Yasumasu Shigeki

Abstract During the fertilization of fish eggs, the hardening of the egg envelope is mediated by transglutaminase (hTGase). After fertilization, TGase undergoes processing. We isolated hTGase from extracts of unfertilized and water-activated rainbow trout eggs. Rainbow trout hTGase (Rt-hTGase) appeared as an 80 kDa protein, and its processed form was 55 kDa. Their N-terminal amino acid sequences were nearly identical, suggesting processing in the C-terminal region. The specific activities were not significantly different, indicating that C-terminal processing does not activate the enzyme itself. We cloned the cDNA by reverse transcription polymerase chain reaction (RT-PCR) using degenerate primers followed by RACE-PCR. The deduced amino acid sequence of the cDNA was similar to that of factor XIII subunit A (FXIIIA). Molecular phylogenetic and gene syntenic analyses clearly showed that hTGase was produced by duplication of FXIIIA during the evolution to Teleostei. The 55 kDa processed form of Rt-hTGase is predominantly composed of an enzyme domain predicted from the amino acid sequence of the cDNA. It is hypothesized that the C-terminal domain of Rt-hTGase binds to egg envelope proteins, and that processing allows the enzyme to move freely within the egg envelope, increasing substrate–enzyme interaction and thereby accelerating hardening.

ABSTRACT False clownfish (Amphiprion ocellaris) employ a hatching strategy regulated by environmental cues, wherein parents provide water flow to encourage embryos to hatch after sunset on the hatching day. Despite previous studies demonstrating the necessity of complete darkness and water agitation for hatching, the regulatory mechanisms underlying these environmental cues remain elusive. This study aimed to investigate how darkness and water agitation affect the secretion of hatching enzymes and the hatching movements of embryos in false clownfish. Assessment of chorion digestion and live imaging of Ca²⁺ in hatching glands using GCaMP6s, a Ca²⁺ indicator, revealed that darkness stimulation triggers the secretion of hatching enzymes by increasing Ca²⁺ levels in hatching gland cells. On the other hand, water agitation primarily stimulated hatching movements in embryos, which led to the rupture of their egg envelopes. These results suggest that changes in light environments following sunset induce embryos to secrete hatching enzymes and that water agitation provided by parents stimulates hatching movements. These responses to environmental cues, light and water agitation, contribute to the rapid and synchronous hatching in false clownfish.