Osmotrophy in fossil protoctists and early animals

Summary

The role of Ca²⁺ in activation and early development of locust eggs was examined through measurement of ooplasmic Ca²⁺ levels before and after fertilization, and through experimental activation of unfertilized eggs. Ooplasmic pCa (i.e. the negative logarithm of Ca²⁺ activity) measured in intact eggs decreased from 5.35 before fertilization, to 4.77 and 3.00 by 1 day and 3 days after fertilization, respectively. pCa was also determined for samples of ooplasm collected by rupturing eggs under paraffin oil. The pCa was 5.10 in ooplasm isolated from unfertilized eggs, and 3.84 in ooplasm collected from eggs within 4 h of fertilization. Ooplasmic pCa remained between 3.97 and 3.12 from 1–6 days after fertilization. Since a decline in pCa indicates an increase in ooplasmic Ca²⁺ activity, the data suggest that regulation of ooplasmic Ca²⁺ during post-fertilization development involves release of Ca²⁺ from internal stores. Experimental egg activation was examined in eggs dissected from the oviducts before fertilization and incubated on moist filter paper. Some eggs were first immersed in experimental solutions for 30–60 minutes before incubation. The presence of an embryo 2 or 4 days after fertilization or experimental treatment was used as an indicator of egg activation. Activation occurred in 92% and 12% of fertilized and untreated eggs, respectively. The percentage of unfertilized eggs which activated increased to 47% if eggs were soaked 30–60 minutes in physiological saline, and to as much as 65%-68% if eggs were injected with Ca²⁺ buffers or if a Ca²⁺ action potential was evoked. Up to 36% and 42% of unfertilized eggs activated after incubation in Ca²⁺-free salines or in the presence of the Ca²⁺-channel blocker Cd²⁺, respectively. Taken together, the results suggest that entry of external Ca²⁺ through voltage dependent channels increases the proportion of eggs which activate, but is not an absolute requirement for activation.     

Effects of egg availability and egg maturity on the ovipositional activity of the parasitic wasp, Coccophagus atratus

ABSTRACT. Effects of egg availability on daily ovipositional activity were determined for Coccophagus atratus Compere (Hymenoptera: Aphelinidae). Females were observed for 6 h per day for their entire adult lives. Observed ovipositional activity was analysed in relation to egg maturation before and after emergence, egg depletion during oviposition and egg replenishment after oviposition. Ovipositional activity, including oviposition, inconsequential probes and searching for hosts, occurred predominantly in the first 30 min of exposure to hosts on the 2nd, 4th, 7th, 10th and 13th days after emergence. The remaining time was spent on non-ovipositional activities (preening, drinking honeydew and sitting on the glass of the experimental arena). Peaks of ovipositional activity were associated with high numbers of mature eggs in the ovaries. Eggs that remained in the ovaries after a bout of oviposition were apparently not mature even though they had attained their maximum size. Non-ovipositional activity continued until females had built up a reserve of about eighteen mature eggs. After each successive bout of oviposition, the rate of oogenesis slowed down. Consequently females took longer to accumulate a supply of eggs and periods of non-ovipositional activity increased.
We conclude that (1) the availability of eggs and the tendency of females to store mature eggs influences ovipositional activity, (2) full-sized eggs are not necessarily mature, (3) future experiments with C.atratus could be restricted to days of high ovipositional activity, and (4) the terms syn- and pro-ovigenic formulated by Flanders (1950) to describe apparent differences in oogenesis between various parasitic Hymenoptera do not apply to C. atratus and are therefore not universally applicable.     

