CLINES FOR HYBRID DYSFUNCTION IN A GRASSHOPPER HYBRID ZONE

Two subspecies of the grasshopper Chorthippus parallelus meet in the P yrenees forming a hybrid zone several kilometers wide. Crosses between the two pure taxa result in sterile male offspring and normal females (i.e., Haldane's rule applies). However, no such dysfunction has been detected in hybrid males collected through the center of the hybrid z one. By assessing the level of dysfunction in the offspring of recipro cal crosses, it was possible to map dines for the genes responsible fo r dysfunction through the zone. This analysis shows that there is no a brupt transition between incompatible genomes in the field. Crosses we re also made between females collected from a transect spanning the hy brid zone and pure males of both subspecies. This reveals noncoinciden t dines for dysfunction near the center of the hybrid zone such that t he dysfunction expressed in the offspring of these crosses is less tha n expected from simple models. More complex models involving interacti on among genes must be invoked. Also, the possibility exists that sinc e the postglacial contact of these two grasshopper taxa, hybrid dysfun ction has become ameliorated by the evolution of modifiers. This hybri d zone is thought to be a tension zone, maintained by a balance betwee n selection against hybrid genotypes and dispersal into the zone cente r. The lessening of hybrid disadvantage over time through the breakdow n of epistatic interactions by recombination or through modification c ould account for the general lack of dysfunction in held collected hyb rids today.