Timing of embryonic quiescence determines viability of embryos from the calanoid copepod, Acartia tonsa (Dana)

Expression and functional analysis of Fushi Tarazu transcription factor 1 (FTZ-F1) in the regulation of steroid hormones during the gonad development of Fujian Oyster, Crassostrea angulata

Crassostrea angulata, a major shellfish cultivated in Southern China, has experienced a notable surge in commercial value in recent years. Understanding the molecular mechanisms governing their reproductive processes holds significant implications for advancing aquaculture practices. In this study, we cloned the orphan nuclear receptor gene, Fushi Tarazu transcription factor 1 (FTZ-F1), of C. angulata and investigated its functional role in the gonadal development. The full-length cDNA of FTZ-F1 spans 2357 bp and encodes a protein sequence of 530 amino acids. Notably, the amino acid sequence of FTZ-F1 in C. angulata shares remarkable similarity with its homologues in other species, particularly in the DNA-binding region (>90%) and ligand-binding region (>44%). In C. angulata, the highest expression level of FTZ-F1 was observed in the ovary, exhibiting more than a 200-fold increase during the maturation stage compared to the initiation stage (P < 0.001). Specifically, FTZ-F1 was mainly expressed in the follicular cells surrounding the oocytes of C. angulata. Upon inhibiting FTZ-F1 gene expression in C. angulata through RNA interference (RNAi), a substantial reduction in the expression of genes involved in the synthesis of sex steroids in the gonads, including 3β-HSD, Cyp17, and follistatin, was observed. In addition, estradiol (E2) and testosterone (T) levels also showed a decrease upon FTZ-F1 silencing, resulting in a delayed gonadal development. These results indicate that FTZ-F1 acts as a steroidogenic factor, participating in the synthesis and regulation of steroid hormones and thus playing an important role in the reproductive and endocrine systems within oysters.

Analysis of the transcriptional pathways associated with the induction of quiescent embryonic arrest in the calanoid copepod Acartia tonsa

The copepod species Acartia tonsa (Dana)(Crustacea) have the unique ability to induce quiescent embryonic dormancy if adverse environmental conditions occur; a characteristic shared by 41 other species belonging to the superfamily Centropagoida in the Calanoida order. However, the transcriptional changes characterizing this process are not known. Here, we compare the transcriptome of embryos in arrested quiescence with the normal development to identify pathways and differentially regulated transcripts involved in quiescent embryogenesis. Quiescence was induced by incubating eggs at 4 °C with anoxia for 26 h(hr), while eggs undergoing normal immediate development were incubated at 16.9 °C in normoxia for 7 h (where gastrulation occurs) or 14 h (where organogenesis occurs) before collecting for RNA extraction and analysis by RNA-sequencing. Results indicate that the expression profile of the quiescent embryo is not as different from the normal embryonic gastrulation as initially expected: None of the mapped transcripts is uniquely expressed in quiescence. Moreover, in quiescence a large proportion of the annotated transcripts display expression values halfway in-between the normal, immediate developmental stages of gastrulation and organogenesis. In depth comparison between the organogenesis stage and quiescent samples, reveal a high degree of divergence, confirming that a developmental arrest has been induced through quiescence. Specifically: Stress response transcripts are prominent in the quiescent phase with a transcript like the mammalian autophagy gene Sequestosome-1/p62 (SQSTM) being upregulated. The present analysis provides a better understanding of the molecular mechanisms characterizing the quiescent embryonic state of A. tonsa.

Embryogenesis of a calanoid copepod analyzed by transcriptomics

The calanoid copepod Acartia tonsa (Dana) has attracted interest because of its use as a copepod model organism as well as its potential economic role as live fish larval feed. While the adult genome and transcriptome of A. tonsa has been investigated, no studies have been performed investigating the genome-wide transcriptional changes during the normal subitaneous embryogenesis. Thus, the aim of the current study was to investigate said transcriptional changes throughout A. tonsa embryonic development. RNA extraction and de novo transcriptome assembly for the subitaneous embryogenesis of the copepod was conducted. The assembly includes for the first-time samples describing quiescent development and overall helps establishing a framework for future studies on the molecular biology of our species of interest. Among the findings reported, sequences annotated to well-known developmental genes, were identified. At the same time are described the molecular changes and gene expression levels throughout the entire 42 h the embryonic development lasts. In conclusion, here we present the most complete genome-wide transcriptional map of early copepod embryonic development to date, enabling further use of A. tonsa as a model organism for crustacean development. Keywords: enrichment of pathways; subitaneous embryogenesis, comparative genomics; transcriptome assembly; invertebrate genomics.

Copepod Embryonic Dormancy: "An Egg Is Not Just an Egg"

Long-lasting embryonic dormancy in invertebrates defies our understanding of what constitutes life because, for example, eggs of some copepods can delay hatching for decades or even centuries. Copepods, often millimeter-sized crustaceans, are some of the most numerous multicellular organisms on earth and are key organisms in most aquatic food webs. Some important free-living marine and estuarine species overwinter or oversummer by arrested embryogenesis in dormancy. The present contribution discusses the complex mechanisms behind embryonic dormancy by compiling knowledge from the 42 calanoid copepods from the superfamily Centropagoidea with well-described embryonic dormancy, which has been of scientific interest for decades. However, the determination of categories of copepod resting eggs-that is, diapause and quiescence, transitions between categories, the mechanisms controlling arrested development by the embryos, and how they interact with their surroundings-is not fully understood. Moreover, a clear link between the presence of the free-swimming population and their resting eggs in sediments is still not convincingly demonstrated. Here I evaluate the relative significance of potential cues driving the production of and the phase shift between egg categories. Understanding the initiation and termination of embryonic dormancy is of great importance for fundamental science-that is, population and food web ecology as well as climate science, aquaculture live feed, and ballast water research. Molecular techniques are developing rapidly, especially within health sciences, thus providing relevant tools applicable for plankton research. Here I suggest that applying molecular methods in addition to traditional physiological approaches in future research will lead to greater understanding of copepod embryonic dormancy, one of nature's wonders.

The Genome and mRNA Transcriptome of the Cosmopolitan Calanoid Copepod Acartia tonsa Dana Improve the Understanding of Copepod Genome Size Evolution

Members of the crustacean subclass Copepoda are likely the most abundant metazoans worldwide. Pelagic marine species are critical in converting planktonic microalgae to animal biomass, supporting oceanic food webs. Despite their abundance and ecological importance, only six copepod genomes are publicly available, owing to a number of factors including large genome size, repetitiveness, GC-content, and small animal size. Here, we report the seventh representative copepod genome and the first genome and the first transcriptome from the calanoid copepod species Acartia tonsa Dana, which is among the most numerous mesozooplankton in boreal coastal and estuarine waters. The ecology, physiology, and behavior of A. tonsa have been studied extensively. The genetic resources contributed in this work will allow researchers to link experimental results to molecular mechanisms. From PCR-free whole genome sequence and mRNA Illumina data, we assemble the largest copepod genome to date. We estimate that A. tonsa has a total genome size of 2.5 Gb including repetitive elements we could not resolve. The nonrepetitive fraction of the genome assembly is estimated to be 566 Mb. Our DNA sequencing-based analyses suggest there is a 14-fold difference in genome size between the six members of Copepoda with available genomic information. This finding complements nucleus staining genome size estimations, where 100-fold difference has been reported within 70 species. We briefly analyze the repeat structure in the existing copepod whole genome sequence data sets. The information presented here confirms the evolution of genome size in Copepoda and expands the scope for evolutionary inferences in Copepoda by providing several levels of genetic information from a key planktonic crustacean species.