Bioinformatic analyses of zona pellucida genes in vertebrates and their expression in Nile tilapia

Zona pellucida (ZP) genes encode ZP glycoproteins which constitute the coat surrounding oocytes and early embryos. Genome-wide identification of ZP genes is still lacking in vertebrates, especially in fish species. Herein, we conducted bioinformatic analyses of the ZP genes of the Nile tilapia and other vertebrates. Totally 16, 9, 17, 27, 21, 20, 26, 19, 14,11, 24, 17, 9, 18, 8, 11, 9, 8, 5, and 4 ZP genes belonging to 5 subfamilies (ZPA, ZPB, ZPC, ZPD, and ZPAX) were found in the sea lamprey, elephant shark, coelacanth, spotted gar, zebrafish, medaka, stickleback, Nile tilapia, Amazon molly, platyfish, seahorse, Northern snakehead, cavefish, tetraodon, clawed frog, turtle, chicken, platypus, kangaroo rat, and human genomes, respectively. The expansion of ZP genes in basal vertebrates was mainly achieved by gene duplication of ZPB, ZPC, and ZPAX subfamilies, while the shrink of ZP gene number in viviparous mammals was achieved by keeping only one copy of the ZP genes in each subfamily or even secondary loss of some subfamilies. The number of ZP gene is related to the environment where the eggs are fertilized and the embryos develop in vertebrates. Transcriptomic analysis showed that 14 ZP genes were expressed in the ovary of Nile tilapia, while two (ZPB2b and ZPC2) were highly expressed in the liver. On the other hand, ZPB1a and ZPB2c were not found to be expressed in any tissue or at any developmental stage of the gonads examined. In the ovary, the expression of ZP genes started from 30 dah (days after hatching), significantly upregulated at 90 dah and maintained this level at 180 dah. The expression of ZPC2 in the liver and ZPC5-2 and ZPAX1 in the ovary was confirmed by in situ hybridization. The ovary- and liver-expressed ZP genes are expressed coordinately with oocyte growth in tilapia.