Preservation of nuclear but not chloroplast DNA in archaeological assemblages of charred wheat grains

We prepared extracts from a sample of charred wheat grains, 2000 years in age, using a standard DNA preparation method in combination with various post-purification procedures designed to remove inhibitors of the polymerase chain reaction (PCR). The resulting extracts gave amplification products when PCRs were directed at the Glu genes, which are located on the wheat nuclear genome. However, repeated attempts to amplify the chloroplast rbcL locus were unsuccessful with these extracts, even though the rbcL PCR system gave positive results with as little as 8.3 pg of modern DNA, compared with a detection limit of 53 pg for the Glu PCRs. We conclude that there is a differential preservation of nuclear and chloroplast DNA in charred wheat grains.