Molecular cloning of Plk1 and Nek2 and their expression in mature gonads of the teleost fish Nile tilapia (Oreochromis niloticus).

Polo-like kinase 1 (Plk1) and NIMA-related kinase 2 (Nek2) are serine/threonine kinases that are involved in the G2/M phase transition of the cell cycle in vertebrates. Although they also play critical roles in the regulation of spermatogenesis and oogenesis, their characterization in meiosis has not been elucidated fully, particularly in teleost fish. To investigate the evolutionary significance of serine/threonine kinases in the reproductive system of fish, we cloned the cDNAs of Plk1 and Nek2 from a Nile tilapia (Oreochromis niloticus) testicular cDNA library. Tilapia Plk1 encodes a protein of 582 amino acids that shares 75% homology with human Plk1, while tilapia Nek2 encodes a putative protein of 446 amino acids that shares 70% homology with human Nek2. Analyses of tissue distribution by RT-PCR and Southern blotting revealed that Plk1 and Nek2 are strongly expressed in the ovary and testis. Northern blot analysis revealed two Nek2 transcripts in the ovary and testis with different expression patterns, which indicates the presence of two structural variants for tilapia Nek2. Moreover, the localization of Plk1 and Nek2 mRNAs in tilapia gonads was determined by in situ hybridization analysis. In the ovary, Plk1 and Nek2 were expressed predominantly in oocytes. In the testis, on the other hand, Plk1 was expressed in primary spermatocytes, while Nek2 was generally expressed in primary and secondary spermatocytes. These results suggest that Plk1 and Nek2 are key factors in the progression of meiosis in fish.