Fruit flies of the Drosophila Melanogaster species were used by Montooth’s team as a model organism for investigating intracellular homeostatic mechanisms. Source: http://whyevolutionistrue.files.wordpress.com/201 1/03/mwedrosophilamelanogaster00001.jpg
Survival of the fittest is a well-established doctrine of nature. This point was reiterated by Kristi Montooth of the Montooth lab in Indiana, who prefaced her research presentation on Friday, October 18 with the following key principles. While the sheer number of diverse species on Earth serves as a testament to Nature’s variability, this seemingly infinite capacity for life is tempered by the ever-present pressures of natural selection. Only those organisms with the most advantageous of attributes will live long enough to pass on their genes to their offspring. Given its reliance on genetic inheritance, natural selection is commonly perceived as a slow process that takes place over the course of generations. However, environments can change drastically in a manner of months, weeks, and sometimes days. To this end, organisms need a way of keeping their internal states at an equilibrium, a process known as homeostasis. The Montooth Lab at Indiana University in Bloomington has performed particularly enlightening work on how homeostasis interacts with natural selection and how this interaction takes an energetic toll on the organism.
The Montooth Lab’s research on natural selection focuses on the adaptive responses of the most basic functional unit of an organism, the cell. Cell membranes are sensitive to changes in temperature. Since the membrane provides the main mechanism by which materials pass into and out of a cell, an organism must take measures to ensure that the fluidity of the membrane stays constant. The fluidity is modulated by changing the concentration of phospholipids, which pack together in varying degrees of tightness. In particular, phosphatidylethanolamine, or PE, is a fairly loosely packed component, whereas phosphatidylcholine, or PC, packs comparatively tightly. The Montooth Lab measures phenotypic plasticity, or the ability of a cell to change expression of genes based on this effect, by measuring the ratio of PC and PE in fruit flies at varying temperatures.
Populations of fruit flies were examined at 16 degrees Celsius environment, a 25 degrees Celsius environment, and an environment whose temperature changed every other generation (1). After 32-64 generations (3 years), each population was independently subjected to temperatures from both ranges: 16 and 25 degrees. The results showed that the “variable” population adapted better to the varying temperature. By having a steeper reaction curve, the “variable” population exhibited a high degree of change in their ratio relative to the change in temperature. This change, interestingly enough, manifested itself in the form of a higher ratio of PC to PE in relation to that of the other two populations. These results demonstrate the effect of natural selection on phenotypic plasticity; the varying temperature environment clearly selected the individuals with the highest plasticity for survival.
The Montooth Lab has also shown that homeostasis can come at a metabolic cost. In fruit fly larvae, one way of approximating the rate of metabolism is by measuring the rate of CO2 release. Since respiration generally involves the consumption of glucose and oxygen to produce carbon dioxide, water, and ATP, one can measure the volume of CO2 to get an idea of the relative rate of respiration, and hence metabolism. The second experiment involved the manipulation of temperature to produce an inducible heat shock response. At high temperatures, proteins begin to denature or unfold. The heat shock response is an adaptive measure used by cells to prevent denaturation. By increasing the transcription of heat shock response proteins (specifically hsp70), cells can ensure that proteins remain folded. Fruit fly larvae were subjected to a 36 degree Celsius heat shock for experimentation. For the most part, researchers found that the metabolic rate of larvae reduced noticeably for around 2 hours after the initial shock. The rate of energy usage was slightly higher than that of walking, but lower than that of flying.
These two studies, alongside further work done by the Montooth Lab, have shown that the principles of natural selection are tightly ingrained in transient adaptations like those involved in homeostasis. Thus, it is clear that while nature selects organisms for survival on a macroscopic scale, there is also a large degree of intracellular adaptation that reflects the pressures of a continually changing environment.
1. Cooper, H. M. (2013). Cellular Adaptation to Thermal Heterogeneity in Natural Populations of Drosophila Melanogaster.