SEX-RATIO DISTORTION CAUSED BY MEIOTIC DRIVE IN MOSQUITOS

Meiotic-drive genes have been described in two species of mosquito, Aedes aegypti and Culex quinquefasciatus. In both species, a Y (M)-linked gene causes a change in sex ratio in favor of males. More is known about the Distorter gene (D) in A. aegypti, but the gene in C. quinquefasciatus appears to be similar. D is located on the right arm of the sex chromosome, is marked with the intercalary Giemsa C-band when present, and is linked closely to the sex locus or region. D undergoes meiotic drive only when in coupling with M, the male-determining gene, which does not recombine with the centromere. Sensitivity to the MD haplotype is controlled at m, the female-determining gene. Therefore, m is considered to be the responder. The m haplotype is polymorphic for sensitivity to MD (m(r1), m(r2), m(s1)-m(s6)). Sensitivity is influenced, in some strains, by a second sex-linked gene t (tolerance to Distorter). Another sex-linked gene, A, enhances the effect of D. Meiotic drive is associated with breaks on the sex chromosomes. First seen at diplotene, mainly at four discrete positions, 90% of these breaks occur on the X (m) chromosome. They usually appear on a chiasmic arm of the bivalent, with the result that most acentric fragments remain attached to the unbroken homologue by a chiasma. Examination of developing spermatozoa in Distorter males reveals extreme sperm depletion, a high degree of abnormality (e.g., multiple axonemes), and an increased DNA content up to the 4C level. Surprisingly, however, the effective fertility of these males is hardly reduced. Evidently, spermatozoa are produced in nondistorter males in vast excess. The Distorter gene has been found in populations from Africa, America, Australia, and Sri Lanka. Resistance to it is much more widespread. The presence of natural resistance argues against the use of D for population control, but its effect has been shown to be enhanced in the presence of certain translocations and the A gene, causing it to become effective in field cage trials, even in the presence of some resistance. There is also experimental support for using D to bring about changes in populations ("population replacement").