Hot alkaline lysis gDNA extraction from formalin-fixed archival tissues.

Formalin fixation of natural history specimens and histopathological material has historically been viewed as an impediment to successful genomic analysis. However, the development of extraction methods specifically tailored to contend with heavily crosslinked archival tissues, re-contextualises millions of previously overlooked specimens as viable molecular assets. Here, we present an easy-to-follow protocol for screening archival wet specimens for molecular viability and subsequent genomic DNA extraction suitable for sequencing. The protocol begins with non-destructive assessment of specimen degradation and preservation media conditions to allow both museum curators and researchers to select specimens most likely to yield an acceptable proportion (20-60%) of mappable endogenous DNA during short-read DNA sequencing. The extraction protocol uses hot alkaline lysis in buffer (0.1M NaOH, 1% SDS, pH 13) to simultaneously lyse and de-crosslink the tissue. To maximise DNA recovery, phenol:chloroform extraction is coupled with a small-fragment optimised SPRI bead clean up. Applied to well-preserved archival tissues, the protocol can yield 1-2 μg DNA per 50 mg of tissue with mean fragment sizes typically ranging from 50-150 bp, which is suitable to recover genomic DNA sufficient to reconstruct complete mitochondrial genomes and achieve up to 25X nuclear genome coverage. We provide guidance for read mapping to a reference genome and discuss the limitations of relying on small fragments for SNP genotyping and de novo genome assembly. This protocol opens the door to broader-scale genetic and phylogenetic analysis of historical specimens, contributing to a deeper understanding of evolutionary trends and adaptation in response to changing environments.