species, such as those that give rise to the eyes. Thus, these findings indicate that equal-cleaving spiralian embryos can utilize different mechanisms of cell fate and axis specification. The implications of these patterns of nemertean development are discussed in relation to experimental work in other spiralian embryos, and a model is presented that accounts for possible evolutionary changes in cell lineage and the process of cell fate specification amongst these protostome phyla.

Multiple origins of anural development in ascidians inferred from rDNA sequences

Ascidians exhibit two different modes of development. A tadpole larva is formed during urodele development, whereas the larval phase is modified or absent during anural development. Anural development is restricted to a small number of species in one or possibly two ascidian families and is probably derived from ancestors with urodele development. Anural and urodele ascidians constitute a model system in which to study the evolution of development, but the phylogeny of anural development has not been resolved. Classification based on larval characters suggests that anural species are monophyletic, whereas classification according to adult morphology suggests they are polyphyletic. In the present study, we have inferred the origin of anural development using rDNA sequences. The central region of 18S rDNA and the hypervariable D2 loop of 28S rDNA were amplified from the genomic DNA of anural and urodele ascidian species by the polymerase chain reaction and sequenced. Phylogenetic trees inferred from 18S rDNA sequences of 21 species placed anural developers into two discrete groups corresponding to the Styelidae and Molgulidae, suggesting that anural development evolved independently in these families. Furthermore, the 18S rDNA trees inferred at least four independent origins of anural development in the family Molgulidae. Phylogenetic trees inferred from the D2 loop sequences of 13 molgulid species confirmed the 18S rDNA phylogeny. Anural development appears to have evolved rapidly because some anural species are placed as closely related sister groups to urodele species. The phylogeny inferred from rDNA sequences is consistent with molgulid systematics according to adult morphology and supports the polyphyletic origin of anural development in ascidians.

Contrasts in crown development of the mistletoesAlepis flavida(Hook. f.) Tiegh. andPeraxilla tetrapetala(L. f.) Tiegh. (Loranthaceae) parasitic onNothofagus solandri(Hook. f.) Oerst., Craigieburn Ecological District, New Zealand

Abstract The developmental features of Alepis flavida and Peraxilla tetrapetala, two parasitic Loranthaceae mistletoes endemic to New Zealand, are described for a better understanding of their comparative ecologies. The two species were found to have strikingly different modes of development. Alepis flavida develops monopodially from preformed buds, with little branching, while Peraxilla tetrapetala develops sympodially, and frequently bifurcates, after shoot-tip abortion from submerged buds with little preformed content, and with much branching. These two patterns produce distinctly different clump forms: loose clumps, with long limbs and few orders of branches, in Alepis flavida, and densely branched clumps, with many orders of branching, in Peraxilla tetrapetala. Both species produce runners, but in Peraxilla tetrapetala these spread to adjacent locations (often the trunk) and produce extremely large, multiple clumps, whereas Alepis flavida remains on the branch on which it established. This difference in development allows these two species to exploit similar niches in Nothofagus solandri forests. The developmental differences observed here support the taxonomic segregation of Alepis and Peraxilla.

An ankryin-like protein in ascidian eggs and its role in the evolution of direct development

