LIFE-HISTORY ADAPTATION AND REPRODUCTIVE ISOLATION IN A GRASSHOPPER HYBRID ZONE

Patterns of life-history adaptation and reproductive isolation were in vestigated in the acridid grasshoppers Melanoplus sanguinipes and M. d evastator, which hybridize along an altitudinal gradient in the Sierra Nevada of California. Melanoplus sanguinipes females crossed with M. devastator males produced eggs that were approximately half as viable as eggs from other crosses. Diminished viability was not attributable either to infection by Wolbachia pipientis or to failure of sperm tran sfer. When offered an opportunity to choose a mate, females from all p opulations discriminated against males of the other species, whereas i n no-choice tests measuring copulation duration only females from the tails of the dines showed preferences. Melanoplus sanguinipes, found a t high elevations where the growing season is short, exhibited faster egg hatch, faster larval development, smaller adult body sizes, and sm aller clutch sizes than M. devastator. Melanoplus devastator, from Cal ifornia's Central Valley, endured a hot and dry summer in a reproducti ve diapause that was absent in M. sanguinipes. Clines in reproductive diapause and clutch size coincided with the region of reproductive inc ompatibility. Development time, body size, and hatch time also changed across the hybrid zone, but the regions of largest transitions in the se traits were either difficult to locate using the limited population s studied here or were not coincident with the zone's center. A method is described for combining ecological and phylogenetic analyses to ad dress the unknown issue of whether life-history divergence has conribu ted to reproductive isolation in this system.