INCREASED B-CHROMOSOME FREQUENCY AND ABSENCE OF DRIVE IN THE FISH PROCHILODUS-LINEATUS

The neotropical freshwater fish species Prochilodus lineatus (Pisces, Prochilodontidae) shows 2n = 54 chromosomes plus supernumerary microch romosomes ranging in number from zero to seven among different animals . The transmission rates of B chromosomes were studied by the analysis of the parental and F-1 generations in 10 controlled crosses performe d with specimens from a natural population. The mean transmission rate observed for B chromosomes (k(B) = 0.511) was consistent with that ex pected from a regular meiotic behaviour of Bs in both sexes and with t he theoretical value under a Mendelian mode of transmission (0.5). Pos sible explanations for the dramatic increase in B frequency observed i n this population during the last 10 years are discussed, bearing in m ind the current absence of drive.