Meiosis is the key process for producing mature gametes.A natural fertile triploid Carassius auratus population (3nDTCC) and an artificially derived sterile triploid crucian carp (3nCC) have been previously observed,providing suitable model organisms for investigating meiosis characteristics in triploid fish.In the present study,the microstructures and ultrastructures of spermato-genesis were studied in these fishes.TdT-mediated dUTP nick end labeling detection was performed to investigate the apoptosis of spermatocytes.Fluorescence in situ hybridization was employed to trace chromatin pairing.In addition,the rnRNA ex-pressions of cell cycle-related genes (i.e.,cell division control 2 and cell cycle protein B) were determined by quantitative real-time polymerase chain reaction to illustrate the molecular mechanism of abnormal meiosis in the 3nCC.The results showed that the 3nCC undergoes an irregular prophase Ⅰ,with the chromosomes distributed in a unipolar radial manner and exhibiting partial pairing,hindered metaphase Ⅰ,and degenerated cells in the subsequent stages.Meanwhile,the 3nDTCC presented a relatively regular meiotic prophase I with complete conjugate chromosome pairs and chromosomes distributed along the karyotheca,which were presented as a ring structure by slicing.Only the spreads with 130-150 irregular chromosomes can be easily detected in the 3nDTCC,suggesting that it may undergo an abnormal metaphase Ⅰ.This study provides new insights into the meiosis of fertile and sterile triploid cyprinid fish.