Molecular events in adaptive evolution of the hatching strategy of ovoviviparous fishes.

Ovoviviparous fish, whose embryonic development and hatching take place in the maternal body, is one of the good model organisms for studying adaptive evolution. Using genome database of the ovoviviparous platy Xiphophorus maculatus, we tried to search hatching enzyme genes (high choriolytic enzyme HCE and low choriolytic enzyme LCE) and egg envelope protein genes (choriogenin H, Hm, and L). Analysis of genes co-localized with them confirmed that shared synteny was found between platy and medaka genome. Both hatching enzyme genes HCE and LCE were pseudogenized in platy. In addition, one of the three choriogenin genes Hm was completely lost from the genome, the other two genes H and L encoded functional proteins. On the other hand, the expression of H and L was very low as compared to oviparous fishes, and the platy egg envelope was extremely thinner. Considering that ovoviviparous fish embryos are protected in the maternal body, an importance of egg envelope for protection of egg/embryo would be reduced in the ovoviviparous fishes. Platy embryos would escape from their thin egg envelope without help of hatching enzymes. In another ovoviviparous fish, black rockfish belonging to different order from the platy, one of the hatching enzyme genes has been reported to be pseudogenized, that is, the embryo of black rockfish can escape from egg envelope by only one hatching enzyme HCE. Adaptive evolution of the hatching strategy of ovoviviparous teleosts may be established by pseudogenization of hatching enzyme genes and/or lowering of expression and/or pseudogenization of hatching enzyme and egg envelope genes.