Zonadhesin-like genes in three fish species: Atlantic salmon (Salmo salar), puffer fish (Takifugu rubripes) and zebrafish (Danio rerio)

The sperm membrane protein zonadhesin (ZAN) has been characterized in mammals and has been implicated in species-specific egg-sperm binding interactions. Zonadhesin is the only protein that contains MAM, mucin and von Willebrand D domains. A zonadhesin-like transcript was obtained from an Atlantic salmon gut tissue cDNA library. This transcript and the corresponding genomic locus were sequenced. A zonadhesin-like gene (ZLG) with a predicted open reading frame of 4,518 nucleotides that encodes MAM, mucin and VWD domains was characterized. Although the predicted protein contains the same major domains as zonadhesin, the domain order is altered. ZLG is expressed in the liver and the gut but not in the testis as a typical zonadhesin. This gene is unlikely to be directly involved in reproduction, because it has a unique domain structure and expression, but is likely to have a novel function. In contrast, zonadhesin-like genes with the same domain content and order as mammalian zonadhesins can be deduced from puffer fish and zebrafish genomic sequencing projects. Zonadhesin-like genes from these three fish species are compared and evolutionary relationships are explored.