Cyclin A2 is differentially expressed during oocyte maturation between gynogenetic silver crucian carp and gonochoristic color crucian carp

Silver crucian carp (Carassius auratus gibelio) is a unique gynogenetic fish. Because of its specific genetic background and reproduction mode, it is an intriguing model system for understanding regulatory mechanism of oocyte maturation division. It keeps its chromosomal integrity by inhibiting the first meiotic division (no extrusion of the first pole body). The spindle behavior during oocyte maturation is significantly different from that in gonochoristic fish. The chromosomes are first arranged in a tripolar spindle, and then they turn around and are reunited mutually to form a normal bipolar spindle. A new member of the fish A-type cyclin gene, cyclin A2, has been isolated by suppression of subtractive hybridization on the basis of its differential transcription in fully-grown oocytes between the gynogenetic silver crucian carp and gonochoristic color crucian carp. There are 18 differing amino acids in the total 428 residues of cyclin A2 between the two forms of crucian carps. In addition, cDNAs of cyclin A1 and cyclin B have also been cloned from them. Thus two members of A-type cyclins, cyclin A1 and cyclin A2, are demonstrated to exist in fish, just as in frog, humans, and mouse. Northern blotting reveals that cyclin A2 mRNA is more than 20-fold and cyclin A1 mRNA is about 2-fold in fully grown oocytes of gynogenetic silver crucian carp compared to gonochoristic color crucian carp. However, cyclin B does not show such a difference between them. Western blot analysis also shows that the cyclin A2 protein stockpiled in fully grown oocytes of gynogenetic crucian carp is much more abundant than in gonochoristic crucian carp. Moreover, two different cyclin A2 expression patterns during oocyte maturation have been revealed in the two closely related crucian carps. For color crucian carp, cyclin A2 protein is translated only after hormone stimulation. For silver crucian carp, cyclin A2 protein can be detected throughout the process of maturation division. The different expression of cyclin A2 may be a clue to understanding the special maturation division of gynogenetic silver crucian carp.