A century of genetic variation inferred from a persistent soil‐stored seed bank

Abstract

Stratigraphic accretion of dormant propagules in soil can result in natural archives useful for studying ecological and evolutionary responses to environmental change. Few attempts have been made, however, to use soil‐stored seed banks as natural archives, in part because of concerns over nonrandom attrition and mixed stratification. Here, we examine the persistent seed bank of Schoenoplectus americanus, a foundational brackish marsh sedge, to determine whether it can serve as a resource for reconstructing historical records of demographic and population genetic variation. After assembling profiles of the seed bank from radionuclide‐dated soil cores, we germinated seeds to “resurrect” cohorts spanning the 20th century. Using microsatellite markers, we assessed genetic diversity and differentiation among depth cohorts, drawing comparisons to extant plants at the study site and in nearby and more distant marshes. We found that seed density peaked at intermediate soil depths. We also detected genotypic differences among cohorts as well as between cohorts and extant plants. Genetic diversity did not decline with depth, indicating that the observed pattern of differentiation is not due to attrition. Patterns of differentiation within and among extant marshes also suggest that local populations persist as aggregates of small clones, likely reflecting repeated seedling recruitment and low immigration from admixed regional gene pools. These findings indicate that persistent and stratified soil‐stored seed banks merit further consideration as resources for reconstructing decadal‐ to century‐long records that can lend insight into the tempo and nature of ecological and evolutionary processes that shape populations over time.

A century-long record of plant evolution reconstructed from a coastal marsh seed bank

“…Accordingly, S. americanus seed banks have been used to infer shifts in genetic variation (Summers et al. 2018), relative abundance (Saunders 2003; Jarrell et al. 2016), ecosystem attributes (e.g., productivity, Saunders 2003), and ecosystem responses to sea level rise over time (e.g., Törnqvist et al.…”

“…2007; Franks and Weis 2008; Franks and Weis 2009; Franks 2011), and recent work (Summers et al. 2018) indicates that persistent and stratified soil‐stored seed banks can similarly serve as resources for examining ecological and evolutionary processes that shape plant populations over time.…”

“…We first assembled a century‐long sequence of “depth” cohorts, taking advantage of prior work on sediment stratigraphy and seed germination (Summers et al. 2018). We then conducted two common garden experiments to assay variation in growth and exposure responses to low and high salinity levels.…”

“…Finally, to gain perspective on possible underlying mechanisms, we leveraged published accounts of relative abundance and multilocus genetic variation (Summers et al. 2018), paleo‐reconstructions (Cronin et al. 2000), and open access environmental records to explore how variation in salinity tolerance corresponds to shifts in genotypic composition, precipitation, and estuarine salinity conditions since the turn of the 20 th century.…”

Plant Love Stories: Share Your Story and Grow a Movement

“…However, what our research group has learned from the marsh is that these seeds can be “resurrected” out of darkness and these ancient plants are essential to understanding evolution (Summers et al. ). Through the support of my own team, I found I could resurrect my happiness as well (Fig.…”