Heat shocking adaptive evolution…

Research bloggingIN evolutionary theory there is a phenomenon known as canalisation, a process in which the phenotype (i.e. the outward physical appearance of an organism) remains unchanged, despite genetic or environmental influences.  This suggests that a mechanism exists to buffer the physical appearance from such changes, which may explain why some species can remain mostly unchanged for millions of years.

The buffering afforded by this mechanism permits the accumulation of genetic variation, in effect storing it up like an evolutionary capacitor. Also, presumably the accumulated genetic variation may be released by an event that overcomes the evolutionary capacitor, releasing fuel (in the form of variation) that provides a substrate for natural selection and potentially accelerating evolution. But how?

The idea of capacitance was first suggested by Rutherford and Lindquist 1 following experiments on a protein called heat shock protein 90 (Hsp90) in fruitflies. Generally speaking, heat shock proteins assist in the maintenance and correct folding of cellular proteins, especially when under temperature stress; Hsp90 plays a particular role in maintaining the unstable signalling proteins that act as key regulators of growth and development.

They suggested that in nature, a stressing event such as high or low temperatures may overcome the protective buffering effect that Hsp90 has on maintaining these key regulators. As Hsp90 becomes diverted from its usual role, due to an increase of stress-damaged proteins in the cell, those cell signalling proteins it normally maintains are free to adopt a range of altered behaviours, interfering with the development of the organism. The result is morphological variants upon which natural selection can act. Rutherford and Lindquist found as much, with chemically and environmentally compromised Hsp90 resulting in flies with abnormal wings, legs or eyes, they observed a broad variety of phenotypes.

Rutherford and Lindquist went on to demonstrate that the capacity for such remarkable variation was pre-existing, i.e. it was encoded genetically prior to the stressing event, but had been silenced. Evolutionary capacitance may therefore provide a mechanism of adaptive evolution in which a population under stress may release previously silent variation, resulting in the appearance of certain individuals with more desirable traits in that changed environment. When such revealed traits are selected for they can become fixed and independently of the buffering action of Hsp90.

This week, in a letter to Nature, Valeria Specchia et al.2 report some fascinating evidence that indicates that beyond merely acting as a gate-keeper to unleash variation, mutations of Hsp90 that compromise its functionality result in new, rather than pre-exisiting, variation. They observed that mutations in Hsp90 affect the production of piRNAs. These are small RNA molecules that are involved in the silencing of genes, particularly those involved in development, i.e. sex cells like eggs and sperm, and all the cell types that give rise to these cells. These piRNAs are also responsible for repressing genetic elements called transposons.

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