A zinc knuckle gene on the Y chromosome (zkY) determines sex in Atlantic cod (Gadus morhua)

The genetic mechanisms determining sex in teleost fishes are highly variable, ranging from a single gene to complex patterns of polygenic regulation. The master sex determining gene has only been identified in very few species and there is no information about the gene in the superorder Paracanthopterygii that includes the codfishes, toadfishes and anglerfishes. Here we characterize a male-specific region of 9 kb on linkage group 11 in Atlantic cod (Gadus morhua) harboring a single gene named zkY for zinc knuckle on the Y chromosome. A diagnostic PCR test of phenotypically sexed males and females of Atlantic cod confirmed the sex-specific nature of the Y-sequence. We searched for autosomal gene copies of zkY and identified twelve highly similar genes, of which eight (zk1-zk8) code for proteins containing the zinc knuckle motif. 3D structure modelling suggests that the amino acid changes observed in six of the eight copies might influence the putative RNA-binding specificity. Cod zkY and the autosomal proteins zk1 and zk2 possess an identical zinc knuckle structure, but only the Y-specific gene zkY was expressed at high levels in the developing larvae before the onset of sex differentiation. We propose that cod zkY functions as the master masculinization gene by coding for a suppressor of germ cell division in males. PCR amplification of Y-sequences in Arctic cod (Arctogadus glacialis) and Greenland cod (Gadus macrocephalus ogac) suggests that this novel sex determining mechanism emerged in codfishes more than 7.5 million years ago.