The robustness of thermal performance curves limits adaptation in growth
rate of wild bacterial strains
Abstract
Thermal adaptation of organisms is a property emerging from the complex
interplay of biophysical constraints and selective forces. The shape of
thermal performance curves has been well investigated but we lack
knowledge of how they may evolve. Two extreme cases can be expected: i)
under the hypothesis of local adaptation, species should shift their
thermal performance curves and have an optimum at the temperature at
which they evolve, or ii) under the hypothesis of thermodynamical
constraints, universal biophysical rules dictate a fixed performance
curve with an optimum at warm temperatures. We perform an evolutionary
experiment to test these two hypotheses on the thermal response of
bacteria growth rate, expecting a strong evolutionary response of the
thermal performance curve. We use four wild bacterial strains and allow
them to evolve at ten different temperatures (ranging from 8.5 to 40°C)
to subsequently measure their growth rate at these ten temperatures. We
investigate the difference in growth rate between evolved lines and
their ancestors. We observe signs of adaptation, as growth rates of
evolved and ancestral strains exhibit small but significant differences.
Our analysis shows however that the shape of the thermal performance
curves does not systematically vary between evolved and ancestral
strains, and none of the evolved lines have an optimal growth rate at
the evolution temperature. One strain grows significantly faster than
its ancestor at the temperature of evolution, but we find that for other
strains, evolution leads to faster as well as slower growth rates. These
differences are repeated between evolutionary replicates, suggesting
they are selected. Our study demonstrates that adaptation does not
always overcome thermodynamical constraints on growth rates, and helps
to better understand how microbes will respond to temperature changes.