A Mg²⁺-dependent high-yield method for extracting high-molecular-weight genomic DNA from a single planarian specimen

The isolation of intact, high-molecular-weight genomic DNA (HMW gDNA) is essential for achieving complete genome assemblies. However, extracting HMW gDNA from a single individual of Dugesia japonica remains a technical challenge using the standard protocol, probably due to the presence of abundant polysaccharides and nucleases.

In this study, we have developed a more robust protocol for preparing HMM gDNA, with high yields and quality, from a single D. japonica. The key step in our protocol involves the use of a Mg²⁺-dependent lysis buffer, rather than using metal cation chelation to block the activities of DNase I as in the standard protocol. Using this approach were able to achieve a yield of about 10-15 µg of HWM gDNA per worm. Our method showed species- and region-specific effectiveness, with optimal results observed at 20 mM Mg²⁺ for our local D. japonica specimens. The extracted HMW gDNA is fully compatible with advanced long-read sequencing platforms such as PacBio HiFi and Oxford Nanopore. However, when applied to Schmidtea mediterranea and D. japonica specimens from Beijing, the method was ineffective and led to progressive gDNA degradation.

This protocol offers a simple and high-yield solution for isolating HMW gDNA from D. japonica. It also provides an alternative for organisms whose gDNA consistently exhibits unexplained degradation using established protocols.