Control of Meiotic Drive of B Chromosomes in the Mealybug, Pseudococcus affinis (obscurus).

Isofemale lines of Pseudococcus affinis (Maskell ) differ in their ability to maintain B chromosomes (Bs) due to the presence of genotypes that affect the rate of transmission (k) of the Bs. The nature of these genotypes was analyzed by comparing ks of males carrying the same B and the same paternal genome (which is heterochromatic), but differing in their maternal genome. In males from line L-60, which maintained the B at a frequency of over 4.0 Bs per individual, the mean k varied between 0.7 and 0.95 in different experiments. Over the same period, the mean k of males with a maternal genome from one of two lines in which the B was rapidly lost (L-119), increased from 0.5 to 0.9, and that of the other line (L-230) decreased gradually from 0.6 to less than 0.1. The ks appear not to be correlated with the geographical or parental origin of the B. The observed changes in k are attributed at least in part to changes in the frequency of genotypes (alleles) which can drastically reduce the transmission of the B and, when present in high frequency, can lead to its rapid loss. The frequency distribution of the ks of sons of F(1) females from the cross L-230 x L-60 suggests that the two lines differ at two unlinked loci with additive effects on k. The genome of L-119 also caused the B to undergo nondisjunction in about 10% of the primary spermatocytes. A comparison between the ks of the males tested and those of males from a natural population suggests that in that population the B is "parasitic" and that the frequency of transmission-reducing genotypes is low.