Genome Expansion of ZPB1(a) and ZPC1(a) in Basal Species or Liver Expression of ZPB1a and ZPC1aa in Advanced Species, Two Different Strategies to Ensure Sufficient ZP Synthesis in Teleosts.

The evolution of ZP (zona pellucida) genes in fish involves both gene expansion and expression organ transition. However, it is unclear when the expansion and transition occurred. Furthermore, it is also unclear whether there is a correlation between ZP gene expansion and expression organ transition. In this study, we identified all ZPs from 15 representative species in Cyclostomata and Neopterygii, and analyzed their expression in available transcriptome data in 8 species. The results showed that the expansion of ZP genes restricted to ZPB1 and ZPC1 first appeared in lamprey and was retained in bowfin and basal teleosts. The expanded ZP genes were highly expressed in teleosts, with ZPB1(a) (7-17 duplicates) and ZPC1(a) (8-32 duplicates), accounting for 82%-92% of the total ZP gene FPKM. The expression organ transition of highly expressed ZP genes occurred in the ancestor of Euteleostei, between Denticipitoidei and Clupeoidei in Clupeiformes. After the transition, only a few copies of ZPB1 and ZPC1 were retained (verified in another 20 species of the Euteleostei), and only ZPB1a and ZPC1aa were expressed in the liver (accounting for 64%-95% of the total ZP gene FPKM) in advanced teleosts. The N-terminus of expanded ZPB1(a) or ZPC1(a) and liver-expressed ZPB1a contains repeated units or low-complexity regions to form helical structures to ensure more elastic egg membranes for better protection of embryos. Taken together, our results demonstrate that teleosts evolved two different strategies to ensure sufficient ZP synthesis: genome expansion of ZPB1(a) and ZPC1(a) in basal species or liver-expression of ZPB1a and ZPC1aa in advanced species.