Independent Initiation of Calcium Dependent Glycosidase Release and Cortical Contractions during the Activation of Ascidian Eggs

During fertilization or ionophore induced activation, ascidian eggs rapidly release cell surface N-acetylglucosaminidase activity used in the block against polyspermy and undergo cortical contractions before they re-initiate meiosis. To better understand the activation process, we probed the relationship between these two processes in Ascidia ceratodes eggs by activating with different agents that increase intracellular Ca levels and under different ionic conditions. Glycosidase activity release was followed by the use of a fluorogenic substrate, and cortical contractions were followed by examining changes in cell shape with light microscopy. Ionomycin (2.7 μM) and thimerosal (1 mM) initiate glycosidase release and cortical contractions when administered in complete sea water (SW) but only the contractions in low Ca SW. Ryanodine (0.67 mM), known to raise free intracellular Ca in a number of cell types by release from the endoplasmic reticulum, causes glycosidase release but fails to initiate cortical contractions in complete SW. Thapsigargin (10 μM), which inhibits Ca dependent ATPase in the ER, causes glycosidase release but induces the contractions only about 50% of the time. These experiments show that, although glycosidase release normally precedes the ooplasmic shape changes that accompany the resumption of meiosis in ascidian eggs, they are not obligately coupled. That both processes can be induced by treatments known to raise intracellular Ca in other systems but under different conditions indicates that there may be a multiplicity of Ca requiring but functionally independent events during egg activation.     

Localization of actin messenger RNA during early ascidian development

The spatial distribution of RNA sequences during early development of the ascidian, Styela plicata, was determined by in situ hybridization with poly(U) and cloned DNA probes. Styela eggs and embryos contain three colored cytoplasmic regions of specific morphogenetic fates, the ectoplasm, endoplasm, and myoplasm. These cytoplasmic regions participate in ooplasmic segregation after fertilization and are distributed to different cell lineages during early embryogenesis. n situ hybridization with poly(U) suggests that poly(A)+RNA is unevenly distributed in eggs and embryos, with about 45% in the ectoplasm, 50% in the endoplasm, and only 5% in the myoplasm. In situ hybridization with a histone DNA probe showed that histone RNA sequences were not localized in eggs or embryos and distributed between the three cytoplasmic regions according to their volumes. In situ hybridization with an actin DNA probe showed actin RNA was localized in the myoplasm and ectoplasm of eggs and embryos with about 45% present in the myoplasm, 40% in the ectoplasm, and only 15% in the endoplasm. These results suggest that a large proportion of the egg actin mRNA is localized in the myoplasm, participates in ooplasmic segregation after fertilization, and is differentially distributed to the mesodermal cell lineages during embryogenesis. Analysis of the translation products of egg mRNA suggests that the localized mRNA codes for a cytoplasmic actin isoform.     

A direct effect of some rifamycin derivatives on the morphology of mammalian mitochondria

Protein synthesis has been investigated in cell-free preparations from mature ovarian oocytes, unfertilized and fertilized eggs, and early embryos of Drosophila melanogaster. Preparations from unfertilized eggs have a specific activity that is 5- to 6-fold higher than the activity of fractions from ovarian oocytes. There is an additional small increase in activity of preparations from fertilized eggs. The specific activity that is rapidly attained in the fertilized egg remains essentially constant for 2 to 2.5 h after fertilization, decreases sharply during blastoderm formation, and again increases during gastrulation. The activities of unfertilized eggs decline slightly during the first 2 h after oviposition, and then decrease more sharply. About 35 % of the ribosomes in preparations from both unfertilized and fertilized eggs sediment in the polyribosome region of sucrose density gradients, whereas no polyribosomes could be detected in preparations from ovarian oocytes. In both ovarian oocytes and fertilized eggs, less than 1 % of the ribosome populations were present as subunits. Additional ribonucleoprotein material of buoyant densities different from those of ribosomal subunits or ribosomes was found throughout the sucrose gradients. About 3.5 % of the ribosomes were found to be membrane-bound in preparations from both unfertilized and fertilized eggs.     

