The effect of enzyme heterozygosity on growth in a strictly outcrossing species, the self-incompatible Arabis petraea (Brassicaceae)

The mating system and the relation between electrophoretic variation and morphological characters were investigated using seed families from two populations of Arabis petraea (Brassicaceae) from Iceland. The mating system analysis revealed that despite the presence of a strong sporophytic self-incompatibility system, gene-flow is restricted and biparental inbreeding common. The amount of isozyme heterozygosity (the number of heterozygous loci in an individual) positively affected the sizes of progeny grown in the greenhouse. Though intrinsic overdominance is commonly used as an explanation for a positive effect of isozyme heterozygosity on fitness in strictly outbreeding species, it is argued that the results in this study can be explained by associative overdominance through gametic disequilibrium created by restricted dispersal. The effect of heterozygosity is due partly to an effect of the mother plant, since more heterozygous mothers produced larger progeny even when the effect of offspring heterozygosity was controlled for. The presence of a maternal effect tends to inflate the effect of heterozygosity, a bias that is rarely controlled for in studies of heterozygosity effects on fitness.