An erythrocyte anti-ankryin antibody was used to investigate the presence and distribution of ankryin in eggs and embryos of ascidian species with different modes of development. In eggs of the indirect developer Ascidia ceratodes, anti-ankryin reacted with a 210 kDa polypeptide which has an electrophoretic mobility similar to the vertebrate ankryins. Immunofluorescence microscopy showed that the ankryin-like protein is co-distributed with the myoplasm throughout development. It is restricted to a thin layer under the plasma membrane in unfertilised eggs, undergoes ooplasmic segregation to the posterior pole of the zygote after fertilisation, and is distributed to the tail muscle cells during cleavage and embryogenesis. After gastrulation and neurulation, lower levels of the ankryin-like protein, presumably of zygotic origin, were observed in brain cells and in the apical margin of epidermal cells. The ankryin-like protein was also localised in the myoplasm in eggs and embryos of another indirect developing species, Halocynthia roretzi. The ankryin-like protein may link the cytoskeleton with the plasma membrane in ascidian eggs, as it does in vertebrate erythrocytes. In contrast to A. ceratodes and H. roretzi, which are members of the families Ascidiidae and Pyuridae respectively, the pattern of ankryin-like protein expression was changed in five species in the family Molgulidae. These molgulid ascidians exhibit either indirect or direct development, and eggs of the direct developing species have lost or modified the myoplasm. The ankryin-like protein was present in young oocytes but failed to persist during oogenesis and disappeared in mature eggs and embryos of these molgulid species. The change in ankryin-like protein expression may be a preadaptation for loss of the myoplasm and the evolution of direct development.

PLASTICITY OF JAW AND SKULL MORPHOLOGY IN THE NEOTROPICAL CICHLIDS GEOPHAGUS BRASILIENSIS AND G. STEINDACHNERI.

I examined plasticity of jaw and skull morphology induced by feeding different diets in two species of the neotropical cichlid genus Geophagus. The two species possess different modes of development, which affect the size at which young begin feeding. I hypothesized that the difference in size at first feeding could lead to a difference in the amount of change inducible in the two species. The young of the substrate-spawning species, G. brasiliensis, which begin feeding at a smaller size, were predicted to be more plastic than those of the mouthbrooding species, G. steindachneri. The two diets used to induce differences were brine shrimp nauplii and chironomid larvae. Numerous measures of the jaw and skull differed significantly between groups fed the two diets but the amount of plasticity induced was small and would not present a problem for taxonomists. Contrary to my prediction, both the magnitude and pattern of plasticity induced in the two species was similar. Thus, mode of parental care and the size at which young begin feeding do not affect the degree of plasticity. Fish fed brine shrimp nauplii were longer in oral jaw region, but were shorter and shallower in the area behind the oral jaws. An additional group of G. brasiliensis was fed flake food to compare the results of this study to other studies. The differences in measures between fish fed brine shrimp diets and flake food diets were greater than those between fish fed brine shrimp and chironomid larvae. A possible role of plasticity for enhancing rather than retarding morphological evolution is discussed.

Distribution patterns and dispersal of prosobranch gastropods along a seamount chain in the Atlantic Ocean

Based on qualitative data on prosobranch gastropods present at eight seamounts and islands of the Vitória-Trindade Seamount Chain off the eastern coast of Brazil, similarities at the species level are examined, and the effects of selection for different modes of development varying with increasing distance from the coast are investigated. Number of species decreases significantly from the continent towards easternmost localities. Similarity coefficients and cluster analysis suggest that similarities are greater among the western seamounts, followed by the eastern, most oceanic localities. Subtidal stations on Trindade Island show less similarity at specific level when compared with the remaining sublittoral stations. Percentages and absolute numbers of species with intracapsular metamorphosis decrease rapidly away from the coast in the sublittoral localities. However, there is little variation for the ratio planktotrophs/lecithotrophs among these localities, and their percentages remained constant over the entire Chain. In spite of the slightly wider range of distribution of planktotrophs within the Chain, the observations suggest that both planktotrophs and lecithotrophs can be effectively dispersed, probably by passive larval transport, in an ‘island-hopping’ pattern across the relatively small distances (100–250 km) that separate summits in the Chain. Notwithstanding shallow, subtidal conditions and intense isolation, percentages of planktotrophs and lecithotrophs also do not vary at the subtidal Trindade stations.

