Molecular characterization and expression analysis of Cyclin B and Cell division cycle 2 in gonads of diploid and triploid bighead catfish, Clarias macrocephalus Günther, 1864

This study investigated the differential expression of genes associated with reproduction in sterile triploid and normal diploid bighead catfish (Clarias macrocephalus Günther, 1864). The triploid fish were produced using cold shock and were reared in the same conditions as the diploid counterpart. The histomicrographs showed completely retarded triploid gonads across the samples aged 2-12 mth, whereas the gonads of the diploids were in developing stages during 2-4 mth, reached the early maturing stage at 6 mth, matured at 8 mth and showed signs of atresia at 10-12 mth. In parallel, the full-length cDNAs of cyclin B1 (CmCcnb1; 1539 bp in length with an open reading frame (ORF) of 1194 bp corresponding to 397 amino acids) and cell division cycle 2 (CmCdc2; 1355 bp, an ORF of 909 bp, 302 amino acids) of bighead catfish (C. macrocephalus Günther, 1864) were isolated. Phylogenetic analysis revealed that the newly characterized CmCcnb1 should be regarded as a member of cyclin B1 rather than cyclin B2. The expression level of CmCcnb1 mRNA was limited in different stages of the ovaries and testes of triploids. In diploid ovaries, its expression was significantly higher than that in triploid ovaries in fish aged 2 mth (513.43 ± 82.22 fold) and in fish aged 8 mth (2430.87 ± 900.06 fold). The CmCcnb1 level in the testes of diploids was significantly greater than that in triploids in fish aged 2 mth (928.85 ± 208.72 fold). Similarly, expression of CmCdc2 mRNA was also reduced in triploids. Its expression was significantly lower than that in diploid females aged 2 mth (7.66 ± 3.42 fold), 4 mth (59.42 ± 10.50 fold) and 8 mth (42.74 ± 8.36 fold). In males, significantly greater expression of CmCdc2 was observed at age 6 mth (58.61 ± 34.64 fold) and 8 mth (72.70 ± 4.36 fold) diploids compared to triploids. The results illustrated that CmCcnb1 and CmCdc2 are functionally involved in oogenesis and spermatogenesis and reduced expression levels of these transcripts affected the reproductive development of triploid C. macrocephalus.