Alteration of Trichostrongylus colubriformis egg permeability by Bacillus thuringiensis israelensis toxin

A toxin from the bacterium Bacillus thuringiensis israelensis is lethal to nematode eggs. Exposure of eggs of the ruminant nematode Trichostrongylus colubriformis to the toxin significantly increased the eggs' permeability to radiolabeled phenylalanine within 2 hr. Calcium chloride inhibited the toxin-induced change in egg permeability. Iodine staining of eggs that were exposed to the microbial toxin revealed that egg permeability was altered within 5 min and was dependent on the dose of toxin. Addition of 34 mM sucrose, 17 mM sodium chloride, or 17 mM potassium chloride to the eggs' medium increased the toxin's lethality. Exopeptidase activity in eggs of T. colubriformis was reduced significantly after exposure to the B. t. israelensis toxin. Tetrodotoxin, tetraethylammonium chloride, ouabain, 4-acetamido-4'-isothiocyano-stilbene-2,2'disulfonic acid (SITS), 4,4'-diisothiocyano-2,2'disulfonic acid stilbene (DIDS), valinomycin, and sodium vanadate, which affect membrane transport, had no significant effect on the activity of B. t. israelensis toxin for eggs. Likewise, a series of nucleotides and their derivatives had no effect on the toxin's activity. Ovicidal activity of the microbial toxin was increased by 4-aminopyridine (4.4 X), but was decreased by furosemide (97 X), nigericin (263 X), or monensin (125 X). Microscopic measurement of T. colubriformis eggs after treatment with the microbial toxin revealed no significant size change.     

Modus operandi of oviposition in Dermacentor reticulatus (Acari: Ixodidae)

The process of oviposition in D. reticulatus was observed and found to be a sequence of exactly coordinated, interlocking events independent of the phase of oviposition. The average period of oviposition in the investigated ticks was 31.6 days at 20 °C and 95% relative humidity. The number of eggs deposited on each day increased until reaching a maximum on the fifth day of oviposition and then decreased continuously. As a result, most of the eggs were deposited during the initial phase of oviposition. The total number of eggs was proportional to the ticks' weight replenishment. Egg-laying commenced with the lowering of the capitulum and the simultaneous spread of the pedipalps which were lowered to the body wall embracing the genital aperture on both sides. Immediately afterwards the cuticular sac of Gene's organ was pushed out and retracted several times. At the cuticular sac's maximum extension, the vestibulum vaginae prolapsed, forming the ovipositor as an extended tube which handed over an egg to the two horns of the cuticular sac after a brief, but intensive, contact with the cuticular sac. Then the vestibulum vaginae invaginated, the pedipalps closed, and the cuticular sac was retracted. Finally, the egg was transported onto the dorsal area of the tick by means of a vigorous rising of the capitulum. During the course of oviposition most of the events, especially the period of egg embracement by the cuticular sac, were prolonged, as was the total time for laying one egg. Similarly, the intervals between successive egg-laying processes increased continuously.The number of eggs deposited was not dependent on the functional ability of Gene's organ, as shown by similar numbers of deposited eggs from ticks with and without mechanical blocking of the cuticular sac. But the participation of the organ in the process of oviposition proved to be a prerequisite for the viability of the eggs. Larvae developed and hatched only from those eggs which were deposited from ticks with an undisturbed Gene's organ. In comparison, eggs without contact to the cuticular sac of Gene's organ dried up and shrivelled immediately after being deposited and did not hatch. Consequently, it strongly suggests, together with the results from other studies, that Gene's organ covers the eggs with a secretion that prevents the loss of water.     

Properties of adenylate cyclase activity during early sea urchin development

The total adenylate cyclase activity in homogenates of eggs of the sea urchins Strongylocentrotus purpuratus and Lytechinus pictus was assayed in vitro and found to remain constant in eggs before and at intervals after fertilization. In S. purpuratus egg homogenates virtually all of the enzyme activity was sedimented by centrifugation at 20 000 g. The enzyme specific activity in the 20 000 g pellet remained unchanged at each point through first cleavage, though it was several-fold higher than in the whole homogenate. The adenylate cyclase from both fertilized and unfertilized eggs was maximally active in vitro when assayed with 10 mM MgSO₄ and 10 mM NaF at pH 8 using 0.2 mM AMP-PNP (an ATP analog) as the substrate. Sucrose density gradient centrifugation of egg homogenates showed that adenylate cyclase activity was present in fractions which sedimented at a variety of densities. The adenylate cyclase specific activity in cortices isolated by the method of Sakai [10] from eggs at first cleavage was 4- to 6-fold higher than in unfertilized egg cortices. The increased enzyme activity in egg cortices at first cleavage suggests that adenylate cyclase-containing membranes may become localized within the egg cortex after fertilization.     

