Sturgeon hatching enzyme and the mechanism of egg envelope digestion: Insight into changes in the mechanism of egg envelope digestion during the evolution of ray-finned fish

Lineage-specific evolution of zona pellucida genes in fish

The zona pellucida (ZP) protein constitutes the egg envelope, which surrounds the vertebrate embryo. We performed a comprehensive study on the molecular evolution of ZP genes in Teleostei by cloning and analyzing the expression of ZP genes in fish of Anguilliformes in Elopomorpha, Osteoglossiformes in Osteoglossomorpha, and Clupeiformes in Otocephala to cover unsurveyed fish groups in Teleostei. The present results confirmed findings from our previous reports that the principal organ of ZP gene expression changed from ovary to liver in the common ancestors of Clupeocephala. Even fish species that synthesize egg envelopes in the liver carry the ovary-expressed ZP proteins as minor egg envelope components that were produced by gene duplication during the early stage of Teleostei evolution. The amino acid repeat sequences located at the N-terminal region of ZP proteins are known to be the substrates of transglutaminase responsible for egg envelope hardening and hatching. A repeat sequence was found in zona pellucida Cs of phylogenetically early diverged fish. After changing the synthesis organ, its role is inherited by the N-terminal Pro-Gln-Xaa repeat sequence in liver-expressed zona pellucida B genes of Clupeocephala. These results suggest that teleost ZP genes have independently evolved to maintain fish-specific functions, such as egg envelope hardening and egg envelope digestion, at hatching.

Molecular evolution of hatching enzymes and their paralogous genes in vertebrates

BACKGROUND

Hatching is identified as one of the most important events in the reproduction of oviparous vertebrates. The genes for hatching enzymes, which are vital in the hatching process, are conserved among vertebrates. However, especially in teleost, it is difficult to trace their molecular evolution in detail due to the presence of other C6astacins, which are the subfamily to which the genes for hatching enzymes belong and are highly diverged. In particular, the hatching enzyme genes are diversified with frequent genome translocations due to retrocopy.

RESULTS

In this study, we took advantage of the rapid expansion of whole-genome data in recent years to examine the molecular evolutionary process of these genes in vertebrates. The phylogenetic analysis and the genomic synteny analysis revealed C6astacin genes other than the hatching enzyme genes, which was previously considered to be retained only in teleosts, was also retained in the genomes of basal ray-finned fishes, coelacanths, and cartilaginous fishes. These results suggest that the common ancestor of these genes can be traced back to at least the common ancestor of the Gnathostomata. Moreover, we also found that many of the C6astacin genes underwent multiple gene duplications during vertebrate evolution, and the results of gene expression analysis in frogs implied that genes derived from hatching enzyme genes underwent neo-functionalization.

CONCLUSIONS

In this study, we describe in detail the molecular evolution of the C6astacin gene in vertebrates, which has not been summarized previously. The results revealed the presence of the previously unknown C6astacin gene in the basal-lineage of jawed vertebrates and large-scale gene duplication of hatching enzyme genes in amphibians. The comprehensive investigation reported in this study will be an important basis for studying the molecular evolution of the vertebrate C6astacin genes, hatching enzyme, and its paralogous genes and for identifying these genes without the need for gene expression and functional analysis.

Toxic effects of ZnSe/ZnS quantum dots on the reproduction and genotoxiticy of rare minnow (Gobiocypris rarus)

ZnSe/ZnS quantum dots (QDs) have excellent optical properties, but researchers have not clearly determined whether they cause harm to organisms. In the present study, the effect of ZnSe/ZnS QDs on the parents and offspring of rare minnow were evaluated for the first time. Exposure to ZnSe/ZnS QDs altered the testicular structure, caused sperm DNA damage and decreased sperm motility in males. They also suppressed the expression of reproduction-related genes, such as androgen receptor (Ar), DM-related transcription factor 1 (Dmrt1), estrogen receptor (Er), and X-ray repair cross complementing gene 1 (Xrcc1). Continued monitoring of the F1 generation revealed that the embryonic development of the F1 generation was abnormal and the growth index of the F1 generation of adult fish showed hormesis. A comet assay showed that the F1 generation still had DNA damage in the 400 and 800 nmol/L groups at 96 h post-fertilization (hpf). Thus, ZnSe/ZnS QDs damaged the reproductive system of the rare minnow, and this effect continued to the F1 generation.

