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 a nd morphological characters were investigated using seed families from two populations of Arabis petraea (Brassicaceae) from Iceland. The ma ting system analysis revealed that despite the presence of a strong sp orophytic self-incompatibility system, gene-flow is restricted and bip arental inbreeding common. The amount of isozyme heterozygosity (the n umber of heterozygous loci in an individual) positively affected the s izes of progeny grown in the greenhouse. Though intrinsic overdominanc e is commonly used as an explanation for a positive effect of isozyme heterozygosity on fitness in strictly outbreeding species, it is argue d that the results in this study can be explained by associative overd ominance through gametic disequilibrium created by restricted dispersa l. The effect of heterozygosity is due partly to an effect of the moth er plant, since more heterozygous mothers produced larger progeny even when the effect of offspring heterozygosity was controlled for. The p resence of a maternal effect tends to inflate the effect of heterozygo sity, a bias that is rarely controlled for in studies of heterozygosit y effects on fitness.