Studies on Unequal Cleavage in Sea Urchins III. Micromere Formation under Compression

When sea urchin embryos at 2-cell stage are flattered between agar plates, the direction of cleavage is rotated by 90° in each division in reference to the preceding cleavage and no micromere is formed. But under this condition, micromeres are formed in 2 cases; 1) When the egg axis is parallel to the plane of flattening, 2 micromeres are formed on one side of a square 16-cell stage. 2) when the egg axis is perpendicular to the plane, 4 micromeres are formed at the center of the square.
When put into a groove, a string of 4 cells is formed showing that the spindle direction is further deflected by the groove. In the following 16-cell stage in the groove, which consists of 2 layers of 8 cells, cases with 2 micromeres on one side and 4 micromeres at the center are still found. If the 2-cell stage is introduced into a groove after the formation of mitotic apparatus, the spindle direction can no longer be changed and the 4-cell stage becomes like 4 pancakes stuck in 2 layers, indicating that 2 asters are holding the ends of a spindle in fixed positions.     

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 lipid-rich 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.__________Translated from Ontogenez, Vol. 36, No. 3, 2005, pp. 182–189.Original Russian Text Copyright © 2005 by Desnitskiy.     

Gamete Recognition and Egg Activation in Sea Urchins

SYNOPSIS. Free-spawning marine invertebrates face the challengeof ensuring that gametes of the same species come into contact,recognize, bind to and fuse with one another once they havebeen released by the adults. Coordinated spawning, chemoattractionand specific cell-cell recognition events help to overcome thischallenge. One marine invertebrate, the sea urchin, has servedas a model system for the study of gamete recognition and fertilizationfor over 100 years. Recent biochemical and molecular advancesin this area have begun to address the questions that have beenraised by the results of elegant physiological observations.The picture of fertilization that is emerging is characterizedby highly specific cell-cell interactions between proteins onthe surfaces of the gametes. These proteins then mediate thebinding and subsequent events that lead to activation of theegg and delivery of the male genetic material. Because of theserecent insights, the sea urchin egg is in a position to provideanswers to one of the central debates in developmental biology—themechanism of egg activation. Does the sperm deliver an activatingfactor? Does sperm binding trigger a receptor-mediated signal?Or is the mechanism a complex combination? With the tools andknowledge gained from the study of sea urchin fertilization,testing of these hypotheses should be feasible in the near future.     

Toxic Diatom Aldehydes Affect Defence Gene Networks in Sea Urchins

Marine organisms possess a series of cellular strategies to counteract the negative effects of toxic compounds, including the massive reorganization of gene expression networks. Here we report the modulated dose-dependent response of activated genes by diatom polyunsaturated aldehydes (PUAs) in the sea urchin Paracentrotus lividus. PUAs are secondary metabolites deriving from the oxidation of fatty acids, inducing deleterious effects on the reproduction and development of planktonic and benthic organisms that feed on these unicellular algae and with anti-cancer activity. Our previous results showed that PUAs target several genes, implicated in different functional processes in this sea urchin. Using interactomic Ingenuity Pathway Analysis we now show that the genes targeted by PUAs are correlated with four HUB genes, NF-κB, p53, δ-2-catenin and HIF1A, which have not been previously reported for P. lividus. We propose a working model describing hypothetical pathways potentially involved in toxic aldehyde stress response in sea urchins. This represents the first report on gene networks affected by PUAs, opening new perspectives in understanding the cellular mechanisms underlying the response of benthic organisms to diatom exposure.     

Some Factors Involved in the Control of Microtubule Assembly in Sea Urchins

Some recent evidence is discussed which suggests that the controlof microtubule assembly during the formation of the mitoticapparatus in sea urchins involves three factors. These include:nucleation sites such as the kinetochores, a mechanism for regulatingintracellular calcium, and a heat stable substance which preventsspontaneous, non-nucleated microtubule polymerization.     

Individual Variability in Reproductive Success Determines Winners and Losers under Ocean Acidification: A Case Study with Sea Urchins

Background

Climate change will lead to intense selection on many organisms, particularly during susceptible early life stages. To date, most studies on the likely biotic effects of climate change have focused on the mean responses of pooled groups of animals. Consequently, the extent to which inter-individual variation mediates different selection responses has not been tested. Investigating this variation is important, since some individuals may be preadapted to future climate scenarios.

