Degradation of DNA in artiticially charred wheat seeds

In this paper we describe artificial heating experiments that were used as a model for the events that give rise to archaeological assemblages of charred grains. Samples of Triticum aestivum L. seeds were heated at 150, 200, 225 and 250 degreesC for periods up to 5 h. There were two distinct patterns of weight loss during heating, one due to expulsion of water and the other involving more substantial organic changes. Blue fluorescing material, typical of the intermediate compounds in the Maillard reaction, was observed after longer periods of heating and/or exposure to the higher temperatures. At each temperature there was a sharp initial decrease in the DNA content of the seeds followed by a gradual reduction in the rate of decay. At 150 and 200 degreesC, the DNA decay appeared to level off with very little additional breakdown occurring after 2 h, but at 225 and 250 degreesC the DNA content of the seeds reached undetectable levels at some point during the heating period. Polymerase chain reactions (PCRs) were used to study the degradation of the nuclear and mitochondrial genomes. PCRs were successful with seeds heated at 150 and 200 degreesC for all time points up to and including 5 h. At 225 and 250 degreesC, both PCRs failed after shorter periods of heating. The mitochondrial product was more resistant to heating, as expected as the mitochondrial genome is present at a higher copy number than the nuclear genome. PCR products from the mitochondrial locus were cloned and sequenced to determine if heating had any effect on the accuracy of sequence information obtained from charred wheat seeds. The frequencies of sequencing errors were similar, and low, in all six samples that were studied. All aspects of seed and DNA degradation were delayed in low-oxygen samples compared with the equivalent aerobic material. (C) 2003 Elsevier Science Ltd. All rights reserved.