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

Abstract

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. Keywords: speciation, reciprocal transplant, habitat association, phenology.