Innovations that changed mammalogy: frozen tissue collections

Abstract Freezing has proven to be an ideal means for preserving tissues for molecular research on mammals. Here, we trace the origins, early practices, and rapid growth of frozen tissue collections to better understand the history, science, and people that led to the early development and recent growth of these internationally valuable research resources. Frozen tissue collections grew gradually from rudimentary beginnings in the early 1970s but have expanded rapidly in recent years, providing data for studies on biodiversity, phylogenetics, emerging pathogens, isotopes, and contaminants, among other avenues of research. Over the decades, field collecting and tissue storage have changed significantly. In the mid-1980s, for example, the development of polymerase chain reaction—which allowed researchers to amplify a specific segment of DNA many times over—reinforced the importance of frozen tissue resources to the burgeoning field of molecular genetics. Ultracold mechanical freezers that hold tissues at −80 °C became, and remain, popular because of their ease of use and delayed DNA degradation. Portable liquid nitrogen dewars have evolved from rudimentary metal tanks to compact “dry shippers” with an absorbent liner that can maintain tissues at low temperatures for 30 d without being refilled and are commonplace in the field. Larger, vapor-phase nitrogen cryotanks are becoming increasingly popular for long-term tissue preservation because they safeguard tissues from thermal fluctuation that may be induced by temporary electrical failures. More than 1.5 million tissue samples associated with museum voucher preparations are now preserved in research collections worldwide, allowing for in-depth study of mammalian genetics and investigation of research questions that were unimaginable even a few years ago.