Molecular mechanism of nutrient uptake in developing embryos of oviparous cloudy catshark (Scyliorhinus torazame)

Two's company: Monozygotic twinning in the small-spotted catshark (Scyliorhinus canicula)

Developmental abnormalities in elasmobranchs (sharks and rays) are rarely documented, with reported cases primarily involving fin or cranial deformities. Monozygotic twinning, the formation of identical twins from a single zygote, is particularly rare in these species and has been observed overwhelmingly in viviparous elasmobranchs. Here, we document a rare case of monozygotic twinning in an oviparous shark, the Small-spotted Catshark Scyliorhinus canicula (Linneaus, 1758). This case was characterized by two embryos connected to a single yolk sac via separate yolk stalks within the egg capsule. The embryos exhibited normal development until mortality at developmental Stage 31 (Ballard et al., 1993) or stage 4 (Musa et al., 2018), likely due to stressors such as oxidative stress and allostatic overload, resulting from shared resources within the capsule. This is the first confirmed instance of mortality in monozygotic elasmobranch twins, highlighting the plausible challenges of polyembryony in oviparous elasmobranchs. These findings underscore the importance of understanding reproductive abnormalities and their implications for fecundity, particularly in light of ongoing anthropogenic pressures that threaten elasmobranch populations globally.

Phylogenetic analysis of viviparity, matrotrophy, and other reproductive patterns in chondrichthyan fishes

The reproductive diversity of extant cartilaginous fishes (class Chondrichthyes) is extraordinarily broad, reflecting more than 400 million years of evolutionary history. Among their many notable reproductive specialisations are viviparity (live-bearing reproduction) and matrotrophy (maternal provision of nutrients during gestation). However, attempts to understand the evolution of these traits have yielded highly discrepant conclusions. Here, we compile and analyse the current knowledge on the evolution of reproductive diversity in Chondrichthyes with particular foci on the frequency, phylogenetic distribution, and directionality of evolutionary changes in their modes of reproduction. To characterise the evolutionary transformations, we amassed the largest empirical data set of reproductive parameters to date covering nearly 800 extant species and analysed it via a comprehensive molecular-based phylogeny. Our phylogenetic reconstructions indicated that the ancestral pattern for Chondrichthyes is 'short single oviparity' (as found in extant holocephalans) in which females lay successive clutches (broods) of one or two eggs. Viviparity has originated at least 12 times, with 10 origins among sharks, one in batoids, and (based on published evidence) another potential origin in a fossil holocephalan. Substantial matrotrophy has evolved at least six times, including one origin of placentotrophy, three separate origins of oophagy (egg ingestion), and two origins of histotrophy (uptake of uterine secretions). In two clades, placentation was replaced by histotrophy. Unlike past reconstructions, our analysis reveals no evidence that viviparity has ever reverted to oviparity in this group. Both viviparity and matrotrophy have arisen by a variety of evolutionary sequences. In addition, the ancestral pattern of oviparity has given rise to three distinct egg-laying patterns that increased clutch (brood) size and/or involved deposition of eggs at advanced stages of development. Geologically, the ancestral oviparous pattern arose in the Paleozoic. Most origins of viviparity and matrotrophy date to the Mesozoic, while a few that are represented at low taxonomic levels are of Cenozoic origin. Coupled with other recent work, this review points the way towards an emerging consensus on reproductive evolution in chondrichthyans while offering a basis for future functional and evolutionary analyses. This review also contributes to conservation efforts by highlighting taxa whose reproductive specialisations reflect distinctive evolutionary trajectories and that deserve special protection and further investigation.

