Different hatching strategies in embryos of two species, pacific herring Clupea pallasii and Japanese anchovy Engraulis japonicus, that belong to the same order Clupeiformes, and their environmental adaptation.

Pacific herring Clupea pallasii and Japanese anchovy Engraulis japonicus, which belong to the same order Clupeiformes, spawn different types of eggs: demersal adherent eggs and pelagic eggs, respectively. We cloned three cDNAs for Pacific herring hatching enzyme and five for Japanese anchovy. Each of them was divided into two groups (group A and B) by phylogenetic analysis. They were expressed specifically in hatching gland cells (HGCs), which differentiated from the pillow and migrated to the edge of the head in both species. HGCs of Japanese anchovy stopped migration at that place, whereas those of Pacific herring continued to migrate dorsally and distributed widely all over the head region. During evolution, the program for the HGC migration would be varied to adapt to different hatching timing. Analysis of the gene expression revealed that Pacific herring embryos synthesized a large amount of hatching enzyme when compared with Japanese anchovy. Chorion of Pacific herring embryo was about 7.5 times thicker than that of Japanese anchovy embryo. Thus, the difference in their gene expression levels between two species is correlated with the difference in the thickness of chorion. These results suggest that the hatching system of each fish adapted to its respective hatching environment. Finally, hatching enzyme genes were cloned from each genomic DNA. The exon-intron structure of group B genes basically conserved that of the ancestral gene, whereas group A genes lost one intron. Several gene-specific changes of the exon-intron structure owing to nucleotide insertion and/or duplication were found in Japanese anchovy genes.