Signaling pathway from [Ca2+]i transients to ooplasmic segregation involves small GTPase rho in the ascidian egg

Intracellular Ca2+ transients occur at fertilization in the eggs of all animal species and are thought to be critical for the initiation of several events in egg activation. The rho family of small GTPases are known to organize and maintain the actin filament-dependent cytoskeleton, and rho is involved in the control mechanism of cytokinesis. In the ascidian Ciona savignyi, the first step of ooplasmic segregation observed just after fertilization is cortical contraction with egg deformation, mediated by the cortical actin filaments. C3 exoenzyme, a rho-specific inhibitor, did not affect the pattern of [Ca2+]i transients in the ascidian egg, but inhibited ooplasmic segregation and cytokinesis at the first cleavage. Injection of inositol 1,4,5-trisphosphate or treatment of Ca2+ ionophore induced deformation of the egg and extrusion of the first polar body, but these phenomena did not occur in the C3 exoenzyme-injected egg. These results suggest that rho proteins are involved in egg deformation, ooplasmic segregation and cytokinesis downstream of the [Ca2+]i transients.     

CHANGES IN ACTIVITIES OF THYMIDYLATE SYNTHASE AND DIHYDROFOLATE REDUCTASE IN SEA URCHIN EGGS AFTER FERTILIZATION

Thymidylate synthase activity in sea urchin eggs increases just after fertilization and decreases 30 min later. Then, cyclic variation in the activity occurs in association with the cleavage cycle. Dihydrofolate reductase activity in fertilized eggs is almost the same as in unfertilized eggs and shows no marked change within 3 hr after fertilization. Aminopterin, an analogue of dihydrofolate, inhibits dihydrofolate reductase, and arrests cleavage. On incubation in sea water containing aminopterin (20-100μM) from the time of fertilization, the development of Clypeaster and Pseudocentrotus eggs was arrested at the 32–64 cell stage, and that of Anthocidaris eggs was arrested at the morula stage. Dihydrofolate (100μM) counteracts the inhibitory effect of aminopterin on egg cleavage. Thymidine at concentrations above 10μM also prevents inhibition by aminopterin. Other deoxyribonucleosides at concentrations of 10μM to 100μM do not affect inhibition of egg cleavage by aminopterin. Deoxyadenosine at concentrations above 5 mM inhibits egg cleavage, but other deoxyribonucleosides have no effect.     

Field Assessment of Adhesion and Hatch ofChrysoperlaEggs Mechanically Applied in Liquid Carriers

A mechanical technique was evaluated for releasing green lacewing eggs in liquid suspensions. Deposited eggs were enclosed within a circle of nonpoisonous adhesive to protect them from predation and to prevent escape of hatched larvae. Released eggs were monitored daily for 5 days after release by measuring three response variables: adhesion rate of eggs to foliage, hatch rate of eggs, and “yield” of larvae from discharged eggs; “yield” was the product of egg adhesion and egg hatch. Factors tested were: egg conditioning prior to release (incubated or refrigerated), carrier (distilled water or commercial carrier solution), application technique (mechanical or hand application), and row facing (North or South). Release technique did not significantly effect egg hatch on any day. Conditioning eggs prior to release had the greatest effect on hatch of eggs and resulting yield of larvae during the 5-day monitoring period. Carrier had a significant effect on adhesion of eggs to leaves and hatch of eggs. Commercial carrier solution increased egg adhesion but decreased egg hatch compared to water. Overall mean yield of larvae from incubated eggs distributed mechanically was not significantly different for eggs suspended in water (36.4% on day 5 post-release) and for eggs suspended in commercial carrier solution (36.1% on day 5 post-release). Hand-applied eggs had a higher hatch and subsequent yield of larvae than mechanically released eggs; however, the hand technique was labor intensive.     

