Small RNA Activity in Archeological Barley Shows Novel Germination Inhibition in Response to Environment.

The recovery of ancient RNA from archeological material could enable the direct study of microevolutionary processes. Small RNAs are a rich source of information because their small size is compatible with biomolecular preservation, and their roles in gene regulation make them likely foci of evolutionary change. We present here the small RNA fraction from a sample of archeological barley generated using high-throughput sequencing that has previously been associated with localized adaptation to drought. Its microRNA profile is broadly similar to 19 globally distributed modern barley samples with the exception of three microRNAs (miRNA159, miRNA319, and miR396), all of which are known to have variable expression under stress conditions. We also found retrotransposon activity to be significantly reduced in the archeological barley compared with the controls, where one would expect the opposite under stress conditions. We suggest that the archeological barley's conflicting stress signals could be the result of long-term adaptation to its local environment.

Genomic methylation patterns in archaeological barley show de-methylation as a time-dependent diagenetic process.

Genomic methylation is variable under biotic and abiotic stresses in plants. In particular, viral infection is thought to significantly increase genomic methylation with particularly high activity around transposable elements. Here we present the genomic methylation profiles of grains of archaeological barley (Hordeum vulgare) from several strata from a site in southern Egypt, from the Napatan to the Islamic periods (800 BCE - 1812 CE). One sample tested positive for viral infection and exhibits an unusually high degree of genomic methylation compared to the rest. A decreasing trend in global methylation levels according to deposition date shows in-situ de-methylation of 5-methylcytosine, which can be described as a diagenetic process. This is most likely a deamination mediated de-methylation process and is expected to lead to 5 mC > T base modifications in addition to the C > U modifications due to cytosine deamination, so represents a time-dependent process of DNA diagenesis in ancient DNA.

Archaeogenomic evidence of punctuated genome evolution in Gossypium.

Transposable elements (TEs) are drivers of evolution resulting in episodic surges of genetic innovation and genomic reorganization (Oliver KR, Greene WK. 2009. TEs: powerful facilitators of evolution. Bioessays 31:703-714.), but there is little evidence of the timescale in which this process has occurred (Gingerich PD. 2009. Rates of evolution. Ann Rev Ecol Evol Syst. 40:657-675.). The paleontological and archaeological records provide direct evidence for how evolution has proceeded in the past, which can be accessed through ancient DNA to examine genomes using high-throughput sequencing technologies (Palmer SA, Smith O, Allaby RG. 2011. The blossoming of plant archaeogenetics. Ann Anat. 194:146-156.). In this study, we report shotgun sequencing of four archaeological samples of cotton using the GS 454 FLX platform, which enabled reconstruction of the TE composition of these past genomes and species identification. From this, a picture of lineage specific evolutionary patterns emerged, even over the relatively short timescale of a few thousand years. Genomic stability was observed between South American Gossypium barbadense samples separated by over 2,000 miles and 3,000 years. In contrast, the TE composition of ancient Nubian cotton, identified as G. herbaceum, differed dramatically from that of modern G. herbaceum and resembled closely the A genome of the New World tetraploids. Our analysis has directly shown that considerable genomic reorganization has occurred within the history of a domesticated plant species while genomic stability has occurred in closely related species. A pattern of episodes of rapid change and periods of stability is expected of punctuated evolution. This observation is important to understanding the process of evolution under domestication.

Archaeogenetic evidence of ancient nubian barley evolution from six to two-row indicates local adaptation.

BACKGROUND: Archaeobotanical samples of barley (Hordeum vulgare L.) found at Qasr Ibrim display a two-row phenotype that is unique to the region of archaeological sites upriver of the first cataract of the Nile, characterised by the development of distinctive lateral bracts. The phenotype occurs throughout all strata at Qasr Ibrim, which range in age from 3000 to a few hundred years. METHODOLOGY AND FINDINGS: We extracted ancient DNA from barley samples from the entire range of occupancy of the site, and studied the Vrs1 gene responsible for row number in extant barley. Surprisingly, we found a discord between the genotype and phenotype in all samples; all the barley had a genotype consistent with the six-row condition. These results indicate a six-row ancestry for the Qasr Ibrim barley, followed by a reassertion of the two-row condition. Modelling demonstrates that this sequence of evolutionary events requires a strong selection pressure. CONCLUSIONS: The two-row phenotype at Qasr Ibrim is caused by a different mechanism to that in extant barley. The strength of selection required for this mechanism to prevail indicates that the barley became locally adapted in the region in response to a local selection pressure. The consistency of the genotype/phenotype discord over time supports a scenario of adoption of this barley type by successive cultures, rather than the importation of new barley varieties associated with individual cultures.