Meiotically driven sex chromosomes can quickly spread to fixation and
cause population extinction unless balanced by selection or suppressed
by genetic modifiers. We report results of genetic analyses that demo
nstrate that extreme female-biased sex ratios in two sister species of
stalk-eyed flies, Cyrtodiopsis dalmanni and C. whitei, are due to a m
eiotic drive element on the X chromosome (X-d). Relatively high freque
ncies of X-d in C. dalmanni and C. whitei (13-17% and 29%, respectivel
y) cause female-biased sex ratios in natural populations of both speci
es. Sex ratio distortion is associated with spermatid degeneration in
male carriers of X-d. Variation in sex ratios is caused by Y-linked an
d autosomal factors that decrease the intensity of meiotic drive. Y-li
nked polymorphism for resistance to drive exists in C. dalmanni in whi
ch a resistant Y chromosome reduces the intensity and reverses the dir
ection of meiotic drive. When paired with X-d, modifying Y chromosomes
(Y-m) cause the transmission of predominantly Y-bearing sperm, and on
average, production of 63% male progeny. The absence of sex ratio dis
tortion in closely related monomorphic outgroup species suggests that
this meiotic drive system may predate the origin of C. whitei and C. d
almanni. We discuss factors likely to be involved in the persistence o
f these sex linked polymorphisms and consider the impact of X-d on the
operational sex ratio and the intensity of sexual selection in these
extremely sexually dimorphic flies.