Frequency increase and mitotic stabilization of a B chromosome in the fish Prochilodus lineatus

Six populations of the fish Prochilodus lineatus were analysed for B chromosome frequency. A study of spermatogenesis revealed the absence of B accumulation during the stages analysed. In one of the populations, from the Mogi-Guacu river where samples have been analysed over a ten-year period, B chromosome frequency doubled between 1979-80 and 1987-89, whereas no additional changes were noticed in samples collected in 1991-92. The analysis of B chromosome mitotic instability, manifested by intraindividual variation in B chromosome number, indicated a very significant decrease during this time period. This suggests that, in the 1980s, this population was in the final stage of B chromosome invasion, and that there was a possible causal relationship between B mitotic instability and the accumulation mechanism that caused its frequency increase. Mitotic stabilization might thus be a way by which a mitotically unstable B chromosome may become neutralized.