Spatial genetic structure and non-equilibrium demographics within plant populations

Abstract

Spatial genetic structure within plant populations is primarily determined by the effects of limited seed and pollen dispersal. In the absence of selfing, spatial genetic structure and limited pollen flow are generally considered to be the primary controlling factors of the dynamics of biparental inbreeding and inbreeding depression. However, in some cases spatial structure may depend on specific demographic processes. Most theoretical studies have focused on the long-term or equilibrium structure of genetic variation, which allows, if demographic considerations are simple enough, a system to be collapsed into a single standardized measure (such as Wright's neighborhood size) of total pollen and seed dispersal distances. Specific demographic considerations may take many forms and strengths. In this paper we examine some particular cases that illustrate a wide range of effects on spatial genetic structure, and identify some of the most critical parameters. For example, Cecropia obtusifolia is a canopy-gap-filling, dioecious species. Perennially, the highly limited availability of habitat forces the removal of seedlings and the associated strong but ephemeral genetic structure, leaving one or a few trees to fill in each canopy gap. The structure among reproductive adults is minor and in line with standard amounts and forms expected for a wind-pollinated species. In contrast, Silene dioica is a dioecious, bee-pollinated, gravity-seed-dispersed perennial, and a colonizer or early successional species. In certain regions of Sweden, S. dioica grows primarily on uplifting islands that undergo rapid succession, and S. dioica exhibits exhibits extreme levels of spatial autocorrelation, caused by highly limited seed and pollen flow at a specific spatial scale that corresponds to demographic patches. Moreover, the types of genotypic concentrations in space vary, being dominated by matrilineal relationships. The availability of habitat rapidly increases, then rapidly decreases (with analogous changes in population growth rates), and, combined with a long-life, causes the genetic structure to exist on such a time scale that it appears to be the main form of within-population genetic structure for the species in this region. Clonal reproduction may have marked effects on the spatial genetic structure of plant populations. For monoecious, self-compatible species, clonal groups may increase effective selfing, whereas in dioecious plants any effects on the development of IBD may be more subtle. We examine a number of dioecious plants with a variety of spatial distributions of clones and populations, existing for various time scales. The demographics of clonal structure may interact with spatial-temporal demographics and structure of the genetic variation.