Article

Stratified analysis of the soil seed bank in the cedar glade endemic Astragalus bibullatus: evidence for historical changes in genetic structure.

Department of Botany and.
American Journal of Botany (Impact Factor: 2.46). 01/2002; 89(1):29-36. DOI: 10.3732/ajb.89.1.29
Source: PubMed

ABSTRACT Persistent seed banks may provide information on historical changes in the genetic composition of populations. We used stratified sampling of the soil seed bank of Astragalus bibullatus (Pyne's ground plum) to assess levels of temporal variation in population genetic structure and to infer historical changes in the levels of inbreeding and relative gene flow. This species has an extremely limited distribution in the Central Basin of Tennessee, where it is found in open areas and along the edges of cedar glades. Protein electrophoresis was conducted on seedlings grown from seeds that had been recovered from three successive 1 cm thick layers of soil sampled from six sites. Analyses of seven polymorphic allozyme loci indicated that there were substantial levels of genetic differentiation among soil layers and sites. Higher levels of genetic diversity were found in seed than in vegetative populations that had been sampled in a previous study. Seed populations from the uppermost soil layer had higher heterozygote deficiencies, displayed higher levels of differentiation among sites, and had higher private allele frequencies than seed populations from the lower two layers. The change in heterozygosity and distribution of genetic variation among sites for the youngest soil layer is consistent with a pattern of increased selfing, sib mating, and decreased gene flow among populations. These changes in inbreeding and relative levels of gene flow are corroborated by information on historical land use practices in the region and support the hypothesis that loss of appropriate habitat has led to smaller population sizes and a more fragmented distribution of this cedar glade endemic.

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    • "There is general agreement that seed banks may reduce the demographic effects of environmental stochasticity (Thompson, 1992; Fenner, 1995), consequently promoting community stability. Seed banks favour plant population persistence by lowering extinction risk (Kalisz and McPeek, 1992) and act as genetic reservoirs (Morris et al., 2002), slowing down the rate of evolutionary change (Nunney, 2002). Despite their role in the promotion of aboveground stability, seed banks have remarkably high temporal variability (Hassan and West, 1986; Bertiller, 1992). "
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    • "Consequently, seed banks are well developed in ecosystems where recurrent drastic perturbations such as wildfire (Valbuena et al., 2000) or floods (Hölzel and Otte, 2004) are responsible for the high mortality of standing individuals or in stressful environments where recruitment conditions are unpredictable (Kemp, 1989). These seed banks would constitute the memory of the vegetation history (Templeton and Levin, 1979) because they accumulate seeds produced over extended periods (Milberg, 1995; Bekker et al., 1999) and also serve as a genetic reservoir of past selective events (Morris et al., 2002) that can slow the rate of evolutionary change (Nunney, 2002). "
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    • "The positive effect of light on germination has been shown for other Astragalus species (Morris et al. 2002), but it is a less common trigger to break dormancy. Temperature and humidity have been demonstrated to be important for the germination of other Astragalus species (Kaye 1999). "
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