Genomic and phylogenetic analysis of choriolysins, and biological activity of hatching liquid in the flatfish Senegalese sole

The hatching enzymes or choriolysins are key proteases in fish life cycle controlling the release of larvae to surrounding environment that have been suggested as target for novel biotechnological uses. Due to the large amounts of eggs released by the flatfish Solea senegalensis, during the spawning season, the hatching liquid properties and choriolysin-encoding genes were investigated in this species. A genomic analysis identified four putative genes referred to as SseHCEa, SseHCEb, SseLCE and SseHE. The phylogenetic analysis classified these paralogs into two clades, the clade I containing SseHCE paralogs and the clade II containing two well-supported subclades named as HE and LCE. The two SseHCE paralogs were intron-less and both genes were tandemly arrayed very close in the genome. The synteny and gene rearrangement identified in the flatfish lineage indicated that the duplication of these two paralogs occurred recently and they are under divergent evolution. The genes SseHE and SseLCE were structured in 8 exons and 7 introns and the synteny was conserved in teleosts. Expression studies confirmed that the four genes were expressed in the hatching gland cells and they migrate co-ordinately from the head to around the yolk sac close to the hatch with specific temporal and intensity expression profiles. Although the mRNA levels of the four genes peaked in the hours previous to larval hatching, the SseHCE and SseLCE paralogs kept a longer expression than SseHE after hatching. These expression patterns were consistent even when larvae were incubated at different temperatures that modified hatching times. The analysis of hatching-liquid using SDS-PAGE and zymography analyses of hatching liquid identified a major band of expected choriolysin size. The optimal pH for protease activity was 8.5 and inhibition assays using EDTA demonstrated that most of the activity in the hatching liquid was due to metalloproteases with Ca²⁺ ions acting as the most effective metal to restore the activity. All these data provide new clues about the choriolysin evolution and function in flatfish with impact in the aquaculture and the blue cosmetic industry.

Translocation of promoter-conserved hatching enzyme genes with intron-loss provides a new insight in the role of retrocopy during teleostean evolution

The hatcing enzyme gene (HE) encodes a protease that is indispensable for the hatching process and is conserved during vertebrate evolution. During teleostean evolution, it is known that HE experienced a drastic transfiguration of gene structure, namely, losing all of its introns. However, these facts are contradiction with each other, since intron-less genes typically lose their original promoter because of duplication via mature mRNA, called retrocopy. Here, using a comparative genomic assay, we showed that HEs have changed their genomic location several times, with the evolutionary timings of these translocations being identical to those of intron-loss. We further showed that HEs maintain the promoter sequence upstream of them after translocation. Therefore, teleostean HEs are unique genes which have changed intra- (exon-intron) and extra-genomic structure (genomic loci) several times, although their indispensability for the reproductive process of hatching implies that HE genes are translocated by retrocopy with their promoter sequence.

Evolutionary Changes in the Developmental Origin of Hatching Gland Cells in Basal Ray-Finned Fishes

Hatching gland cells (HGCs) originate from different germ layers between frogs and teleosts, although the hatching enzyme genes are orthologous. Teleostei HGCs differentiate in the mesoendodermal cells at the anterior end of the involved hypoblast layer (known as the polster) in late gastrula embryos. Conversely, frog HGCs differentiate in the epidermal cells at the neural plate border in early neurula embryos. To infer the transition in the developmental origin of HGCs, we studied two basal ray-finned fishes, bichir (Polypterus) and sturgeon. We observed expression patterns of their hatching enzyme (HE) and that of three transcription factors that are critical for HGC differentiation: KLF17 is common to both teleosts and frogs; whereas FoxA3 and Pax3 are specific to teleosts and frogs, respectively. We then inferred the transition in the developmental origin of HGCs. In sturgeon, the KLF17, FoxA3, and HE genes were expressed during the tailbud stage in the cell mass at the anterior region of the body axis, a region corresponding to the polster in teleost embryos. In contrast, the bichir was suggested to possess both teleost- and amphibian-type HGCs, i.e. the KLF17 and FoxA3 genes were expressed in the anterior cell mass corresponding to the polster, and the KLF17, Pax3 and HE genes were expressed in dorsal epidermal layer of the head. The change in developmental origin is thought to have occurred during the evolution of basal ray-finned fish, because bichir has two HGCs, while sturgeon only has the teleost-type.