Most people remember learning about chlorophyll in school. Chlorophyll (Klor-o-fill) of course is the molecule in plants that makes them appear green, and it is central to the process by which plants capture light and turn it into energy. That energy is stored by the plant in foods like oils, starches, proteins, and Apple Jacks that are the basis of all life on earth (especially the life of my 10-year-old son). What you may not remember learning is that chlorophyll is kept inside little compartments in the cell called chloroplasts.
As a geneticist, I think chloroplasts are especially cool because in addition to chlorophyll, they also contain DNA. Chloroplast DNA is not the usual DNA, which is stored in a special compartment called a nucleus which, in a way, is like the brain of a cell. DNA in the nucleus of a plant cell is like our DNA- one copy from our female parent (in humans “mom”) and another from our male parent (“Dad”). But the DNA in chloroplasts only comes from the female parent; a special present just from mom, so to speak. For plant geneticists, this characteristic of the DNA in chloroplasts, along with some other peculiar features of chloroplast DNA, make it especially useful for understanding the long-term history of a species. Chloroplast DNA can tell us things about the relatedness of organisms that the “normal” DNA (in the nucleus) can’t tell us. For example, chloroplast DNA is often used to determine how many places a species used as a refuge during the most recent time when glaciers covered Indiana (that was about 40,000 to about 12,000 years ago). During that glacial period, the native plants of Indiana were pushed into small pockets where there were habitats that allowed them to hang on during the long (25,000 year) wintry episode. These little pockets of Indiana-like habitat were called refugia. In how many refugia did black walnut hang out, and why does it matter? It turns out, chloroplast DNA can tell us, and it matters because during those 25,000 years the walnut in one refuge evolved and changed compared to the walnut in another refuge.
Later, as the ice receded and walnut and other trees advanced out of their refugia to occupy the eastern U.S. again, the genetic diversity of the trees from the different refugia was distinct, though the trees were still the same species. Those trees whose mothers had been in one refuge inherited chloroplast DNA different than those trees whose mothers had been in a different refuge. Often, trees whose ancestors resided in one refuge can be found to occupy specific regions or habitat types different than trees derived from a different refuge. For example, if one of the glacial refugia was a little dryer and warmer than the others, then trees that derived from those mother trees might be better adapted to dryer and warmer environments. All sorts of genetic differences in walnut might be traced back to those years spent in glacial refugia, so it would be convenient for us to know how many refugia there were, and where the descendants of each refuge ended up. To do that, we need more information about the DNA in the chloroplast of walnut. That’s just one of my summer projects, and part of the research at the Hardwood Tree Improvement and Regeneration Center (HTIRC) in the Department of Forestry and Natural Resources at Purdue University.