Sex-linked Correlated Responses in Female Reproductive Traits to Selection on Male Eye Span in Stalk-eyed Flies

Coevolution between male and female traits can result from correlatedresponses to selection or correlated selection on geneticallyindependent traits. This study examines the possibility thattraits involved in precopulatory sexual selection may influencethe evolution of traits involved in postcopulatory sexual selectiondue to the existence of correlated selection or correlated responsesto selection. Artificial selection on male eye span in Cyrtodiopsisdalmanni, a sexually dimorphic stalk-eyed fly, is used to testfor correlated changes in reproductive traits of male and femaleflies. Flies from replicate lines that had been under selectionfor 57 generations were matched for age and genotyped at fourX-linked microsatellite loci. Egg number and testis size increasedwith age, but did not differ among lines. Spermathecal areasand duct lengths differed among replicates, but not among selectiontreatments. Female relative eye span, size of the ventral receptacleand egg size exhibited significant correlated responses to selectionon male relative eye span. The absence of any change in spermlength or testis size between lines indicates that changes infemale traits are unlikely due to correlated selection mediatedby sperm competition. Significant effects of X-linked microsatellitegenotypes indicate instead that the correlated responses toselection were due, in part, to X-linked genes in linkage disequilibriumor that exhibit pleiotropy. The presence of nonadditive alleliceffects on genetically correlated female traits combined withadditive allelic effects on a male ornament provides a previouslyunrecognized mechanism by which genetic variation could be maintaineddespite strong sexual selection.