Meiotic drive in Aedes aegypti (L.) is shown by a Giemsa C-banding technique to be associated with. preferential isochromatid breakage of the X chromosome during male meiosis. These breaks remain open at least until anaphase-I and, since the range of cells affected is proportional to the sensitivity of the X chromosome to the Distorter gene, it is argued that they are directly related to the decreased number of spermatozoa found in distorting males. This reduction is considered to be attributable to the degeneration of more X-than Y-bearing spermatids but it is probable that some non-functional X-bearing spermatozoa are also produced. Chromosome breakage is almost completely confined to four sites, two adjacent to the centromere, one just proximal to the intercalary band and another about the centre of the unbanded arm. Although the first three of these lie within a region in which crossing-over does not take place, fragmentation occurs more frequently in a chiasmate arm than in one devoid of chromatid exchange.

The separate identities of the male-determining factor, M, and the sex-linked Distorter gene, D, are established in an Accra strain ofAedes aegypti. Their positions in the Y chromosome are defined in relation to each other and to Giemsa C-bands. Thus, M is invariably inherited with the centromere, whereas D lies towards the intercalary band. Approximately 1.2% recombination occurs between M and D but, in a chromosome known for its distal localization of chiasmata, it is argued that the two are not necessarily as closely linked cytologically as this might imply. Evidence on the genetic effects of recombina. tion in the region of M and D is also considered.

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