Domain-specific changes of ciliary striated rootlets during the cell cycle in the hypotrich ciliate Paraurostyla weissei

ABSTRACT The dynamics of striated ciliary rootlets (cr) during development of ciliary structures in cells of the hypotrich ciliate Paraurostyla weissei was studied by immunostaining with polyclonal antibody raised against isolated cr of Paramecium tetraurelia. Wildtype cells and two mutants: mlm showing multiple left marginal cirri and mlm/pl showing in addition a certain degree of pattern lability were used to study the boundaries of particular cortex domains. In interphase cells, cr are attached to all left marginal and caudal cirri and to only the posterior third of the right marginal cirri, cr appear in all nonoral primordia during the first wave of basal body proliferation. After nucleation and early elongation of cr, some cr undergo site-specific regression or stabilization. The spatial deployment of these different modes of development corresponds to specific cortical domains. In mlm/pl mutants, where specific cortical domains are broadened, changes in the cr system are characteristic for a given domain, regardless of its broadening, but boundaries between adjacent domains are weakened.

Single-copy DNA distance between two congeneric sea urchin species exhibiting radically different modes of development.

We have investigated the differences between nuclear genomes of two purportedly congeneric species of sea urchin that differ radically in early development. Heliocidaris tuberculata develops by means of a typical pluteus larva, whereas H. erythrogramma develops directly from an egg that is 100-fold the volume of the H. tuberculata egg. Reassociation kinetic analysis shows that the kinetic components of the genomic DNA from the two species are essentially the same. No single repeat component explains the 30% difference between the H. erythrogramma and H. tuberculata genomes. Reciprocal hybridization of tracer-labeled single-copy DNA fractions between these species indicates that approximately 50% of the single-copy DNA is sufficiently similar to form hybrids at standard hybridization criterion. Thermal denaturation profiles of the hybridized single-copy DNA sequence yields median (T50H) values of 13.8 degrees-16.5 degrees C. This result suggests a divergence time of 10-13 Mya, which is comparable to divergence times between congeneric sea urchin species in other genera that do not differ significantly in development. Radical differences in early developmental processes can evolve rapidly between closely related forms.

Electrophoretic separation of morphologically similar species of the genusRissoa(Gastropoda: Prosobranchia)

Abstract Morphologically Rissoa labiosa and R. membranacea can be separated only by differences in apical dimensions, i.e. the diameter of the initial embryonic shell and the diameter of the first half whorl. However, since different apical dimensions indicate different modes of development, the question has been raised whether the two types belong to the same species. Electrophoretic investigations demonstrated differences in genetic composition of sympatric populations of R. labiosa and R. membranacea, which supports the hypothesis of distinct species.

Change in Setal Pattern during Early Development ofEupolymnia Nebulosa(Polychaeta: Terebellidae) Grown in Simulated Natural Conditions

Eupolymnia nebulosa(Montagu) is a tubicolous worm with its body divided into two regions: thorax with biramous segments and three pairs of gills; abdomen with numerous segments lacking notopodia. The head bears grooved foodgathering tentacles which are not retractile into the mouth. The worm, a typical deposit feeder, lives in mucous tubes encrusted with sand. Although the family Terebellidae is homogeneous in terms of body organization and feeding ecology, there is a remarkable heterogeneity of developmental pattern. In the generaLaniceandLoimia, the life-cycle involves two pelagic stages separated by a short benthic larval stage; in other genera the development is mixed or direct (Bhaudet al.1987; Pechenik, 1979; Caswell, 1981). It is important to study the development of representatives of the family which may reveal the adaptive significance of the different modes of development or may yield information about evolutionary relationships within the family. In several previous papers larval development (Bhaud & Grémare, 1988a,b) and reproductive biology (Bhaudet al.1987) ofE. nebulosahave been reported from the Mediterranean coast. The present article reports observations on juvenile growth and morphological organization in natural conditions. This is the first step to growth rate studies in terms of temperature and food supply (Bhaud, 1988).

An Assessment of Poecilogony in Marine Invertebrates: Phenomenon or Fantasy?

