Achiasmate segregation of a B chromosome from the X chromosome in two species of psyllids (Psylloidea, Homoptera)

The segregation of a B chromosome from the X chromosome was studied in male meiosis in two psyllid species, Rhinocola aceris (L.) and Psylla foersteri (Flor.) (Psylloidea, Homoptera). The frequency of segregation was determin ed from cells at metaphase II. In R. aceris, the B chromosome was mitotical ly stable and segregated quite regularly from the X chromosome in four geog raphically distant populations, while it showed less regular, but preferent ial segregation in one population. This was attributed to the presence of B chromosome variants that differ in their ability to interact with the X ch romosome in segregation. In P. foersteri, the B chromosome was mitotically unstable and segregated preferentially from the X chromosome in spermatocyt e cysts, which displayed one B chromosome in every cell. Behaviour of the B chromosome and X chromosome univalents during meiotic prophase and at meta phase I in R. aceris, and during anaphase I in P. foersteri suggested that the regular segregation resulted from the incorporation of B chromosomes in achiasmate segregation mechanisms with the X chromosome in the place occup ied by the Y chromosome in species with XY system. The regular segregation of a B chromosome from the X chromosome may obscure the distinction of a B chromosome and an achiasmate Y chromosome in some cases.