Enabling Measurement of Darwinian Evolution in Space

A common definition of life is a “self-sustaining chemical system capable of Darwinian evolution”, or natural selection of inherited variations that contribute to survival and reproduction. Thus, measuring Darwinian evolution would seem highly relevant to searching for life beyond Earth. While it is now feasible to track evolution in the laboratory, such an experiment has not yet been reported in space. Prior work has demonstrated the ability of microorganisms to adapt to multiple extremes relevant to space and potentially habitable niches beyond Earth. For example, Wassmann and colleagues cultured the gram-positive bacterium Bacillus subtilis 168 under the selective pressure of ultraviolet (UV) radiation (200–400 nm) akin to that found on Mars over 700 generations; the resulting population was found to be significantly more resistant than the ancestral line to both UV and other stresses, including increased salinity, vacuum, desiccation, and ionizing radiation. More recently, Tirumalai et al. cultured Escherichia coli in ground-based low-shear modeled microgravity for 1000 generations revealing 5 coding mutations of not-yet-characterized significance. Miniaturization of nucleic acid extraction and sequencing technologies is enabling development of space instruments targeting nucleic acids, including work by our group and recent use of a nanopore sequencer on the International Space Station (ISS). In addition, NASA plans to deploy a deep space cubesat, BioSentinel, with a biological payload, to characterize the ability of yeast to carry out DNA repair in space. Critically, this system demonstrates the capability to initiate, sustain, and characterize biological systems in space-compatible formats. Here we describe how these advances can be integrated to enable in-situ measurement of Darwinian evolution, with applications for understanding adaptation to space and for future life detection missions. Specifically, we focus on applying nanopore sequencing to detect and characterize evolution in the lab and propose a system to autonomously measure evolution in space as an extension of the Search for Extraterrestrial Genomes (SETG), an instrument under development for in-situ nucleic acid-based life detection. The minimal approach would involve sequencing before and after a culturing period during which organisms of interest would be exposed to a simulated or actual stressor. An integrated system for measuring Darwinian evolution in space would not only allow for definitive measurement of nucleic-acid based life; it could also be used to improve understanding of microbial life's ability to adapt to the harsh conditions of space and, in doing so, support human health beyond Earth and inform future use of synthetic-biology during deep space missions.