Between family variation in sex-ratio in Trinidad (T-30) strain of Aedes-aegypti (L) indicating differences in sensitivity to meiotic drive gene MD

Sex ratio in the Trinidad (T-30) strain of Aedes aegypti has remained constant at around 43%? during seventeen years of laboratory culture. The divergence from 50% is due to meiotic drive by the MD gene on the Y chromosome. The driving Y chromosome gives a much more distorted sex ratio (mean = 5.7%?) when coupled with the highly sensitive X chromosomes from strain 64. This was demonstrated in all of 98 families tested, indicating that all or most of the Y chromosomes in T-30 carry the MD gene. Consequently the low level of sex ratio distortion in T-30 must be due to resistance to MD.; ; Crosses made within T-30 demonstrated wide differences in sex ratio between families, depending on the sensitivity of the male parent’s X chromosome to MD. However, sex ratios were not continuously variable but fell within fairly discrete categories. Thus, X chromosomes could be classified according to the modal sex ratios associated with them: ms3 (12.5%?), ms2 (32.5%?), ms1 (40%?), mr1 (47.5%?) mr2 (57.5%?).; ; The different sex ratio categories were more discrete in the families of sib matings than from random matings, suggesting the possibility of background modification of what is essentially a balanced polymorphism. Evidence is presented suggesting that the polymorphism could be due to interaction at two loci. A further X variant, ms4 (<10%?) characterised strain 64 but was absent from T-30.; ; A comparison of fertility between the different sex ratio categories in T-30 established that sex ratio distortion was not caused by differential mortality after fertilisation.