Fine-Scale Genetic Structure of a Turkey Oak Forest

Abstract

Abstract.-Theoretical models and computer simulations of the genetic structure of a continuous population predict the existence of patches of highly inbred individuals when gene flow within the population is limited. A map of the three genotypes of a two-allele locus is expected to exhibit patches of homozygotes embedded in a matrix of heter ozygotes, when gene flow is limited. A search for such patch structure was made on a 160 X 160 m plot within a continuous 60+ ha old-growth stand of Qiiercus laevis (turkey oak). Approximately 3400 trees were genotyped for 9 polymorphic loci using starch-gel electrophoresis, and the genetic structure was analyzed with spatial autocorrelation (both nominal and interval), hierarchical E statistics, and number-of-alleles-in-common. Adults (diameter at breast height > 0) and juveniles were analyzed separately but showed similar structure. While no distinct patch structure was found, a greater degree of relatedness was observed on a scale of 5 m-10 m than at greater distances, probably because of the limited acorn dispersal from maternal trees and a small amount of cloning by root sprouts. A computer simulation of a 10,000 tree forest breeding for 10,000 yr indicates that the effective neighborhood sizes (of randomly drawn seed-and pollen-donors) are both in excess of 440 individuals. The model thus cannot distinguish the observed data from panmictic mating.Key words.-Allozyme, F statistics, gene flow, genetic structure, isolation-by-distance, neighborhood size, numberof-alleles-in-common, Qiierciis laevis, randomization, spatial autocorrelation.Received December 22, 1992. Accepted March 30, 1994 Plant species and their populations are rarely randomly distributed within communities. Similarly, the spatial distri bution of individuals within populations is often nonrandom, as are the distributions of genotypes and alleles. Genetic structure can result from several factors, including isolation in small patches, limited pollen and seed dispersal, and se lection acting at the microhabitat level. Sewall Wright (1943aWright ( , 1946 argued that reproductive isolation can take place in a continuous population simply by means of limited gene flow. He described this process as " isolation by dis tance" and proposed that limited gene flow would generate patches of genetically similar individuals, which he called neighborhoods or demes. According to Wright's (1982) " shifting-balance" theory of evolution, if demes become ecologically adapted, then genetic structure can be enhanced. When environmental conditions change, some demes adapt to new conditions and are thus selected, whereas others be come extinct. It is plausible that deme-structure can arise in physically isolated populations, but it is less easy to visualize its creation in a continuously distributed population.Wright (1943b) used his isolation-by-distance model to describe the genetic differentiation of patches of white and blue flowers of the diminutive annual Linanthiis parryae in the Mojave Desert. He divided the 130-km long...

Joint analysis of spatial genetic structure and inbreeding in a managed population of Scots pine

“…The availability of highly variable molecular markers has facilitated the analysis of fine-scale genetic structure in natural tree populations. The fine-scale structure has been found in some forest tree species (Sork et al, 1993;Berg and Hamrick, 1995;Streiff et al, 1998;Dutech et al, 2002;Hardy et al, 2006). These tree species are characterized by either limited seed dispersal or restricted pollen and seed dispersal.…”

Spatial structure of genetic variability in natural stands of Fagus sylvatica L. (beech) in Italy

“…(1994) observed similar values. A patch size of 28 m was reported for Quercus laevis (Berg & Hamrick, 1995). In Q.…”

“…This measure, probably reasonable for square patches, has been reported to overestimate patches of irregular shape (Berg & Hamrick, 1995). Patch size was estimated as the minimum distance at which statistically significant correlograms showed values below the chosen probability threshold.…”

“…Similar results were obtained in other forest tree species: Picea mariana (Knowles, 1991), Acer saccharum (Perry & Knowles, 1991), Quercus petraea and Q. robur (Bacilieri ci' a!., 1994). Studies with intensive sampling of relatively limited areas also revealed scarce spatial structuring in lodgepole pine (Epperson & Allard, 1989), Picea abies (Leonardi et al, 1996) and Turkey oak (Berg & Hamrick, 1995).…”

Fine-scale spatial genetic dynamics over the life cycle of the tropical tree Prunus africana

“…SGS in adults (ADLTS) was slightly stronger than in old juvenile stages. Other studies looking at SGS from a multistage perspective have either found an increase (for example, Jacquemyn et al, 2006), decrease (for example, Zhou and Chen, 2010) or no change (for example, Berg and Hamrick, 1995) in SGS with age. This absence of a common pattern highlights the importance of speciesand site-specific postdispersal and early selection, as well as differences across generations (for example, owing to different levels of gene flow).…”