Poecilogony, defined as more than one mode of reproduction within a single species, has been reported in various invertebrates, including mollusks and polychaetes. Many cases that have been described involve planktotrophic and non-planktotrophic development in allopatric populations, or instances of planktonic larval stages and benthic juveniles being found together, but not associated with adults. There is always the possibility of mis-assignment of larvae to adults of the wrong species. Most cases that offer these kinds of evidence are now known to involve cryptic species, not poecilogony.There are a few species in which release of young occurs both at metamorphosis and a day or so before. There may be cases in which extracellular yolk or nurse-egg production is variable and allows the release of larvae at different stages, but no actual instance is known. A few instances are known of allopatric populations with different modes of development and other differences in reproductive characters that lack reproductive isolation when brought into the laboratory. The polychaetes Streblospio benedicti, Cirriformia tentaculata, Boccardia proboscidea, and the opisthobranch Elysia chlorotica are in this category. All examples of poecilogony require further genetic substantiation.Despite the scarcity of proven examples of poecilogony, the presence of more than one mode of reproduction within a genus is the rule in most invertebrate phyla. The evolutionary and ecological significance of these patterns is discussed.

Gastric choriocarcinoma and yolk sac tumor in a man: Observations about its possible origin

The case of a 65-year-old man who had a gastric tumor with components of choriocarcinoma and yolk sac tumor is presented. The presence of a small component of adenocarcinoma in the primary tumor and the absence of any gonadal or other primary tumor at autopsy confirm its origin. Despite the common association of embryonal and trophoblastic elements in gonadal tumors, this is the first case in which such an association was clearly depicted in the stomach. This and the usual coexistence of adenocarcinoma with choriocarcinoma in the stomach suggest the possibility of a different mode of development of these tumors in the two sites: forward differentiation of neoplastic germ cells in the gonads and retrodifferentiation (opisthoplasia) of neoplastic mucosal epithelial cells in the stomach.

The activity of Balbiani rings 1 and 2 in salivary glands of Chironomus tentans larvae under different modes of development and after pilocarpine treatment

The activity of the Balbiani rings 1 and 2 (BR1 and BR2) in the salivary gland was followed during development of fourth instar larvae of Chironomus tentans under different modes of development, with or without a previous pilocarpine treatment. The activity was determined in parallel by two different methods, by incorporation of [ 3H]uridine into BR-RNA (75 S) and by morphometry of BR1 and of BR2. In glands of untreated larvae BR2 does not change dramatically except for a depression of activity during oligopause (resting phase). BR1 is completely inactive during this phase but exhibits a pronounced activity maximum in the middle of the prepupal period, in subitaneously developing (i.e., uninterrupted) as well as in postoligopause cultures. After pilocarpine treatment the activity of BR2 (rather than of BR1) is generally increased. The extent of this stimulation, however, is strongly development dependent. A most striking activity difference is observed in postoligopause between animals of stage 5 and of stage 6. The relationship between BR2 activity and degree of emptying of the salivary gland lumen was investigated. A model is proposed in which BR2 activity is conceived as being regulated by two parameters: by the degree of filling of the gland lumen and by the stage and mode of development of the larva.

Coarsening in the Cu-Ni-Cr system

A comprehensive metallographic study of coarsening in the (γ 1 + γ 2) region of the Cu-Ni-Cr system in reported. Particles of γ 2 form and coarsen at the extreme Cu rich side but all the other alloy compositions form interconnected structures arising from spinodal decomposition. These are shown to coarsen by increasing their uniform interphase spacings. It is argued that this involves some diffusion controlling interaction between adjacent arms of the structure in which the phases move bodily. Subsequent stages involve the break up into particles, increasing the interface area and finally particle coarsening and incoherency. Rates of all stages have been measured. A comprehensive coarsening sequence is proposed for different modes of development for such initially interconnected spinoda´l structures.

