Genotypes affecting the condensation and transmission of heterochromtic B chromosomes in the mealybug Pseudococcus affinis

In male mealybugs (Pseudococcide: Homoptera) the set of chromosomes of paternal origin becomes heterochromatic (H) and genetically inactive in early embryogenesis. During spermatogenesis the two sets segregate and only the meiotic products with the euchromatic (E) set form sperm. Individuals of Pseudococcus affinis (Maskell) may carry a B chromosome (B) which is usually heterochromatic. During prophase I of spermatogenesis, however, the B becomes even less condensed than the E set and usually segregates with the E set. We have previously shown that natural populations of P. affinis contain genotypes that can reduce the rate of transmission (k) of Bs from more than 0.9 to less than 0.1. We now demonstrate that these genotypes suppress k either by enhancing or by preventing the decondensation of the B, which in turn affects the position of the B on the metaphase plate and its segregation. We also demonstrate that radiation-induced fragment of the B, and a piece of a B which has been translocated onto a piece of an H chromosome, retain their characteristic pattern of condensation and thus that the condensation of the B is not controlled by one or a few cis-acting centers or loci.