The effect of light exposure on buoyancy of halibut eggs

Effects of light-exposure on eggs of the Atlantic halibut ( Hippoglossus hippoglossus ) were investigated. Egg buoyancy, yolk sac osmolality and perivitelline space (PVS) in light and dark-exposed eggs were followed during 3–12 days after fertilization. In light-exposed eggs, the density increased to a maximum at day 6 while the density in dark exposed eggs significantly decreased between day 4 and 10. There was no significant difference between treatments at day 12. The pattern of yolk osmolality reflected these changes in density. Embryonic volume, calculated from estimates of total volume and PVS, was found to decrease rapidly at days 3–4 after fertilization in the light exposed group, whereas the control group during the same period showed no change. After this period, the embryonic volume showed a parallel decrease in both groups. The increased egg density is caused by both the loss of water from the embryonic compartment and by increased yolk osmolality. These results are discussed in relation to changes in vertical distribution in both natural and culture systems.     

Oocyte fertilization triggers acid phosphatase activity during Rhodnius prolixus embryogenesis

Acid phosphatase activity, previously identified in Rhodnius prolixus oocytes, was studied during egg development. Fertilized eggs exhibited a five fold increase of total acid phosphatase activity during the first days of development. In contrast non-fertilized oviposited eggs showed no activation of this enzyme. An optimum pH of 4.0 for pNPP hydrolysis in a saturable linear reaction and a strong inhibition by lysosomal acid phosphatase inhibitors such as NaF (10 mM) and Na(+)/K(+) tartrate (0.5 mM) are the major biochemical properties of this enzyme. Fractionation of egg homogenates through gel filtration chromatography revealed a single peak of activity with a molecular mass of 94 kDa. The role of this enzyme in VT dephosphorylation was next evaluated. Western blots probed with anti-phosphoserine polyclonal antibody demonstrated that VT phosphoaminoacid content decreases during egg development. In vivo dephosphorylation during egg development was confirmed by following the removal of (32)P from (32)P-VT in metabolically labeled eggs. Vitellin was the only phosphorylated molecule able to inhibit pNPPase activity of partially purified acid phosphatase. These data indicate that acid phosphatase activation follows oocyte fertilization and this enzyme seems to be involved in VT dephosphorylation.     

Propagation of transient Ca2+ increase in sea urchin eggs upon fertilization and its regulation by microinjecting EGTA solution

Upon fertilization, the concentration of intracellular Ca2+ (Cai) in sea urchin eggs increased up to 3 microM when measured with fura-2, a fluorescent Ca indicator and the increase in Cai traversed from the sperm entry point as a wave over the entire egg at the mean propagation velocities of 5.0 microns/sec in C. japonicus egg and 5.3 microns/sec in H. pulcherrimus egg. However, the velocity was not uniform; i.e., it was rapid in the vicinity of the sperm entry point and the opposite point, but slow in the central region of the egg. Microinjecting a Ca-EGTA buffer and an IP3 solution into the C. japonicus egg induced the transient Cai increase more rapidly than that upon fertilization, due perhaps to the diffusion of the injectates. In order to investigate Ca2+ release during Cai increase upon fertilization, EGTA solutions were microinjected into unfertilized or fertilizing eggs. Microinjecting 100 mM EGTA (final concentration of 1 mM) not only suppressed the transient Cai increase, but also reduced the increased Cai rapidly, and never induced egg activation after insemination, whereas 10 mM EGTA (final concentration of 0.1 mM) did not significantly affect the Cai increase or the activation. Ca2+ released upon fertilization was estimated to be 150-170 microM in the egg cytoplasm from the amount of microinjected EGTA and fura-2. It was concluded that although more than 150 microM of Ca2+ was released intracellularly upon fertilization, Cai increased to only a few microM because most of the released Ca2+ was sequestered by intracellular Ca2+ binding substances.     