Two modes of development in the morphospecies Crepidula dilatata (Gastropoda: Calyptraeidae) from Southern Chile

In Chinquihue Bay, a sheltered locality in Southern Chile, female Crepidula dilatata Lamarck brood egg masses which differ in intracapsular development. In some, all eggs develop and are hatched as free veliger larvae (indirect development). In others, only some of the eggs develop and hatch as young adults (direct development), the rest being consumed as nurse eggs. Two possible interpretations are considered: intrapopulation variation of developmental pattern in a species, and the coexistence of two sibling species. Stages of the intracapsular development are described and illustrated. Fecundity varies according to size of the female. For females brooding eggs with indirect development it is estimated at between 3840 and 85575 embryos per spawn; for females with eggs undergoing direct development, embryo production per egg mass is estimated at between 70 and 812. A comparison is made between females brooding the two kinds of egg masses. Minor differences exist in adult coloration, shape of egg capsules, egg diameters, spawning season and their abundance and distribution in the intertidal. Brooding females with eggs displaying indirect development are significantly larger than those with eggs of direct development. The latter females form chains subtidally but not intertidally as do females bearing eggs with indirect development. On these females with pelagic larvae, chains frequently include sedentary males which reach large sizes. On basal females with direct development, chains contain mostly other females, suggesting that matings are temporary and by errant males. These results are compared with existing information in the literature for other species both in this genus and in other marine invertebrates. It is concluded that two sympatric sibling species with different modes of development are present in the morphospecies C. dilatata Lamarck in Southern Chile.

The Self—Determinism of Teleogenic Systems

Abstract The existence of uncontrollable state variables is shown to be necessary for a system to be teleogenic, capable of generating its own goals. Since human beings and other living organisms are manifestly teleogenic, the behavioristic paradigm is fundamentally flawed. A triadic theory of human development, in which heredity and environment are joined by a self-determined component consisting of the evolving uncontrollable state variables, is outlined. The behavior of a simple teleoid under two different modes of development is described.

LIVER MORPHOLOGY

SummaryThe liver is constructed according to one fundamental plan in all vertebrates. It is a continuous mass of cells tunnelled by a labyrinth of lacunae, in which the network of sinusoids is suspended. This structure is termed a muralium (‘wallwork’, system of walls). The walls are predominantly two cells thick in lower vertebrates and in some primitive birds. They are only one cell thick in a few other primitive birds and in all song birds and mammals.Where the liver parenchyma touches the portal and hepatic canals, and the external capsule, it is bounded by the limiting plate, a continuous sheet of liver cells.The portal veins send short inlet venules through holes in the limiting plate to supply the sinusoids. Individual sinusoids empty into the central veins.The sinusoids are lined by one kind of cells: potential phagocytes. These cells never bridge the lumen of sinusoids.The hepatic arteries have direct inlets into the paraportal, intermediate and centrolobular sinusoids.Meshes of bile canaliculi surround each individual liver cell in mammals. In lower vertebrates they rarely form meshes. They drain into intralobular and periportal ductules.The liver lobules are ephemeral. They are only expressions of the temporary blood‐pressure conditions. Reduction of the portohepatic blood‐pressure gradient abolishes hepatic lobules and creates ‘portal’ lobules. This process is reversible.In man, each portal triad (portal vein branch, hepatic artery branch, hepatic duct tributary) has its own sharply delimited territory. There are normally no anastomoses between territories.These portal segments are united by the hepatic veins which cross the boundaries of portal segments.The major branches and roots of these vessels and ducts have been mapped and named. Applications to surgery are suggested.The uniform structure of adult livers arises from a great diversity of developmental processes. Twelve fundamentally different modes of development have been found among thirty species of twenty‐eight genera in sixteen orders out of eight classes of vertebrates. In ten species the mesoderm participates in the formation of the hepatic parenchyma.The ductal system develops in some cases as an outgrowth from the gut; in more cases it develops from liver cells and joins the intestine secondarily.Von Baer's first and second law of embryogenesis, as well as the biogenetic law of Müller and Haeckel, are contradicted by these observations. New evidence is presented for the non‐specificity of germ layers.