Scientific researchers have recently challenged previously-held beliefs about the origins of the Schizophyllum commune strain 20R-7-F01, long thought to be derived from subseafloor sediments over 20 million years old. New genomic analysis indicates this strain likely originated from terrestrial environments, highlighting the significance of genetic exchange and adaptation mechanisms.
During the Integrated Ocean Drilling Program's Expedition 337, scientists drilled Hole C0020A off the coast of Japan, reaching depths of 2466 meters below the seafloor. This expedition unearthed fungal and microbial communities, serving as evidence for life existing at extreme depths under extreme conditions, such as high pressure and temperature. Among the findings was strain 20R-7-F01, which was isolated from sediments believed to date back millions of years.
Historically, researchers proposed this strain could be used as a high-quality genomic model for studying evolution and environmental adaptation mechanisms. Liu et al. previously observed characteristics of the S. commune strain 20R-7-F01, noting its cultural growth over 20 million years after burial. These assertions led the scientific community to believe the strain was isolated and adapted to deep-sea environments.
To ascertain the true origins of the strain, researchers conducted extensive analyses using simple sequence repeats (SSR) of DNA. These SSRs serve as genetic markers due to their high variability and reproducibility, allowing scientists to explore genetic diversity among populations. The results showed no indicators of spatial or temporal genetic isolation for strain 20R-7-F01, contradicting its purported longstanding separation from terrestrial analogs.
This investigation revealed the strain's significant genetic similarity to contemporary East Asian species, showing particularly high rates of similarity at trinucleotide and tetranucleotide loci, which supports the hypothesis of intensive genetic exchange. This raised questions among researchers about how such genetic fluidity and commonality could exist, considering the strain's supposed isolation for millions of years.
Challenges to the status quo didn't end there. While previous studies asserted the geological age of the sediments contained indigenous S. commune members, the new findings suggest external contamination may have occurred during drilling. The drilling process was noted to inject biological material from surrounding waters, which could introduce significant variables undermining the accuracy of ancient origins claimed previously.
Conducting phylogenetic analyses on DNA from thirty-five strains revealed the evolutionary trajectories of S. commune strain 20R-7-F01 align more closely with strains from terrestrial origins rather than the deep-sea sediments. The genetic markers produced through SSR DNA analysis indicated it does not belong to the presumed ancient lineage but is more appropriately placed within the contemporary genetic diversity seen within terrestrial populations.
Sustaining the argument for the strain's terrestrial origin is corroborated by the fact many fungi, including S. commune, are adapted to survive harsh conditions but require specific environments and factors to reproduce. The extreme conditions of the ocean floor, such as high pressure and temperature, do not support the normal growth processes of this fungus. Therefore, the current evidence compels researchers to reconsider their approach to classifying the origins of such ancient microbial life.
Given these revelations, the study of S. commune strain 20R-7-F01 invites new discussions within the scientific community. The evolution of fungi, particularly their gene flow and adaptation concerning environmental pressures, remains under examination. The findings advocate for advanced methodologies when assessing the origins of organisms found at great geological depths. Such methodologies will contribute to dialogue about organism classification, evolutionary changes, and the proposed relationships between historical geographic distributions.
Consequently, this reevaluation challenges established paradigms about ancient organisms and their evolutionary paths. The researchers' work reflects a growing consensus to refine and redefine the interpretations of microbial findings unearthed through ocean drilling expeditions.