Coevolution of sperm and female reproductive tract morphology in stalk-eyed flies

Sperm and female reproductive tract morphology are among the most rapidly e volving characters known in insects. To investigate whether interspecific v ariation in these traits results from divergent coevolution we examined tes tis size, sperm length and female reproductive tract morphology for evidenc e of correlated evolution using 13 species of diopsid stalk-eyed flies. We found that sperm dimorphism (the simultaneous production of two size classe s of sperm by individual males) is ancestral and occurs in four genera whil e sperm monomorphism evolved once and persists in one genus. The length of 'long sperm' types, though unrelated to male body or testis size, exhibits correlated evolution with two regions of the female reproductive tract, the spermathecae and ventral receptacle, where sperm are typically stored and used for fertilization, respectively. Two lines of evidence indicate that ' short sperm: which are probably incapable of fertilization, coevolve with s permathecae. First, loss of sperm dimorphism coincides phylogenetically wit h reduction or loss of spermathecae. Second, evolutionary change in short-s perm length correlates with change in spermathecal size but not spermatheca l duct length or ventral receptacle length. Morphological coevolution betwe en sperm and female reproductive tracts is consistent with a history of fem ale-mediated selection on sperm length.