Effects of stand density on outcrossing rate in two Norway spruce (Picea abies) populations

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Abstract
The mating system was investigated in two neighbouring Norway spruce (Picea abies (L.) Karst.) populations with markedly different stand densities. The amount of outcrossing was estimated using open-pollinated array data at four enzyme loci. Multilocus estimates of the proportion of viable progeny owing to outcrossing were 0.956 and 0.955 for the low- and high-density stand, respectively, indicating that the large reduction in stand density had no impact on outcrossing rates. The multilocus estimate was higher than the mean single-locus estimate only for the low-density stand. The lowering of the mean estimate in the low-density stand is probably a consequence of the clustering of related individuals. The regression of pollen allele frequencies on ovule genotype, which is a direct measure of the effective selfing caused by consanguineous matings, had a significant coefficient for the low-density stand and a nonsignificant one for the high-density stand. This obvservation confirms that the major effect of low-stand density in Norway spruce is the occurrence of consanguineous matings. Key words: mating system, outcrossing rate, inbreeding, Norway spruce, stand density.

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