The relationship between a novel NAD(P)H oxidase activity of ovoperoxidase and the CN- -resistant respiratory burst that follows fertilization of sea urchin eggs

The extracellular protein coat of the sea urchin egg is cross-linked after fertilization via dityrosyl linkages made by an exocytosed ovoperoxidase. The source of oxidant for this reaction is unknown, but eggs produce H2O2 in amounts equivalent to the cyanide-insensitive O2 uptake "respiratory burst" that follows fertilization. Several possible H2O2-forming oxidase activities, including glucose, xanthine, fatty acyl, and fatty-acyl CoA oxidases, were absent from the egg cortex. However, an NAD(P)H-O2 oxidoreductase activity was found in the egg cortex and was completely accounted for by ovoperoxidase. Homogeneous ovoperoxidase exhibits two types of NAD(P)H oxidase activity. One of these activities is similar to that of horseradish peroxidase and lactoperoxidase; it is dependent on Mn2+ ions and catalytic amounts of phenols, such as 2,4-dichlorophenol and N-acetyltyrosinamide, and is greater than 95% inhibited by 0.1 mM cyanide. A second, novel oxidase activity utilizes Ca2+ and an unidentified, heat-stable, Mr less than 1000 factor that can be extracted by ethanol from egg homogenates. This NADH oxidase activity is only 40% inhibited by 0.1 mM cyanide and is maximally stimulated by 10 mM Ca2+. It has an apparent Km for NADH of 50 microM. The stoichiometry of NADH:O2 consumption is 1.6:1, but approaches 2:1 in the presence of 20 micrograms/ml superoxide dismutase or 200 micrograms/ml catalase. This indicates that complete reduction of O2 to water occurs and that the reaction does not produce H2O2 stoichiometrically. However, nearly complete inhibition of the reaction by higher catalase concentrations suggests that H2O2 is an intermediate. The properties of this novel oxidase activity suggest that it may play such a role in vivo.     

Changes in intracellular content of glutathione and thiols associated with gamma-glutamyl cycle during sperm penetration and pronuclear formation in rat oocytes

The content of glutathione and other thiols in rat eggs was examined during sperm penetration and pronuclear formation by high-performance liquid chromatography with fluorescence detection. Reduced glutathione (GSH) content was higher in unfertilized oocytes (8.50 +/- 0.29 pmol/egg) and penetrated eggs with a decondensed sperm nucleus (DSH eggs; 7.72 +/- 0.56 pmol/egg) than eggs at the pronuclear stage (PN eggs; 5.93 +/- 0.10 pmol/egg). The content of oxidised glutathione (GSSG) was not different among experimental groups (152.6 +/- 74.1 nmol/egg in unfertilized eggs, 146.0 +/- 50.0 nmol/egg in DSH eggs and 39.7 +/- 17.3 nmol/egg in PN eggs). The GSSG/GSH ratio did not change during fertilization. Although the reduced cysteinylglycine content of eggs did not change among experimental groups, the oxidised form of cysteinylglycine increased (p < 0.025) between sperm decondensation (6.9 +/- 1.5 nmol/egg in unfertilized oocytes and 10.1 +/- 2.1 nmol/egg in DSH eggs) and pronuclear formation (40.5 +/- 11.5 nmol/egg in PN eggs). Low contents of cystine were detected during fertilization but cysteine and gamma-glutamylcysteine were not detected in any treatment groups. These results demonstrate that GSH content in rat eggs decreases between sperm decondensation and pronuclear formation, probably due to the increased activity of gamma-glutamyl transpeptidase.     

Polarity and reorganization of the endoplasmic reticulum during fertilization and ooplasmic segregation in the ascidian egg

