STUDIES ON THE EVOLUTION OF BISEXUAL REPRODUCTION IN CRUCIAN CARP (CARASSIUS-AURATUS-GIBELIO BLOCH)

Populations of crucian carp (Carassius auratus gibelio Bloch) from northeast China have a zygotic chromosome number of 140-156 and appear to be triploid in origin. However, measurements of the DNA content of sperm and somatic cells show this species to be diploid in its chromosomal behaviour. Populations consist predominantly of females, but include a small and variable proportion of males (up to 25%). Reproduction can be by gynogenesis or it can be bisexual, and sex determination is by an XX-XY mechanism. An hypothesis is proposed to account for the alternative forms of reproduction. It is suggested that females may produce two types of eggs, a 'G' type that cannot complete the meiotic division, which leads to gynogenetic diploid female offspring, and a 'B' type that does complete meiosis, is fertilized normally, and gives rise to both male and female offspring. © 1990.