The Evolution of Self-Pollination in Clarkia xantiana

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Abstract
Normally outcrossing, pink-flowered, diploid Clarkia xantiana has given rise to two distinct, localized, self-pollinating populations in an ecologically marginal area. These two populations are morphologically very similar, except that one is pink and the other white. The selfers have smaller flowers than the outcrossers, the stigma is mature at anthesis when it is in contact with the dehiscing anthers, the style does not elongate after anthesis, flowering is earlier, and the habitat in which they occur is visibly more xeric. The genomes of the outcrosser and the pink selfer appear to be structurally identical; the hybrids are highly fertile and regularly show nine bivalents at meiosis. The white selfer differs from both of the others by a translocation and hybrids with them have a somewhat lower fertility. The essential change effecting selfpollination has been early maturation of the stigma, which appears to have a simple genetic basis. F2 data also indicate that when the stigma matures the style ceases to elongate, suggesting that the phenomena are causally related. Inheritance of flower size, on the other hand, is multifactorial and not genetically correlated with self-pollination. It is argued that the initial event leading to self-pollination was the reduction of the population to extremely small size during a growing season truncated by water stress, and that the genotypes surviving this catastrophic selection were those produced by the earliest flowering individuals. In a very small population genes promoting self-pollination would be at an advantage. Once established, an inbreeding race would become self-perpetuating. Reduction in flower size followed from relaxed selection for large flowers and positive selection for smaller flowers. The whiteflowered selfer is derived from the pink selfer.

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