Recent studies of DNA sequences of land plants have uncovered a phenomenon that is rare in the tree of life. Tropical ferns in the family Marattiaceae, an ancient lineage of plants with fossils dating back roughly 300 million years, are hypothesized to have extremely slow rates of DNA sequence change through evolutionary time, relative to other plants. These ferns have often been called "living fossils" in the traditional morphological sense: living plants are nearly indistinguishable from some ancient fossils. This is occasionally observed in plant groups, however marattioid ferns are theorized to differ from conventional patterns of molecular evolution observed in other living fossil groups by also having low rates of sequence change through time, making these ferns "molecular living fossils". This research builds an extensive library of DNA sequences for living representatives of the marattioid ferns to: (1) assess if the molecular living fossil hypothesis holds up under close scrutiny with thorough sampling, (2) locate the area(s) within the genome where this phenomenon is taking place, (3) isolate potential causal factors for constraints on DNA sequence mutation, and (4) better understand the evolutionary history of this unique family of plants. In order to achieve these goals, DNA sequences and morphological characters will be used to create a family tree or "phylogeny" of the marattioid ferns and statistical methods will be used in combination with known fossil dates to allow assessment of rates of change through time for this family of plants. Using DNA sequences from different areas of the genome will allow detection of rate differences within the genome telling us where the hypothesized molecular living fossil phenomenon is acting and with what strength.
While it has long been understood by biologists that branches of the tree of life exhibit different rates of change through time, little is known about the mechanisms that underlie these rate differences. Significant amounts of research have gone into studying groups that appear to evolve more quickly than others, yet few have approached this problem from the opposite angle by studying groups that appear to hardly change at all through time. Understanding mechanisms that constrain the rate of mutation in DNA sequence is currently an important area of study; this research is a first step in this direction and lays the groundwork for future explorations in this area. This project will perform the traditional function of a systematic study by providing details of the evolutionary relationships of an important but poorly studied group of plants, and will extend the boundaries of systematics by providing insights into the underlying machinery of molecular evolution.
