Meeting Abstract
Two evolutionary events in the egg envelope system affected the egg envelope digestion system of the hatching enzyme(s). The first event was the acquisition of the egg envelope hardening mechanism mediated by transglutaminase that occurred before the divergence of Teleostei fish, resulting in the acquisition of a toughened hard egg envelope. At this time, a switch in the substrate of the hatching enzyme occurred; the amphibian and sturgeon hatching enzymes selectively cleaved one of five egg envelope proteins, ZPAX, while Teleostei hatching enzymes cleaved the repeated sequences at the N-terminal regions of ZPB and ZPC, where most of the cross-links formed by transglutaminase were located. The second event was the acquisition of liver-synthesized ZP proteins, which occurred in the common ancestor of the Otocephala and Euteleostei. The ZP proteins in fish were originally synthesized in the ovary. The ZP-synthesizing proteins in the liver conferred the ability to develop a thick and protective egg envelope. Around that time, duplication of the gene encoding hatching enzyme occurred. Consequently, the fishes in the Otocephala and Euteleostei possess two types of hatching enzymes. In the Euteleostei, the two enzymes have developed into an efficient egg envelope digestion system that completely solubilizes the egg envelope; one duplicate has maintained the ancestral activity to swell and soften the egg envelope, and the other has acquired a new function to solubilize the swollen egg envelope. However, this type of efficient egg envelope digestion system did not develop in the Otocephala. One duplicate was lost in the Otophysi, and the liver-expressing ZP genes were also lost. Therefore, the evolutionary process of the egg envelope digestion system corresponds well with that of the egg envelope, suggesting co-evolution between these two systems.