Functional characterization of follicle-stimulating hormone and luteinizing hormone receptors in cloudy catshark, Scyliorhinus torazame

The follicle-stimulating hormone receptor (FSHR) and luteinizing hormone receptor (LHR) in cloudy catshark were cloned, and recombinant FSHR and LHR were expressed for characterization. Ventral lobe extract (VLE) from the pituitary contains homologous FSH and LH, and it stimulated the cAMP signaling of FSHR and LHR dose-dependently. Two transcript variants of LHR (LHR-L with exon 10 and LHR-S without) were identified, and LHR-S was the dominant form with higher basal cAMP activity without VLE stimulation. Among various developmental stages of follicles, FSHR expression was mainly associated with the pre-vitellogenic and early white follicles. When follicles were recruited into vitellogenesis, the expression of FSHR decreased while of LHR was upregulated reciprocally, suggesting that LHR may also be responsible for the control of vitellogenesis in chondrichthyans. The expression of LHR-L was upregulated among maturing follicles before ovulation, indicating LHR-L could have a specific role in receiving the LH surge signal for final maturation. Plasma LH-like activity was transiently increased prior to the progesterone (P4)-surge and testosterone-drop at the beginning of P4-phase, supporting a pituitary control of follicle-maturation via LH signaling in chondrichthyans. The expression of follicular LHR was downregulated during the P4-phase when LH-like activity was high, indicating that the LH-dependent downregulation of LHR is conserved in chondrichthyans as it is in other vertebrate lineages. (213 words).

Gluconeogenesis in the yolk syncytial layer-like tissue of cloudy catshark (Scyliorhinus torazame)

Glucose has important roles in the development of zebrafish, the vertebrate animal model; however, in most oviparous animals, the amount of maternally provided glucose in the yolk is scarce. For these reasons, developing animals need some ways to supplement glucose. Recently, it was found that developing zebrafish, a teleost fish, undergo gluconeogenesis in the yolk syncytial layer (YSL), an extraembryonic tissue that surrounds the yolk. However, teleost YSL is evolutionarily unique, and it is not clear how other vertebrates supplement glucose. In this study, we used cloudy catshark (or Torazame catshark), an elasmobranch species which possesses a YSL-like tissue during development, and sought for possible gluconeogenic activities in this tissue. In their yolk sac, glucose increased, and our isotope tracking analysis detected gluconeogenic activities with glycerol most preferred substrate. In addition, many of gluconeogenic genes were expressed at the YSL-like tissue, suggesting that cloudy catshark engages in gluconeogenesis in this tissue. The gluconeogenesis in teleost YSL and a similar tissue in elasmobranch species implies conserved mechanisms of yolk metabolism between these two lineages. Future studies on other vertebrate taxa will be helpful to understand the evolutionary changes in the modes of yolk metabolism that vertebrates have experienced.

Gluconeogenesis in the extraembryonic yolk syncytial layer of the zebrafish embryo

Yolk-consuming (lecithotrophic) embryos of oviparous animals, such as those of fish, need to make do with the maternally derived yolk. However, in many cases, yolk possesses little carbohydrates and sugars, including glucose, the essential monosaccharide. Interestingly, increases in the glucose content were found in embryos of some teleost fishes; however, the origin of this glucose has been unknown. Unveiling new metabolic strategies in fish embryos has a potential for better aquaculture technologies. In the present study, using zebrafish, we assessed how these embryos obtain the glucose. We employed stable isotope (13C)-labeled substrates and injected them to the zebrafish embryos. Our liquid chromatography-mass spectrometry-based isotope tracking revealed that among all tested substrate, glutamate was most actively metabolized to produce glucose in the zebrafish embryos. Expression analysis for gluconeogenic genes found that many of these were expressed in the yolk syncytial layer (YSL), an extraembryonic tissue found in teleost fishes. Generation 0 (G0) knockout of pck2, a gene encoding the key enzyme for gluconeogenesis from Krebs cycle intermediates, reduced gluconeogenesis from glutamate, suggesting that this gene is responsible for gluconeogenesis from glutamate in the zebrafish embryos. These results showed that teleost YSL undergoes gluconeogenesis, likely contributing to the glucose supplementation to the embryos with limited glucose source. Since many other animal lineages lack YSL, further comparative analysis will be interesting.

