Tales of a living fossil: from evolution to past environmental stress to persistence to future global change

Rates of biodiversity loss are greater than at any other time in history. This loss has been linked to human activities such as gas emissions increasing global warming, use of pesticides and herbicides leading to eutrophication and pollution, urbanization and deforestation leading to habitat fragmentation. Because of this rapid environmental change exasperated by human activities, resilience of natural populations is at risk. Yet, we know remarkably little about the mechanisms of resilience, and how to predict when it might occur. This is because long-term studies are lacking. To circumvent the lack of long-term studies, the use of ‘space-for-time’ is often adopted, although this substitution is quite controversial.
Travelling back in time to study processes that triggered evolutionary responses and led to population persistence is impossible unless we chose the resurrection route, by which organisms from different times in the past are brought back to life and their evolutionary responses measured. This sounds like Jurassic Park but it is possible for a number of organisms, including the waterflea Daphnia. Daphnia is a keystone species in inland water habitats and adopts a unique life cycle which involves the production of dormant embryos in times of stress. Many of these embryos become buried in the mud at the bottom of lakes before they have a chance to hatch and remain viable for decades or centuries. Time travelling using dormant stages is one of the most powerful ways of tracking long-term evolutionary dynamics in response to environmental change. The study of resurrected dormant embryos enables us to study population persistence under pressure from past environmental changes and to predict adaptation to future global change as a function of each population’s evolutionary memory.

Capitalizing on the well-defined role of Daphnia spp. in sustaining freshwater ecosystems and its unique life cycle that enables us to perform common garden experiments on ancestral (extinct) and modern populations, we have gained unprecedented insight into natural patterns of evolution and into how history of local adaptation and phenotypic plasticity can orchestrate a response to future climate change. Using high throughput technologies we have uncovered the gene regulatory networks that coordinate early adaptive response to environmental stress revealing that genes are organized into co-responsive modules coherently co-expressed across numerous conditions and genetic backgrounds. Capitalizing on the ability of Daphnia to ‘remember’ past exposure to environmental stress, my coworkers and I suggest to use extinct Daphnia populations that experienced extreme environmental shifts or artificially evolved populations under controlled laboratory settings as an effective bioremediation strategy for polluted inland waters.