Methodology/Principal Findings

We examined the effect of CO₂-induced pH changes (“ocean acidification”) in sperm swimming behaviour on the fertilization success of the Australasian sea urchin Heliocidaris erythrogramma, focusing on the responses of separate individuals and pairs. Acidification significantly decreased the proportion of motile sperm but had no effect on sperm swimming speed. Subsequent fertilization experiments showed strong inter-individual variation in responses to ocean acidification, ranging from a 44% decrease to a 14% increase in fertilization success. This was partly explained by the significant relationship between decreases in percent sperm motility and fertilization success at ΔpH = 0.3, but not at ΔpH = 0.5.

Conclusions and Significance

The effects of ocean acidification on reproductive success varied markedly between individuals. Our results suggest that some individuals will exhibit enhanced fertilization success in acidified oceans, supporting the concept of ‘winners’ and ‘losers’ of climate change at an individual level. If these differences are heritable it is likely that ocean acidification will lead to selection against susceptible phenotypes as well as to rapid fixation of alleles that allow reproduction under more acidic conditions. This selection may ameliorate the biotic effects of climate change if taxa have sufficient extant genetic variation upon which selection can act.     

Distribution Patterns of Strongylocentrotus Sea Urchins along the Coast of Eastern Kamchatka

The distribution patterns of three sea urchin species of the genus Strongylocentrotus in relation to depth, type of substrate, surf action, and to some hydrological parameters were studied along the coast of eastern Kamchatka and in adjacent waters in 1984–1996. The sea urchin Strongylocentrotus polyacanthus dominated the open coast on rocky subintertidal sites with increased surf action and standard oceanic salinity. S. droebachiensis prevailed in shoals with lower salinity, higher water temperature and lower turbulence (enclosed and semienclosed bays). S. pallidus dominated at depths corresponding to the position of the cold intermediate layer deeper than 30–50 m with normal oceanic salinity and lower temperature.     

Gonads of Sea Urchins as the Source of Medication Stimulating Sexual Behavior

In experiments on male rats it was established that native and lyophilized gonads of sea urchins have a stimulating effect on sexual behavior. An effective amount and optimal duration of a dose of the gonads of sea urchins were determined. It is suggested that sexual behavior is connected with the influence of the components of gonads on the adrenoreceptors of the reproductive system of animals.     

Ontogeny of an Extracellular Matrix Component of Sea Urchins and its Role in Morphogenesis

A monoclonal antibody, Sp14, recognizes fibers that form a complex meshwork within the blastocoel of embryos of the sea urchin, Strongylocentrotus purpuratus . The fibers first appear as the blastocoel begins to form and increase in density throughout development. Ultrastructural localizations using the immunoperoxidase method show bundles of 20 nm fibers that are continuous with the basal lamina and have an indistinct axial periodicity. Embryos treated with tunicamycin, β-D-xylopyranoside, β-aminoproprionitrile, proline analogues, or deprived of sulfate all form immunoreactive fibers although in some treatments the pattern formed is abnormal. Immunoreactivity of extracted fibers is not affected by digestion with chondroitinase ABC, hyaluronidase, collagenase or heparinase. However, proteinase K readily destroys immunoreactivity. Fibers will form in cultures of micromeres or mesenchyme 24 to 48 hr after plating with or without horse serum. In embryos in which the blastocoelar matrix has been altered by injection with Sp14, there is inhibition of the release of secondary mesenchyme from the tip of the archenteron and in some embryos supernumerary skeletal elements are formed. It is proposed that Sp14 recognizes a component of the blastocoelar extracellular matrix that is required for the migration of mesenchyme.     

Studies on Unequal Cleavage in Sea Urchins II. Surface Differentiation and the Direction of Nuclear Migration

Cortical features of the meso- and macromeres differ from those of the micromeres in sea urchins. At the end of the 8-cell stage, the four animal cells have a continuous row of vesicles lining the free surface of the cell by transmission electron microscopy (TEM) and the nuclei and the resulting mitotic apparatuses (MA) remain at the cell centers and eventually divide equally into eight mesomeres. In the four vegetal cells, narrow gaps can be seen in the vesicular rows near the vegetal pole. The resting nuclei migrate to these gaps and on forming the spindles, they point directly to the gaps. The result is formation of vesicle-free micromeres and vesicle-covered macromeres by unequal divisions.     

