Cohort 2 - Spartina salt marsh grass and growth form in microhabitats vs. latitudinal gradients


Spartina salt marsh grass and growth form in microhabitats vs. latitudinal gradients

Salt marshes fringe the land-water interface across the Atlantic and Gulf coasts of North America, supporting biologically productive, estuarine ecosystems. The perennial cordgrass Spartina alterniflora, dominates the intertidal of these marshes, creating both habitat and marsh sediment structure. However, human development and global climate change threaten the vitality and productivity of these critical coastal ecosystems.

S. alterniflora exhibits two distinct growth forms across its latitudinal range; a short form (10-40 cm) and a tall form (1-3 m). There is much debate and uncertainty surrounding the environmental and or genetic underpinnings of the two observed growth forms (Smart 1986, Gallagher et. al 1988, Freshwater 1988, Proffitt et. al 2003). Are the two phenotypes of S. alterniflora a plastic response to environmental variables or are the observed differences genetic? Furthermore, if there are genetic differences between the two morphotypes, are they consistent across S. alterniflora’s geographic range?

In this study we aim to resolve the morphotype variation in S. alterniflora alongits latitudinal cline, from Georgia to Massachusetts. Using a double digest RAD-tag sequencing approach (Peterson et. al 2012), we plan to genotype many individuals (tall- and short-form at each of the 7 sites) producing a range of markers (SNPs) across homologous sites. This approach allows us to adequately and efficiently characterize underlying genomic differences among populations. These differences can help inform our knowledge of how populations will respond to new conditions imminent in a changing climate.

Leaf tissue samples (20cm) of 20 individuals of both tall and short Spartina alterniflora were collected from seven sites along the US east coast for later genetic analysis. Sites were selected along the latitudinal gradient to encompass topographic, climatic and marsh structural diversity. We will look at differences in SNPs across a genome, using double digest Restriction Associated DNA sequencing (ddRADseq), a protocol that sequences only regions generated by double digests, we can reduce the fraction of each individual genome sequenced, but increase the number of individuals sampled, thus making the problem tractable (Peterson et. al 2012).