Abstract
Stephanomeria exigua subsp. coronaria (COR) and S. malheurensis (MAL) are related as progenitor and recent derivative and occur sympatrically at the only known site of MAL in eastern Oregon. MAL is predominantly self-pollinating, but COR is obligately outcrossing because it has a sporophytic self-incompatibility system. Self-pollination is an important component that contrib- utes to the reproductive isolation of MAL and evolved following a breakdown in the self-incom- patibility (SI) system it inherited from COR. We found a single COR plant derived from the parent population growing in our greenhouse that was self-compatible and self-pollinating. It was used to study the genetic basis of the change in compatibility within COR. We also analyzed the change in interspecific hybrids between COR and MAL. In both cases, the difference in compatibility was governed by a single gene, probably the S locus which controls the incompatibility reactions between the pollen grain and the stigmatic surface. The discovery of a SC allele and accompanying self- pollination in the progenitor population of COR bolsters the model that the origin of MAL required few genetic changes.
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