New insights from fine‐scale spatial genetic structure analyses in plant populations

TLDR

Fine‑scale spatial genetic structure (SGS) in plant populations has traditionally been described qualitatively, but isolation‑by‑distance theory now offers quantitative methods to estimate SGS and infer gene dispersal parameters. This review examines the theoretical foundations of SGS quantification, addresses practical issues such as relatedness statistics, sampling design, and autocorrelogram interpretation, and proposes extensions to compare SGS with direct gene‑dispersal estimates, assess biparental inbreeding, and evaluate dispersal‑distribution kurtosis. The authors introduce the Sp statistic, which relates the decline of pairwise kinship with log distance in two dimensions, estimates the reciprocal of neighbourhood size under suitable conditions, and apply it to 47 plant species to demonstrate its utility and potential extensions. Sp values were significantly higher in selfing species, in herbs compared to trees, and in low‑density populations, indicating that mating system, life form, and.

Abstract

Abstract Many empirical studies have assessed fine‐scale spatial genetic structure (SGS), i.e. the nonrandom spatial distribution of genotypes, within plant populations using genetic markers and spatial autocorrelation techniques. These studies mostly provided qualitative descriptions of SGS, rendering quantitative comparisons among studies difficult. The theory of isolation by distance can predict the pattern of SGS under limited gene dispersal, suggesting new approaches, based on the relationship between pairwise relatedness coefficients and the spatial distance between individuals, to quantify SGS and infer gene dispersal parameters. Here we review the theory underlying such methods and discuss issues about their application to plant populations, such as the choice of the relatedness statistics, the sampling scheme to adopt, the procedure to test SGS, and the interpretation of spatial autocorrelograms. We propose to quantify SGS by an ‘ Sp ’ statistic primarily dependent upon the rate of decrease of pairwise kinship coefficients between individuals with the logarithm of the distance in two dimensions. Under certain conditions, this statistic estimates the reciprocal of the neighbourhood size. Reanalysing data from, mostly, published studies, the Sp statistic was assessed for 47 plant species. It was found to be significantly related to the mating system (higher in selfing species) and to the life form (higher in herbs than trees), as well as to the population density (higher under low density). We discuss the necessity for comparing SGS with direct estimates of gene dispersal distances, and show how the approach presented can be extended to assess (i) the level of biparental inbreeding, and (ii) the kurtosis of the gene dispersal distribution.