ENZYME POLYMORPHISM IN PLANT-POPULATIONS

This review encompasses a decade of studies of enzyme polymorphism in plant populations, in the light of both general theory and specific, simplified models. The patterns of the observed frequency of heterozygotes, compared with panmictic expectations adjusted only for inbreeding, are summarised for 23 outbreeding and 7 inbreeding plant species. There is a trend for outbreeders to show less heterozygosity than expected, and inbreeders to show more, despite the contrary evolutionary pressures on the mating system (the so-called heterozygosity paradox). An annual life cycle and pollination by animal vectors seem to increase the discrepancy in outbreeders. Of the several forces which might account for this paradox, the effects of intense microgeographic differentiation, of low gene flow, of self compatibility and of overdominance of linked segments are predominant. The evidence indicates that inbreeding plant species show more intense geographic and microgeographic differentiation, and more intense multilocus associations than outbreeders. Recent attempts to describe selection operating on variants by the analysis of life cycle components, of physiological processes, and of genetic demography are discussed. The fundamental importance of mating systems and their variation, as a distinctive feature of plant populations is already clear from the studies in hand. Therefore a closer integration of the joint microevolution of mating systems, and of genetic variation is required in both theoretical and experimental studies. © 1979.