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: 2.6). 01/2002; 89(1):29-36. DOI: 10.3732/ajb.89.1.29
Source: PubMed


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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    • "Besides this, it has the highest reproductive output, and its seedling populations are the densest, so self-thinning can alter them substantially, leading to differentiation among stages (see also [15] for Atriplex tatarica). As pointed out by Morris et al. [9], the capacity of seed banks to retain higher levels of genetic diversity may depend on (1) seed dormancy characteristics and potential multigenerational contributions to the seed bank, and (2) the relative size of seed bank populations. Species producing large seed banks with dormant seeds would be expected to contain higher levels of genetic diversity in their seed banks in comparison to other life history stages [8], [9], [13]. "
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    ABSTRACT: We attempted to confirm that seed banks can be viewed as an important genetic reservoir by testing the hypothesis that standing (aboveground) plants represent a nonrandom sample of the seed bank. We sampled multilocus allozyme genotypes from three species with different life history strategies: Amaranthus retroflexus, Carduus acanthoides, Pastinaca sativa. In four populations of each species we analysed the extent to which allele and genotype frequencies vary in consecutive life history stages including the summer seed bank, which has been overlooked up to now. We compared the winter seed bank (i.e., seeds collected before the spring germination peak), seedlings, rosettes, the summer seed bank (i.e., seeds collected after the spring germination peak) and fruiting plants. We found that: (1) All three species partitioned most of their genetic diversity within life history stages and less among stages within populations and among populations. (2) All genetic diversity parameters, except for allele frequencies, were similar among all life history stages across all populations in different species. (3) There were differences in allele frequencies among life history stages at all localities in Amaranthus retroflexus and at three localities in both Carduus acanthoides and Pastinaca sativa. (4) Allele frequencies did not differ between the winter and summer seed bank in most Carduus acanthoides and Pastinaca sativa populations, but there was a marked difference in Amaranthus retroflexus. In conclusion, we have shown that the summer seed bank is not genetically depleted by spring germination and that a majority of genetic diversity remains in the soil through summer. We suggest that seed banks in the species investigated play an important role by maintaining genetic diversity sufficient for recovery rather than by accumulating new genetic diversity at each locality.
    PLoS ONE 11/2012; 7(11):e49471. DOI:10.1371/journal.pone.0049471 · 3.23 Impact Factor
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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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    ABSTRACT: Seed banks are critical in arid ecosystems and ensure the persistence of species. Despite the importance of seed banks, knowledge about their formation and the extent to which a seed bank can recover after severe perturbation remains scarce. If undisturbed, soil seed banks reflect a long vegetation history; therefore, we would expect that new soil seed banks and those of undisturbed soils require long periods to become similar with respect to both density and composition. In contrast, if soil seed banks are only a short- to mid-term reservoir in which long-term accumulation constitutes only a tiny fraction, they will recover rapidly from the vegetation. To shed light on this question, we evaluated seed bank formation in a semi-arid gypsum community. Soils from 300 plots were replaced with sterilized soil in an undisturbed semi-arid Mediterranean community. Seasonal changes in seed bank density and composition were monitored for 3 years by comparing paired sterilized and control soil samples at each plot. Differences in seed bank density between sterilized and control soil disappeared after 18 months. The composition of sterilized seed banks was correlated with that of the control plots from the first sampling date, and both were highly correlated with vegetation. Nearly 24 % of the seed bank density could be attributed to secondary dispersal. Most seeds died before emergence (66·41-71·33 %), whereas the rest either emerged (14·08-15·48 %) or persisted in the soil (14·59-18·11 %). Seed banks can recover very rapidly even under the limiting and stressful conditions of semi-arid environments. This recovery is based mainly on the seed rain at small scales together with secondary dispersal from intact seed banks in the vicinity. These results emphasize the relevance of processes occurring on short spatial scales in determining community structure.
    Annals of Botany 01/2012; 109(1):299-307. DOI:10.1093/aob/mcr260 · 3.65 Impact Factor
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