NARROW HYBRID ZONE BETWEEN 2 SUBSPECIES OF BIG SAGEBRUSH, ARTEMISIA-TRIDENTATA (ASTERACEAE) .3. DEVELOPMENTAL INSTABILITY

Stable hybrid zones are believed to occur because gene dispersal is op posed by selection, but the nature of this selection remains unclear. The dynamic equilibrium model postulates that hybridization disrupts c oadapted gene complexes, leading to alterations in development and sub sequent hybrid unfitness. Alternatively, the bounded hybrid superiorit y model assumes that selection is due to exogenous factors. Here, we e xamine the developmental instability of sagebrush across a narrow hybr id zone. The results show no difference among the parental and hybrid taxa for the majority of comparisons examined. When significant differ ences in developmental instability were observed, one of the parental taxa (mountain big sagebrush) was typically the least developmentally stable. Hybrids were found to be the least developmentally stable for only two of the 28 comparisons studied. Interestingly, for some compar isons, hybrids were the most developmentally stable. These results con tradict predictions of the dynamic equilibrium model. The sagebrush hy brid zone does not appear to be maintained by endogenous selection, as there is little indication that development is disrupted by hybridiza tion.