Academic journal article The American Midland Naturalist

Population Genetic Structure of the Self-Compatible Annual Herb; Polygonum Thunbergii (Polygonaceae) Detected by Multilocus DNA Fingerprinting

Academic journal article The American Midland Naturalist

Population Genetic Structure of the Self-Compatible Annual Herb; Polygonum Thunbergii (Polygonaceae) Detected by Multilocus DNA Fingerprinting

Article excerpt

ABSTRACT-The spatial genetic structure of Polygonum thunbergii was investigated by DNA fingerprinting, using an oligonucleotide probe (GACA)4 and restriction enzyme Dra I. The genetic similarity of DNA fingerprints among 15 plants collected over a distance of 230 m continuously decreased with increasing distance. This significant negative correlation between similarity of DNA fingerprint and distance means that there is a spatial genetic structure within the population of P thunbergii because similarity of DNA fingerprint and genetic relatedness are strongly correlated. The genetic structure would be caused by the strictly limited seed dispersal in this species, which has cleistogamous and chasmogamous flowers with no specialized seed dispersal mechanism.

INTRODUCTION

Almost all higher plants are sessile, and "migration" of plants occurs only through pollen and seed dispersal, mediated by biotic and abiotic vectors such as animals, wind and water. Simulations have demonstrated that limited gene dispersal of both pollen and seed results in genetic structure, which is the clustering of genetically related individuals among plants within a population (Berg and Hamrick, 1995; Epperson and Li, 1997). Thus, spatial genetic structure within a population is one of the major interests in studies of plant populations.

In spite of the expectation of genetic structure within populations that have limited gene dispersal, previous empirical studies detected weak or no genetic structure among adult plants within populations of tree species that have limited seed dispersal (e.g., Hamrick et al., 1993; Berg and Hamrick, 1995; Epperson and Alvarez-Buylla, 1997; Streiff et al., 1998; Ueno et al., 2000). This can be explained by the effect of high mortality in offspring, resulting in few survivors among genetically related individuals (Hamrick and Nason, 1996). Because of this demographic thinning of offspring, adjacent plants become less closely related genetically in the adult stage (Hamrick and Nason, 1996). However, spatial autocorrelation analysis of flower color phenotypes, which are certainly under genetic control, has shown the clustering of identical phenotypes among adult plants of Ipomea purpurea that have nondispersible seeds (Epperson and Clegg, 1986) and Impatiens pallida, which mainly reproduce through cleistogamous flowers that produce nondispersible seeds (Schoen and Latta, 1989). These studies of two herbaceous species suggest that the seed dispersal distances of the tree species mentioned above might not be strictly limited and that the spatial genetic structure among adult plants within a population can be observed only under conditions of strictly limited seed dispersal, even though the demographic thinning of offspring occurs.

We investigated the spatial genetic distribution of the herb Polygonum thunbergii Sieb. et Zucc. (Polygonaceae) using a multilocus DNA fingerprinting method. Seed dispersal in P thunbergii is strictly limited. This species always has some cleistogamous flowers (Kawano et aL, 1990), the seeds of which are not dispersed but laid underground near the mother plant. Even the seeds produced from its chasmogamous flowers have no specialized mechanism of dispersal and, thus, are also dispersed around maternal plants. The intensive demographic thinning of offspring was found in a natural population of this spices (Kawano et aL, 1990). If the demographic thinning of offspring is the major cause of the low genetic relatedness among near-neighbors in adult stage, the spatial genetic structure should not be observed within a population. Here we address the question: is spatial genetic structure observed within a population of P thunbergii under conditions of strictly limited seed dispersal?

We used an artificial oligonucleotide as a probe to detect DNA fingerprints (Ali et al., 1986). Each probe consisted of a short tandem repeat (one unit is 2-4 bases) that is abundant in eukaryote genomes (Tautz and Renz, 1984). …

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