A comparative analysis of the evolution of the egg envelopes and the origin of the yolk

Fine structure of micropylar regions of Cobitis hankugensis, Iksookimia longicorpa and their unisexual natural hybrids

Iksookimia longicorpa and Cobitis hankugensis are two species of fish distributed only on the Korean Peninsula. They have a unique reproductive ecology that naturally hybridizes into three widely known unisexual types, maintaining populations of almost all females. In this study, the fine structure of the micropyles of I. longicorpa, C. hankugensis and their hybrids was analyzed to find out how egg–sperm interaction, a common interspecies isolation mechanism, is possible between heterogeneous species. Analysis of 30 eggs from five females of each species revealed that all had one funnel-shaped micropylar region and a manhole-shaped micropyle canal. With the exception of C. hankugensis, which had no spiral grooves or ridges, the rest had counterclockwise spiral grooves and ridges on the micropylar region. All five species, however, showed identical groove patterns for the micropyle canal. The egg size was the largest in HL (one from the C. hankugensis locus with one from the I. longicorpa locus) and the smallest in C. hankugensis. In the hybrids, the HL type had the largest egg and HHL (two from the C. hankugensis locus with one from the I. longicorpa locus) type the smallest. For the diameter of the micropylar region and micropyle canal, the diploid I. longicorpa, C. hankugensis and HL were smaller than those of the triploid. In addition, as the ratio of the canal diameter to the eggs was lower in I. longicorpa than in C. hankugensis, it was confirmed that I. longicorpa has a relatively small micropyle canal compared with C. hankugensis.

HREM, RNAseq and Cell Cycle Analyses Reveal the Role of the G2/M-Regulatory Protein, WEE1, on the Survivability of Chicken Embryos during Diapause

Avian blastoderm can enter into diapause when kept at low temperatures and successfully resume development (SRD) when re-incubated in body temperature. These abilities, which are largely affected by the temperature and duration of the diapause, are poorly understood at the cellular and molecular level. To determine how temperature affects embryonic morphology during diapause, high-resolution episcopic microscopy (HREM) analysis was utilized. While blastoderms diapausing at 12 °C for 28 days presented typical cytoarchitecture, similar to non-diapaused embryos, at 18 °C, much thicker blastoderms with higher cell number were observed. RNAseq was conducted to discover the genes underlying these phenotypes, revealing differentially expressed cell cycle regulatory genes. Among them, WEE1, a negative regulator of G2/M transition, was highly expressed at 12 °C compared to 18 °C. This finding suggested that cells at 12 °C are arrested at the G2/M phase, as supported by bromodeoxyuridine incorporation (BrdU) assay and phospho-histone H3 (pH 3) immunostaining. Inhibition of WEE1 during diapause at 12 °C resulted in cell cycle progression beyond the G2/M and augmented tissue volume, resembling the morphology of 18 °C-diapaused embryos. These findings suggest that diapause at low temperatures leads to WEE1 upregulation, which arrests the cell cycle at the G2/M phase, promoting the perseverance of embryonic cytoarchitecture and future SRD. In contrast, WEE1 is not upregulated during diapause at higher temperature, leading to continuous proliferation and maladaptive morphology associated with poor survivability. Combining HREM-based analysis with RNAseq and molecular manipulations, we present a novel mechanism that regulates the ability of diapaused avian embryos to maintain their cytoarchitecture via cell cycle arrest, which enables their SRD.

Toxicity of nanoplastics during the embryogenesis of the ascidian Ciona robusta (Phylum Chordata)

