Breeding Systems of Clarkia sect. Phaeostoma (Onagraceae): I. Pollen-Ovule Ratios

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Abstract
Six annual, self-compatible species of section Phaeostoma fall into two breeding system groups: one of outcrossers and the other of selfers. The outcrossers, including Clarkia unguiculata, C. springvillensis, and most populations of C. tembloriensis and C. xantiana, are both protandrous and herkogamous, and have large flowers which require pollen vectors for successful pollination. The selfers, including Clarkia calientensis, C. exilis, and some populations of C. xantiana and C. tembloriensis, are homogamous and autogamous, and have small flowers capable of unaided self pollination. Both groups include species with mixed mating systems and, hence, they are interpreted to be facultative outcrossers and facultative selfers. On average, the outcrossers produce about 12,000 to 14,500 pollen grains and 90 to 95 ovules per flower, with mean pollen/ovule ratios (P/O) ranging from about 127 to 159. The selfers produce about 4000 to 6000 pollen grains and 67 to 116 ovules per flower, with mean P/O's ranging from 38 to 62. Population to population differences are significant within species of the outcrossing group, but interspecific differences are not significant. Similarly, interpopulational differences are significant within species of the selfing group, but interspecific differences are not significant. Therefore, P/O's in Clarkia are useful in determining the general breeding system type, but cannot readily resolve differences in the degree of outcrossing or the degree of selfing within those types. Seeds and pollen grains are generally larger in outcrossers than in selfers, but vary extensively in both groups and, therefore, are not reliable indicators of sex allocation.