Understanding the Genetic Structure of Plant Populations: Some Old Problems and a New Approach

Abstract

The genetic structure of natural plant populations results from the interaction of selection, gene flow and genetic drift. Reviews of the plant allozyme literature demonstrate that the distribution of allozyme variation within and among plant populations is closely associated with the species’ mating system, pollination ecology and seed dispersal mechanism. Yet, there are relatively few species for which dependable estimates of the mating system or of pollen or seed dispersal are available.

Estimates of plant mating systems based on the mixed mating model have provided insights into the breeding structure of a few species. There are, however, relatively few data available on temporal and spatial variation in the mating system. Also, the assumptions of the mixed mating model are often violated in natural populations. Finally, the mixed mating model is limited in its ability to provide information about the breeding structure of populations.

Our understanding of gene flow via pollen or seed in natural populations is poor at best. Estimation procedures based on pollinator movements have underestimated pollen movement by not adequately dealing with pollen carryover and do not measure the effective movement of genes. Procedures using genetic markers, although giving a more accurate measure of pollen flow in natural or artificial populations typically produce results which are limited in scope and generalizations are difficult.

The use of paternity analysis to identify the father of individual seeds or seedlings removes many of the problems inherent to estimates of gene flow or the mating system. Although requiring considerable effort, paternity analysis can determine several genetic parameters that have previously been difficult or impossible to measure. From such analyses a detailed picture of the mechanisms which interact to produce the genetic structure of plant populations can be obtained.