How did genetic diversity get established in northern range limits of trees?
Temperate tree species in Eastern North America, including American beech, show consistently high genetic diversity in their northern range limit, in contrast to prevailing population genetics theory of latitudinal (poleward) decrease in genetic diversity. This raises questions about the emergence and establishment of genetic diversity as species colonized their northern range limit a few thousand years after the last ice age (21,000 years ago). In American beech, I tested the hypothesis that small, local beech populations have been established long before the known demographic expansion identified in pollen records in its northwestern range margin in Michigan. These small populations already carried most of the genetic diversity and have persisted in microrefugia farther north than commonly inferred. With training from our collaborator Dr. Poinar at McMaster University, I successfully extracted beech chloroplast DNA (cpDNA) from mud and macrofossils preserved in lake sediments for five thousand years (ancient DNA). Most of the modern cpDNA haplotypes found in existing beech populations were recovered in our ancient samples, indicating early establishment of high genetic diversity in early beech populations.
Holocene vegetation shifts: insights from ancient DNA analysis
Rapid changes in climate are causing species to shift their ranges. Plants are known to have individualistic response to climatic changes over the past 10,000 years, where climate warmed as much as it is warming today, leading to shifting plant community compositions in a given space and time. Long-term records of forest or vegetation shifts throughout Holocene are inferred from pollen and macrofossil analyses, however, these analyses have inherent limitations e.g. long pollen dispersal and uncertainty in detecting small populations. Ancient DNA from lake sediments can complement these paleoecological analyses. I cored lakes in Michigan and extracted DNA from lake sediments to characterize how forests have changed over the last 10,000 years. Using a metagenomics approach, I targeted ancient chloroplast DNA from different plant species and sequenced them in Illumina. Characterizing the magnitude of past vegetation changes in response to past climate changes provides an empirical long-term record of how biodiversity was shaped through time and lends insights into the population process of range shifts.