An analysis of the causes of genetic isolation in two Pacific Coast iris hybrid zones

In two Pacific Coast Iris hybrid zones, the causes of genetic isolation appear to differ substantially. The Iris douglasiana - Iris innominata hybrid zone follows an ecotone a few kilometres inland from the ocean, implying different habitat associations for the two species, perhaps because of climate. Reciprocal transplant experiments showed that habitat association plays a major role in isolation. Each species survives best in its own habitat. Additional differences in perianth-tube length and flowering time between the two species have not developed into significant genetic isolating factors. The second hybrid zone occurs where species with different perianth-tube lengths co-occur (Iris chrysophylla - Iris tenax). Crosses between long- and short-tubed species suggest that differences in perianth-tube length can limit gene flow. Gene flow is also limited in the other direction, because the long-tubed species, I. chrysophylla, blooms earlier (though the flowering periods overlap). Coupled with the fact that these irises are all protandrous (anthers mature about 3 days before stigmas), relatively few days will be available when the long-tubed species can pollinate the short-tubed species. This combination of factors is probably a major form of genetic isolation in the I. chrysophylla - I. tenax zone.