Ancient DNA extraction methods for herbarium specimens: When is it worth the effort?

From Dormant Collections to Repositories for the Study of Habitat Changes: The Importance of Herbaria in Modern Life Sciences

In recent decades, the advent of new technologies for massive and automatized digitization, together with the availability of new methods for DNA sequencing, strongly increased the interest and relevance of herbarium collections for the study of plant biodiversity and evolution. These new approaches prompted new projects aimed at the creation of a large dataset of molecular and phenological data. This review discusses new challenges and opportunities for herbaria in the context of the numerous national projects that are currently ongoing, prompting the study of herbarium specimens for the understanding of biodiversity loss and habitat shifts as a consequence of climate changes and habitat destruction due to human activities. With regard to this, the National Biodiversity Future Center (active in Italy since 2022) started a large-scale digitization project of the Herbarium Centrale Italicum in Florence (Italy), which is the most important Italian botanical collection, consisting of more than 4 million samples at present.

Retrieval of long DNA reads from herbarium specimens

High-throughput sequencing of herbarium specimens' DNA with short-read platforms has helped explore many biological questions. Here, for the first time, we investigate the potential of using herbarium specimens as a resource for long-read DNA sequencing technologies. We use target capture of 48 low-copy nuclear loci in 12 herbarium specimens of Silene as a basis for long-read sequencing using SMRT PacBio Sequel. The samples were collected between 1932 and 2019. A simple optimization of size selection protocol enabled the retrieval of both long DNA fragments (>1 kb) and long on-target reads for nine of them. The limited sampling size does not enable statistical evaluation of the influence of specimen age to the DNA fragmentation, but our results confirm that younger samples, that is, collected after 1990, are less fragmented and have better sequencing success than specimens collected before this date. Specimens collected between 1990 and 2019 yield between 167 and 3403 on-target reads > 1 kb. They enabled recovering between 34 loci and 48 (i.e. all loci recovered). Three samples from specimens collected before 1990 did not yield on-target reads > 1 kb. The four other samples collected before this date yielded up to 144 reads and recovered up to 25 loci. Young herbarium specimens seem promising for long-read sequencing. However, older ones have partly failed. Further exploration would be necessary to statistically test and understand the potential of older material in the quest for long reads. We would encourage greatly expanding the sampling size and comparing different taxonomic groups.

Challenges and Opportunities behind the Use of Herbaria in Paleogenomics Studies

Paleogenomics focuses on the recovery, manipulation, and analysis of ancient DNA (aDNA) from historical or long-dead organisms to reconstruct and analyze their genomes. The aDNA is commonly obtained from remains found in paleontological and archaeological sites, conserved in museums, and in other archival collections. Herbarium collections represent a great source of phenotypic and genotypic information, and their exploitation has allowed for inference and clarification of previously unsolved taxonomic and systematic relationships. Moreover, herbarium specimens offered a new source for studying phenological traits in plants and for disentangling biogeography and evolutionary scenarios of species. More recently, advances in molecular technologies went in parallel with the decreasing costs of next-generation sequencing (NGS) approaches, which paved the way to the utilization of aDNA for whole-genome studies. Although many studies have been carried out combining modern analytic techniques and ancient samples, such as herbarium specimens, this research field is still relatively unexplored due to the need for improving strategies for aDNA manipulation and exploitation from ancient samples. The higher susceptibility of aDNA to degradation and contamination during herbarium conservation and manipulation and the occurrence of biochemical postmortem damage can result in a more challenging reconstruction of the original DNA sequence. Here, we review the methodological approaches that have been developed for the exploitation of historical herbarium plant materials, such as best practices for aDNA extraction, amplification, and genotyping. We also focus on some strategies to overcome the main problems related to the utilization of herbarium specimens for their exploitation in plant evolutionary studies.

A comparison of freezer-stored DNA and herbarium tissue samples for chloroplast assembly and genome skimming

Premise

The use of DNA from herbarium specimens is an increasingly important source for evolutionary studies in plant biology, particularly in cases where species are rare or difficult to obtain. Here we compare the utility of DNA from herbarium tissues to their freezer-stored DNA counterparts via the Hawaiian Plant DNA Library.

Methods

Plants collected for the Hawaiian Plant DNA Library were simultaneously accessioned as herbarium specimens at the time of collection, from 1994-2019. Paired samples were sequenced using short-read sequencing and assessed for chloroplast assembly and nuclear gene recovery.

Results

Herbarium specimen-derived DNA was statistically more fragmented than freezer-stored DNA derived from fresh tissue, leading to poorer chloroplast assembly and overall lower coverage. The number of nuclear targets recovered varied mostly by total sequencing reads per library and age of specimen, but not by storage method (herbarium or long-term freezer). Although there was evidence of DNA damage in the samples, there was no evidence that it was related to the length of time in storage, whether frozen or as herbarium specimens.

Discussion

DNA extracted from herbarium tissues will continue to be invaluable, despite being highly fragmented and degraded. Rare floras would benefit from both traditional herbarium storage methods and extracted DNA freezer banks.

Relevance of DNA preservation for future botany and ecology

The use of molecular methods in plant systematics and taxonomy has increased during the last decades; however, the accessibility of curated genetic samples and their metadata is a bottleneck for DNA-based genetic studies in botany. Plant biodiversity DNA banks and DNA-friendly collections could be critical suppliers of curated genetic material for researchers in the current context of plant biodiversity loss. Here, we aimed to understand the potential of plant DNA banks and DNA-friendly collections to enhance the growth and openness of scientific knowledge. The preservation of genetic material should become part of a natural collection's process for the generation of extended specimens enabling the preservation of both the phenotype and genotype and contributing to the generation of data networks which cross-fertilize other fields. These curated collections are advantageous in endangered species research, detecting processes related to extinction, giving a genetic dimension to IUCN assessments or completing the Leipzig Catalogue of Vascular Plants. Therefore, DNA collections are fundamental in producing FAIR data, responsible research and innovation (RRI) and meeting the goals of international conservation programmes. The completion of natural collections is important for current research efforts and furthermore vital to support future research in an era of ongoing plant biodiversity loss.