Tissue Regeneration and Biomineralization in Sea Urchins: Role of Notch Signaling and Presence of Stem Cell Markers

Echinoderms represent a phylum with exceptional regenerative capabilities that can reconstruct both external appendages and internal organs. Mechanistic understanding of the cellular pathways involved in regeneration in these animals has been hampered by the limited genomic tools and limited ability to manipulate regenerative processes. We present a functional assay to investigate mechanisms of tissue regeneration and biomineralization by measuring the regrowth of amputated tube feet (sensory and motor appendages) and spines in the sea urchin, Lytechinus variegatus. The ability to manipulate regeneration was demonstrated by concentration-dependent inhibition of regrowth of spines and tube feet by treatment with the mitotic inhibitor, vincristine. Treatment with the gamma-secretase inhibitor DAPT resulted in a concentration-dependent inhibition of regrowth, indicating that both tube feet and spine regeneration require functional Notch signaling. Stem cell markers (Piwi and Vasa) were expressed in tube feet and spine tissue, and Vasa-positive cells were localized throughout the epidermis of tube feet by immunohistochemistry, suggesting the existence of multipotent progenitor cells in these highly regenerative appendages. The presence of Vasa protein in other somatic tissues (e.g. esophagus, radial nerve, and a sub-population of coelomocytes) suggests that multipotent cells are present throughout adult sea urchins and may contribute to normal homeostasis in addition to regeneration. Mechanistic insight into the cellular pathways governing the tremendous regenerative capacity of echinoderms may reveal processes that can be modulated for regenerative therapies, shed light on the evolution of regeneration, and enable the ability to predict how these processes will respond to changing environmental conditions.     

Nitrate Assimilation in Rice Grown under Lowland Conditions

Pattern and extent to which the main shoot of rice (Oryza sativa L.) cv. Pusa 33 assimilates NO₃^- when grown under lowland conditions was determined in a field study. The in vivo NR (nitrate reductase) activity is low as compared to the value in other cereals grown under aerobic soil conditions. The leaf blades had higher NR activity (g fr. wt.)^-1 than the sheaths and stem. Calculation of total NO₃^- (mol) reduced in the main shoot, obtained by integrating the in vivo NR assay values per plant part and per day over the duration for which the various plant parts on the main shoot remained metabolically active, showed that out of the total reduced N at harvest, 16.6% was assimilated via the enzyme nitrate reductase. In the leaf sheaths and stem the NO₃^- was reduced to slightly over 50% of the total NO₃^- that was reduced in the main shoot. The rest of the amount was reduced in the leaf blades.     

Apoptosis in Wheat Seedlings Grown under Normal Daylight

Apoptosis was observed in the coleoptile and initial leaf in 5-8-day-old wheat seedlings grown under normal daylight. Apoptosis is an obligatory event in early wheat plant ontogenesis, and it is characterized by cytoplasmic structural reorganization and fragmentation, in particular, with the appearance in vacuoles of specific vesicles containing intact organelles, chromatin condensation and margination in the nucleus, and internucleosomal fragmentation of nuclear DNA. The earliest signs of programmed cell death (PCD) were observed in the cytoplasm, but the elements of apoptotic degradation in the nucleus appeared later. Nuclear DNA fragmentation was detected after chromatin condensation and the appearance in vacuoles of specific vesicles containing mitochondria. Two PCD varieties were observed in the initial leaf of 5-day-old seedlings grown under normal daylight: a proper apoptosis and vacuolar collapse. On the contrary, PCD in coleoptiles under various growing (light) conditions and in the initial leaf of etiolated seedlings is only a classical plant apoptosis. Therefore, various tissue-specific and light-dependent PCD forms do exist in plants. Amounts of O2*- and H2O2 evolved by seedlings grown under normal daylight are less than that evolved by etiolated seedlings. The amount of H2O2 formed in the presence of sodium salicylate or azide by seedlings grown under normal daylight was increased. Contrary to etiolated seedlings, the antioxidant BHT (ionol) did not inhibit O2*- formation and apoptosis and it had no influence on ontogenesis in the seedlings grown under normal daylight. Thus, in plants grown under the normal light regime the powerful system controlling the balance between formation and inactivation of reactive oxygen species (ROS) does exist and it effectively functions. This system is responsible for maintenance of cell homeostasis, and it regulates the crucial ROS level controlling plant growth and development. In etiolated plants, this system seems to be absent, or it is much less effective.     

