Direct-developing sea urchins and the evolutionary reorganization of early development.

The evolution of development can be made accessible to study by exploiting closely related species that exhibit distinct ontogenies. The direct-developing sea urchin Heliocidaris erythrogramma is closely related to indirect-developing sea urchins that develop via a feeding larval stage. Superficial consideration would suggest that simple heterochronies resulting in loss of larval features and acceleration of adult features could explain the substitution of direct for indirect development. However, our experiments show that early development has in fact been extensively remodeled, with modified localization of maternal determinants coupled with dissociation of cell cleavage from axis formation resulting in novel patterns of cell lineage differentiation and fate map. Gene expression has undergone concomitant changes.     

Effects of oil pollution on the development of sex cells in sea urchins

The sea urchinStrongylocentrotus nudus is highly sensitive to oil pollution. Experiments were performed in winter, spring and summer over periods of 15 to 45 days. Experimental urchins were kept in water with hydrocarbon concentrations of 10 to 30 mg l⁻¹, and control urchins in pure sea water. Thermal stimulation by Evdokimov's method was applied to obtain mature sexual products during winter and spring tests. Summer investigations were conducted at temperatures of 17 to 18 °C. The gonads were studied histologically and morphometrically, and the sexual cells obtained were analyzed at the embryological level. No histological and morphometrical differences were recorded between sexual cells of controls and experimentals. However, marked hydrocarbon effects were observed in the embryonic development of artificially fertilized cells from experimental urchins. Control embryos developed normally. Embryogenesis of artificially fertilized gametes from control females and experimental males, and vice versa, was found to be distinctly abnormal. Many abnormalities were identified at the first cleavage stage, as well as in blastula, gastrula and pluteus. Fertilization of experimental eggs with experimental sperm resulted in serious disturbances of embryos, followed by the development of non-viable larvae. On the whole, embryogenesis of sexual cells from experimental urchins was characterized by prominent delay, asynchronism and presence of abnormal non-viable larvae. Consequently, long-term effects of sublethal hydrocarbon concentrations resulted in the formation of defective sex cells and high larval mortality.     

Embryonic expression of engrailed in sea urchins

Neural patterning genes that are expressed along the anterior-posterior axis of deuterostomes are expressed late in larval development in echinoderms and are thought to function in establishing the highly-derived, adult body plan. We have used genomic resources to clone an engrailed gene (SpEn) from Strongylocentrotus purpuratus, and with this we have developed an antibody specific for SpEn. SpEn is expressed late in embryogenesis in the developing larval nervous system. At the prism stage, a small number of neuroblasts in the oral ectoderm on the edge of the larval mouth begin expressing SpEn. The cells are in bilaterally symmetric positions. The expression of SpEn precedes the expression of the neural markers, synaptotagmin and serotonin in the SpEn immunoreactive cells. The SpEn cells are located on the margin of the domain of cells expressing SpNK2.1, but they do not have nuclear SpNK2.1. Expression of engrailed in a pair of bilateral neural structures in early development appears to be a shared feature of bilaterians.     

The role of proteoglycan synthesis in the development of sea urchins : II. The effect of administration of exogenous proteoglycan

The role of proteoglycan synthesis in the early development of sea urchins was studied by treating the embryos with a variety of inhibitors of proteoglycan synthesis and also with proteoglycan of exogenous origin. Developmental arrest at the blastula stage caused by p-nitrophenyl-β- -xyloside (p-NP-xyl) was cancelled by the administration of proteoglycan of exogenous origin. Proteoglycan of post-gastrular origin was effective for cancellation, but proteoglycan of blastular origin was not effective. This suggests that the hindrance of development by p-NP-xyl was due to the lack of proteoglycan synthesis at the late blastula stage. On the other hand, developmental arrest caused by 2-deoxy- -glucose (deoxy-glucose) and Na-selenate was not cancelled by the administration of proteoglycan under any condition tested. Apart from the cancelling action of proteoglycan, solely administered proteoglycan caused a blockage in development at a stage corresponding to the stage from which it was extracted, indicating that proteoglycan may be characterized by a kind of stage specificity.     

Effects of low-intensity pulsed electromagnetic fields on the early development of sea urchins.

The effects of weak electromagnetic signals on the early development of the sea urchin Paracentrotus lividus have been studied. The duration and repetition of the pulses were similar to those used for bone healing in clinical practice. A sequence of pulses, applied for a time ranging from 2 to 4 h, accelerates the cleavages of sea urchin embryo cells. This effect can be quantitatively assessed by determining the time shifts induced by the applied electromagnetic field on the completion of the first and second cleavages in a population of fertilized eggs. The exposed embryos were allowed to develop up to the pluteus stage, showing no abnormalities.     

