Adaptation to abiotic stressors and environmental change is imperative to survival in a rapidly changing world. Dr. Reid Brennan, an ecological geneticist and postdoctoral scholar in the Pespeni Lab at University of Vermont, presented his research in aquatic systems to explore the genomic basis of populations’ responses to these stressors over short- and long- time periods.
Dr. Brennan then talked about the rapid adaptation to ocean acidification in sea urchins. He pointed out another aspect of climate change in addition to the constant increase in temperatures is ocean acidification. More CO2 dissolved in the atmosphere results in higher concentrations of CO2 in oceans. This acidification causes a decrease in available minerals that some marine animals use to make their shells. Therefore, this is a selective pressure on those species. To see how these organisms will respond to this change over time, he and colleagues used a single generation experiment with the purple sea urchins. Purple sea urchins occupy the entire western coast of the North America. They also respond to experimental pH selection in the lab. Their ability to adapt makes them useful to try to understand what limits or drives the adaptation mechanism. They used two different treatments representing the two selective pressures that might occur in the wild; moderate acidification (pH 8.0), extreme events (pH 7.5).
Dr. Brennan first talked about the morphological consequences of these selective pressures. Urchins under extreme conditions had lower mean body length (Figure 2) . He stated that the low pH levels stunt the growth of the sea urchins. Under different treatments the density of allele frequencies shifted towards low starting allele frequency. Meaning that although some genetic variants are rare, under different treatments, they might become adaptive. He then stated this shift towards rare genetic variation indicates that large population sizes are important for preserving this adaptive genetic variation.
These results demonstrate that after only three generations, copepods adapted to their new environmental conditions through changes in their genetic structure. However, the mechanisms of adaptation were different for each group. In the ambient line, copepods adapted to their environment through shifts in gene expression over multiple generations. Conversely, copepods originating in the high temperature, high CO2 environment thrived in the ambient conditions due to shifts in allele frequency. This equates to an overall loss of genetic diversity in the population. So, the rapid changes undertaken to adapt to simulated climate change conditions actually reduced genetic diversity. Consequently, this population would have less ability to respond to future environmental changes.
These studies demonstrate that impacts of global change on oceans could result in rapid adaptation, selection, and evolution of aquatic organisms. These changes are evident on a genomic level, even after just one or a few generations. Changes of this type and magnitude can reduce overall population genetic diversity, and potential loss of species-wide resilience to subsequent changes.
Brennan, R.S., Garrett, A.D., Huber, K.E., Hargarten, H., Pespeni, M.H., (2018). Rare genetic variation is important for survival in extreme low pH conditions. BioRXiv. doi: https://doi.org/10.1101/422261
Brennan, R.S., Healy, T.M., Bryant, H.J., Van La, M., Schulte, P.M. and Whitehead, A.,. Genome-wide selection scans integrated with association mapping reveal mechanisms of physiological adaptation across a salinity gradient in killifish. Molecular Biology and Evolution. In press.