Relationships among Egg Size, Composition, and Energy: A Comparative Study of Geminate Sea Urchins

Egg size is one of the fundamental parameters in the life histories of marine organisms. However, few studies have examined the relationships among egg size, composition, and energetic content in a phylogenetically controlled context. We investigated the associations among egg size, composition, and energy using a comparative system, geminate species formed by the closure of the Central American Seaway. We examined western Atlantic (WA) and eastern Pacific (EP) species in three echinoid genera, Echinometra, Eucidaris, and Diadema. In the genus with the largest difference in egg size between geminates (Echinometra), the eggs of WA species were larger, lipid rich and protein poor compared to the smaller eggs of their EP geminate. In addition, the larger WA eggs had significantly greater total egg energy and summed biochemical constituents yet significantly lower egg energy density (energy-per-unit-volume). However, the genera with smaller (Eucidaris) or no (Diadema) differences in egg size were not significantly different in summed biochemical constituents, total egg energy, or energy density. Theoretical models generally assume a strong tradeoff between egg size and fecundity that limits energetic investment and constrains life history evolution. We show that even among closely-related taxa, large eggs cannot be assumed to be scaled-up small eggs either in terms of energy or composition. Although our data comes exclusively from echinoid echinoderms, this pattern may be generalizable to other marine invertebrate taxa. Because egg composition and egg size do not necessarily evolve in lockstep, selective factors such as sperm limitation could act on egg volume without necessarily affecting maternal or larval energetics.     

Phenotypic evolution under gene-culture transmission in structured populations.

I consider a simple model for the evolution of a quantitative character is structured populations when an offspring's phenotype is determined partly by his or her genetic constitution and partly by cultural transmission of the parental phenotype. Analysis of the model indicates that when individual and group selection are in the same direction, phenotypic evolution always proceeds faster under gene-culture vs. purely genetic transmission. When individual and group selection are countervailing, altruistic characters evolve faster under gene-culture transmission when individual selection is weak and migration among groups is limited, with increased individual selection and migration tending to decrease the advantage of gene-culture transmission over purely genetic transmission. Given the prevalence of cultural transmission in higher species, these results suggest that contrary to what is often assumed, group selection may indeed by a potent evolutionary force in the evolution of altruistic characters.     

Light-controlled Leaf Expansion in Peas Grown under Different Light Conditions

Several photosystems control leaf expansion in Alaska peas (Pisum sativum). Phytochrome is known to control expansion in dark-grown peas. But plants exposed briefly to red light are insensitive to phytochrome, an insensitivity that is itself phytochrome-produced. Leaf expansion in these plants is promoted by 440 or 630 nm of light (probably mediated by protochlorophyll). Plants grown in white fluorescent light required simultaneous exposure to high intensity blue and yellow light for promotion of leaf expansion. Since these results parallel studies on light-controlled inhibition of stem elongation, shoot growth as a whole is coordinated by these photosystems. Such coordination might be a mechanism of plant competition for light.     

Carbon Isotope Discrimination in Coffee Genotypes Grown under Limited Water Supply

Photosynthetic gas exchange, plant-water relations characteristics, and stable carbon isotope discrimination (Δ) were evaluated for five Coffea arabica L. genotypes growing under two soil moisture regimes in the field. The Δ of leaf tissue was strongly correlated (r = −0.95) with inherent water use efficiency (ratio of assimilation to stomatal conductance; A/g). The variation in inherent water use efficiency (WUE) among genotypes was 30% for plants irrigated weekly. The higher WUE exhibited by some of these plants resulted from reduced g rather than increased photosynthetic capacity at a given g. Withholding irrigation for 1 month caused Δ to decline substantially in expanding leaf tissue of all genotypes. A strong correlation (r = 0.92) was found between Δ and plant hydraulic efficiency estimated as the ratio of g to the diurnal range in leaf water potential (Ψ_l). The Δ values for plants irrigated weekly adequately predicted drought-induced changes in Δ (r = 0.99) and midday Ψ_l (r = 0.95). The results indicated that Δ might be used to evaluate several aspects of plant performance and response to specific environmental conditions, once suitable background physiological data have been gathered.