During the first cell cycle of the ascidian egg, two phases of ooplasmic segregation create distinct cytoplasmic domains that are crucial for later development. We recently defined a domain enriched in ER in the vegetal region of Phallusia mammillata eggs. To explore the possible physiological and developmental function of this ER domain, we here investigate its organization and fate by labeling the ER network in vivo with DiIC16(3), and observing its distribution before and after fertilization in the living egg. In unfertilized eggs, the ER-rich vegetal cortex is overlaid by the ER-poor but mitochondria-rich subcortical myoplasm. Fertilization results in striking rearrangements of the ER network. First, ER accumulates at the vegetal-contraction pole as a thick layer between the plasma membrane and the myoplasm. This accompanies the relocation of the myoplasm toward that region during the first phase of ooplasmic segregation. In other parts of the cytoplasm, ER becomes progressively redistributed into ER-rich and ER- poor microdomains. As the sperm aster grows, ER accumulates in its centrosomal area and along its astral rays. During the second phase of ooplasmic segregation, which takes place once meiosis is completed, the concentrated ER domain at the vegetal-contraction pole moves with the sperm aster and the bulk of the myoplasm toward the future posterior side of the embryo. These results show that after fertilization, ER first accumulates in the vegetal area from which repetitive calcium waves are known to originate (Speksnijder, J. E. 1992. Dev. Biol. 153:259-271). This ER domain subsequently colocalizes with the myoplasm to the presumptive primary muscle cell region.     

Surface polarization in loach eggs and two-cell embryos: correlations between surface relief, endocytosis and cortex contractility

The aim of this study was to examine the reorganization of the microfilamentous cortical layer (MC) accompanying ooplasmic segregation in loach eggs. Using scanning (SEM) and transmission electron microscopy (TEM), we found that the MC is thicker in folded areas. Prior to fertilization, surface microvilli are distributed more or less uniformly throughout the egg. A similar, more or less uniform, distribution of endocytotic events was observed in the eggs 5-15 min after insemination using fluorescence microscopy of Lucifer yellow CH uptake. During ooplasmic segregation, the surface is progressively polarized so that before the first cleavage onset (50-60 min after insemination) only the blastodisc surface is folded and undergoes endocytosis, whereas the vegetal surface is smooth and does not show internalization. In two-cell embryos, the blastomeric surface is also regionalized according to its relief and endocytosis. When surface tension was lowered by sucking most yolk granules out of the egg, we observed contractile responses only in the animal folded surface. These data suggest that a polar distribution of contractile structures is established in the loach egg undergoing ooplasmic segregation.     

Field oviposition rates and egg load dynamics of pollen beetles (Meligethes aeneus Fab.) (Colepotera: Nitidulidae)

Abstract 1 Egg loads from field collected pollen beetles (Meligethes aeneus Fab., Coleoptera: Nitidulidae) were determined by dissecting beetles caught at the beginning and end of the putative daily oviposition period. Field collected beetles were offered Brassica napus (L.) plants in cages for 8 (morning and early afternoon), 16 (overnight), and 24 h to ascertain the number of eggs laid during these time periods. 2 Most eggs were laid in the morning and early afternoon. The proportion of gravid females was higher at the beginning of the oviposition period than at the end. Most females in the morning carried two eggs, whereas one egg was more common in the afternoon. 3 We hypothesized that the number of eggs laid during the oviposition period would be equivalent to the difference between egg loads at the beginning and end of oviposition. This was not the case; differences in egg loads were significantly lower than number of eggs laid. However, the number of eggs laid was equivalent to the egg load at the beginning of the oviposition period, suggesting that eggs available in the morning are laid during the following day. 4 Population estimates of daily oviposition rates, approximately 0.7 eggs per beetle and day, were close to estimates from laboratory studies when the proportion of gravid females was taken into consideration.     

Avoidance of egg parasitism through submerged oviposition by tandem pairs in the water strider, Aquarius paludum insularis (Heteroptera: Gerridae)

Abstract.  1. Females of the water strider Aquarius paludum insularis (Motschulsky) (Heteroptera: Gerridae) carry males on their backs and oviposit under water after copulation. This study focuses on the benefit  A. paludum insularis receives by ovipositing in tandem.
2. Males guarded females in tandem through to the end of oviposition in 85% of copulations. Females in tandem dived deeper than single females, and the density of A. paludum insularis eggs increased with water depth. The proportion of eggs parasitized by a scelionid wasp, Tiphodytes gerriphagus Marchal (Hymenoptera: Scelionidae) decreased with increasing water depth.
3. These results suggest that during oviposition guarding by males is beneficial for females, because it enables pairs to dive and lay eggs deeper and in oviposition sites where the risk of egg parasitism is lower.