Effects of fecal microbiota transplantation on metabolic health of DBA mice

Introduction

Numerous studies have demonstrated that C57BL/6 mice exhibit superior growth rates and overall growth performance compared to DBA mice. To investigate whether this discrepancy in growth performance is linked to the composition of gut microorganisms, we conducted fecal microbiome transplantation (FMT) experiments.

Methods

Specifically, we transplanted fecal fluids from adult C57BL/6 mice, high-fat C57BL/6 mice, and Wistar rats into weaned DBA mice (0.2mL/d), and subsequently analyzed their gut contents and gene expression through 16S rRNA sequencing and transcriptome sequencing. During the test period, C57BL/6 mice and Wistar rats were provided with a normal diet, and high-fat C57BL/6 mice were provided with a high-fat diet.

Results

The results of our study revealed that mice receiving FMT from all three donor groups exhibited significantly higher daily weight gain and serum triglyceride (TG) levels compared to mice of CK group. 16S rRNA sequensing unveiled substantial differences in the abundance and function of the gut microbiota between the FMT groups and the CK group. Transcriptome analysis revealed a total of 988 differential genes, consisting of 759 up-regulated genes and 187 down-regulated genes, between the three experimental groups and the CK group. Functional Gene Ontology (GO) annotation suggested that these genes were primarily linked to lipid metabolism, coagulation, and immunity. Pearson correlation analysis was performed on the differential genes and clusters, and it revealed significant correlations, mainly related to processes such as fatty acid metabolism, fat digestion and absorption, and cholesterol metabolism.

Discussion

In summary, FMT from dominant strains improved the growth performance of DBA mice, including body weight gain, institutional growth, and immune performance. This change may be due to the increase of probiotic content in the intestinal tract by FMT and subsequent alteration of intestinal gene expression. However, the effects of cross-species fecal transplantation on the intestinal flora and gene expression of recipient mice were not significant.

Effects of high incubation temperature on tight junction proteins in the yolk sac and small intestine of embryonic broilers

During the transition from incubation to hatch, the chicks shift from obtaining nutrients from the yolk sac to the intestine. The yolk sac tissue (YST) and small intestine serve as biological barriers between the yolk or gut contents and the blood circulation. These barriers must maintain structural integrity for optimal nutrient uptake as well as protection from pathogens. The objective of this study was to investigate the effect of high incubation temperature on mRNA abundance of the tight junction (TJ) proteins zona occludens 1 (ZO1), occludin (OCLN), claudin 1 (CLDN1), and junctional adhesion molecules A and 2 (JAMA, JAM2) and the heat shock proteins (HSP70 and HSP90) in the YST and small intestine of embryonic broilers. Broiler eggs were incubated at 37.5°C. On embryonic day 12 (E12), half of the eggs were switched to 39.5°C. YST samples were collected from E7 to day of hatch (DOH), while small intestinal samples were collected from E17 to DOH. The temporal expression of TJ protein mRNA from E7 to DOH at 37.5°C and the effect of incubation temperature from E13 to DOH were analyzed by one-way and two-way ANOVA, respectively and Tukey's test. Significance was set at P < 0.05. The temporal expression pattern of ZO1, OCLN, and CLDN1 mRNA showed a pattern of decreased expression from E7 to E13 followed by an increase to DOH. High incubation temperature caused an upregulation of ZO1 and JAM2 mRNA in the YST and small intestine. Using in situ hybridization, OCLN and JAMA mRNA were detected in the epithelial cells of the YST. In addition, JAMA mRNA was detected in epithelial cells of the small intestine, whereas JAM2 mRNA was detected in the vascular system of the villi and lamina propria. In conclusion, the YST expressed mRNA for TJ proteins and high incubation temperature increased ZO1 and JAM2 mRNA. This suggests that the TJ in the vasculature of the YST and intestine is affected by high incubation temperature.