Evolutionary reorganizations of ontogenesis in sea urchins

The data published during recent 15-20 years on comparative, experimental and molecular embryology of unusually developing sea urchins have been reviewed. These animals are characterized by large lipidrich eggs, highly modified embryogenesis, and the absence of a planktotrophic larva. Such a type of development is evolutionary advanced and arose independently in various phylogenetic lineages of the sea urchins.     

Nuclear activity during oogenesis in sea urchins

Summary Early meiotic stages of Arbacia punctulata oocytes have revealed the presence of synaptinemal complexes in the chromosomes, which persist through zygotene-pachytene. The synaptinemal complexes conform broadly to the usual tripartite structures found in other higher forms. In addition, nuclei at these stages consist of a small nucleolus and dense bodies of varying sizes. The nucleolus is fibrillar in texture throughout and does not seem to incorporate Uridine-5-³H after pulse labeling, whereas the chromosomes are labeled. The nucleolar label is visualized at diplotene stages and onwards. The nuclear envelope differentiates by the appearance of numerous nuclear pore complexes with dense material in the annuli, and the chromosomes become markedly diffused. At vitellogenesis stage the nucleolus and chromatin become highly labeled after pulse incorporation of Uridine, indicating synthesis of ribosomal and chromosomal RNAs.This investigation was supported by grants No. A-5049, A-3624 and D-17 from National Research Council, Canada, grant No. DRB-9340-05 (U6) from Defense Research Board, Canada, and grant No. DRG-918 AT from Damon Runyon Memorial Fund for Cancer Research.     

Heterochrony and heterotopy in the phylogeny of sea urchins

The role of heterochrony is evident in ontogeny and phylogeny of irregular (exocyclic) sea urchins. After metamorphosis, a juvenile passes the stage of regular (endocyclic) sea urchin, in which the periproct is surrounded by plates of the apical system. A shift of the periproct in the area of the fifth interambulacrum occurs in extant taxa at early stages of postlarval development and is accompanied by the reduction of genital plate 5. In some ancient (Jurassic) adult irregular sea urchins, the endocyclic state of the apical system is retained for a long time and the derivative of the fifth genital plate is sometimes observed even in Early Cretaceous species. Considerable transformations in the structure of the lower test surface in members of the order Spatangoida are manifested in changes in the relative positions of plastron plates and ambulacral areas I and V, separation of sternal plates from the labrum, etc. The mechanism of these changes is connected with translocation or “sliding” of sutures of particular plates as a result of nonuniform growth and partial resorption. The study of evolutionary lineages of Cretaceous and Cenozoic sea urchins has shown that the evolution was connected with the directional changes in some morphological characters at late ontogenetic stages. The process was accompanied by either extension, peramorphosis (lineages of the genera Micraster, Infulaster–Hagenowia in the European Province), or the loss of these stages, paedomorphosis (Hemiaster (Bolbaster) lineage, Late Eocene–Middle Miocene of Australia). The phenomena of heterochrony and heterotopy in the development of peripetal, marginal, and lateroanal fascioles in the Late Cretaceous and Paleocene families Hemiasteridae, Schizasteridae, and Paleopneustidae are described. The heterotopy is also illustrated by the example of the development of additional genital pores on ocular plates II and IV of the Middle Jurassic species Pygomalus analis (Disasteroida); its apical system has five pores instead of four. In the Late Cretaceous species Guettaria roccardi (Holasteroida), ocular plates II and IV have two pores each; in the apical system, there are eight genital pores instead of four. In some members of the order Holectypoida, the place of lost genital plate 5 is occupied by a new plate sometimes pierced by a pore, but judging from crystallographic data, it is not homologous to other genital plates. The order Clypeasteroida is characterized by the development of very small pores in both ambulacral and interambulacral fields; they provide passage for numerous accessory tube feet.     

Constraint, flexibility, and phylogenetic history in the evolution of direct development in sea urchins

Development in sea urchins typically involves the production of an elaborate feeding larva, the pluteus, within which the juvenile sea urchin grows. However, a significant fraction of sea urchins have completely or partially eliminated the pluteus, and instead undergo direct development from a large egg. Direct development is achieved primarily by heterochrony, that is, by the abbreviation or elimination of larval developmental processes and the acceleration of processes involved in development of adult features. Direct development has evolved independently several times, and in several ways. These radically altered ontogenies offer remarkable opportunities for the study of the mechanisms by which early development undergoes evolutionary modification. The recent availability of monoclonal antibody and cDNA probes that recognize homologous cells in embryos of closely related typical and direct developing species makes possible an experimental analysis of the cellular and molecular bases for heterochronic changes in development.