Nanoplastics are considered contaminants of emerging concern at the global scale. The recent evidence of their occurrence in seawater from the Mediterranean Sea calls for a thorough evaluation of their impact on marine life and in particular on vulnerable life stages such as planktonic embryos. Here, we investigated the impact of increasing nominal concentrations of 50 nm amino-modified (PS-NH₂) and 60 nm carboxy-modified (PS-COOH) polystyrene nanoparticles (PS NPs) on the embryonic development of the ascidian Ciona robusta (phylum Chordata), a common benthic invertebrate living in Mediterranean coastal areas with the peculiarity of being an early chordate developmental model. A strong agglomeration of PS-COOH (approx. 1 µm) was observed in natural sea water (NSW) already at time 0, while PS-NH₂ resulted still monodispersed (approx. 130 nm) but largely aggregated after 22 h with a microscale dimension similar to those negatively charged. However, their effect on C. robusta embryos development largely differed at 22 h: PS-COOH did not affect larvae phenotypes nor their development, while PS-NH₂ caused a dose-dependent effect (EC₅₀ (22 h) of 7.52 μg mL^-1) with various degrees of phenotype malformations (from mild to severe) and impairment of larval swimming. Embryos (up to 30%) exposed to 15 µg mL^-1PS-NH₂ resulted not developed and the majority was unable to hatch. Calculated PS-NH₂ EC₅₀ resulted higher than those available for other marine invertebrate species, suggesting a protective role of the egg envelopes surrounding C. robusta embryos toward nanoplastics exposure.

The female reproductive system and oogenesis in Thulinius ruffoi (Tardigrada, Eutardigrada, Isohypsibiidae)

In this study, we describe the female reproductive system organization and oogenesis in the eutardigrade Thulinius ruffoi. Light, confocal and electron microscopy was used in this study. During oogenesis, three phases can be distinguished: previtellogenesis, vitellogenesis, and choriogenesis. Germ-line cells form cell clusters in which the cells are connected by intercellular (cytoplasmic) bridges. These structures are crucial for delivering the yolk materials, macromolecules, ribosomes, and organelles to the developing oocyte. Vitellogenesis is of a mixed type. Autosynthesis and heterosynthesis of the yolk material occur. Yolk precursors that have been synthesized outside the ovary are delivered to the oocyte via endocytosis. We also present data on cortical granules, and moreover, we describe the cortical reaction in tardigrades, possibly for the first time.

A monoclonal antibody against chorion proteins of the sea bass Dicentrarchus labrax (Linnaeus, 1758): studies of chorion precursors and applicability in immunoassays

The monoclonal antibody DLE7 was obtained against 44- to 50-kDa polypeptides solubilized from the vitelline envelope of the Mediterranean sea bass Dicentrarchus labrax. In Western blot analysis of chorion lysates it recognized cross-reactive bands at 44 kDa, 48 kDa, and 110 kDa. Previous affinity blotting with concanavalin-A showed that most of solubilized bands were glycosylated. Enzymatic deglycosylation of chorion proteins followed by Western blot analysis with DLE7 showed that the 48-kDa and 110-kDa antigens were differentially affected by endoglycosidase-F treatment. When DLE7 was employed in immunofluorescence analysis, isolated chorions and ovarian cryosections stained intensely. Positivity was also observed in liver cryosections of spawning females but not in liver of males and nonspawning females. To study the origin and delivery of chorion proteins, DLE7 was used in Western blot analysis of liver homogenates and blood serum of spawning females. Cross-reacting bands were detected in liver (90 kDa) and serum (180 kDa, 50 kDa). DLE7 was also used for the first time to set up an indirect ELISA assay to detect egg antigens in the blood of egg-producing females, raising the possibility of using DLE7 as a female-specific marker of spawning for sea bass.

Characterization of the main egg envelope proteins of the sea bass Dicentrarchus labrax L. (Teleostea, Serranidae)

Fish eggs are surrounded by a resistant acellular coat commonly called the chorion or zona radiata. This study characterizes the eggshell proteinaceous content of unfertilized eggs of the sea bass Dicentrarchus labrax, with a view to the preparation of immunogens. Solubilization of the purified eggshells was achieved in 8 M urea followed by one- and two-dimensional gel electrophoresis. Glycoproteins were detected using concanavalin-A in one and two-dimensional gels, and the principal glycoproteins had a molecular weight of 47 kDa and 170 kDa. Partial purification of a few polypeptides in the 45 kDa to 55 kDa range was achieved by gel filtration chromatography. Although whole eggshells were relatively insoluble even in 8 M urea, partial purification of these polypeptides enable them to dissolve completely in solutions at low ionic strength.