On BioScience and Life and Such

Posts Tagged ‘canalization’

Once again Hsp90 changes how we think about evolution

In Uncategorized on July 14, 2008 at 1:37 pm

post to news.thinkgene.com

Hsp90 and it’s possible role in evolution (as a capacitor for rapid change), I have covered extensively in the  5 post series for JustScience week 08 (Revolution Evolution, Presenting….Hsp90, How can chaperones act in evolution, Evidence for Hsp90 involvement in rapid evolution of new traits and Hsp90 to end controversies in evolution theory). Recently I found this thorough review on the subject from which I would like to share the essentials (review written by Roberta L Millstein at University of California, Davis):

Recent work on the heat-shock protein Hsp90 by Rutherford and Lindquist …. has been included among the pieces of evidence taken to show the essential role of developmental processes in evolution;

To recap, the theory is that heat shock proteins can hide genetic variation until a stressful environment exposes them to allow rapid change (evolution) of morphology and subsequently, traits.

Hsp90 acts as a buffer against phenotypic variation, allowing genotypic variation to build. When the buffering capacity of Hsp90 is altered (e.g., in nature, by mutation or environmental stress), the genetic variation is “revealed,” manifesting itself as phenotypic variation.

The theory is backed up by genetic experiments on Drosophila and Arabidopsis. Results from this research on Hsp90 lends support to channeling and “hopeful monster” theory and as such, follows the more controversial line of evolution-thinking. The review sums up many of the controversial sides of conclusions from the Hsp90 research:

This phenomenon raises questions about the genetic variation before and after what I will call a “revelation event”: Is it neutral, nearly neutral, or non-neutral (i.e., strongly deleterious or strongly advantageous)? Moreover, what kinds of evolutionary processes do we take to be at work?

My goal with the previous posts on Hsp90 was to show that the data lends sufficient support to alter and revise how we think about evolution. It seems this is the goal of the review as well.

The primary goal of this paper is to illuminate the alternative scenarios and the processes operating in each. At the end, I raise the possibility of a synthesis between evo-devo and nearly neutral evolution.

Evolution I strongly believe, is not entirely and exclusively driven by a random (and slow) constant mutation rate, but rather controlled by a number of additional mechanisms to ensure that an organism can evolve rapidly. To me, this is not controversial at all, – it does not overturn any Darwinian principles, but serves as an extention to explain the speed of evolution that has sometimes baffled us. Nevertheless, conclusions drawn from Hsp90 research remains controversial to many evolutionists, and Millstein sums it up with:

I find it somewhat ironic that people who are otherwise unorthodox in their thinking with respect to evolution are so orthodox when it comes to adaptationism. After all, as the late Gould argued, nonadaptive approaches were left out of the evolutionary synthesis (Gould 1983) just as developmental processes were (Gould 2002).

Which to me, a molecular biologist gone amateur evolutionist, is a good ending note. Reviews like this, one can only hope, will lend credibility to alternative thoughts on mechanisms of Darwinian evolution. Which is surely needed to fully understand the beauty and complexity of the molecular mechanisms that shapes our world.

Note: the review I have linked to is open access, but apparantly only a draft, the final version is available here, but isn’t open access (shame on you Biological Theory and MIT press Journals).

Hsp90 to end controversies in evolution theory (final chapter, blogging in Just Science 08)

In Uncategorized on February 8, 2008 at 9:42 am

Previous posts have shown Hsp90 to be a molecular buffer allowing rapid morphological changes in times of stress. As will be discussed below, such a buffering function supports the evolutionary theories of punctuated equilibria, hopeful monsters and canalization.

So…, this last post will end with the final conclusions based on the arguments presented in the previous 4 posts. But, first….Two fundamental questions:

1. Even if Hsp90 can promote rapid changes in phenotype (appearance) how is this change retained (fixed) for future generations ?

This fixation has been demonstrated to occur (see Sangster TA et al.), and the traits become independent of Hsp90. The exact mechanism(s) however remains to be elucidated.

Nevertheless, temporarily compromising Hsp90 function (either by drugs or by temperature rise) is sufficient for fixing new traits. Simulations seem to show that knocking out the genes for key proteins (not necessarily heat shock proteins) lead to increased phenotypic diversity, and thus the underlying cause may be genetic fixation. However, interplay between epigenetic and genetic mechanisms has been suggested and been backed up by experiments. Thus fixation probably happens through yet to determined genetic as well as epigenetic mechanisms, or a combination of both. A model for epigenetic fixation is given in the thumbnail below:

Epigenetic evolution through Hsp90

Models for genetic fixation follows the theory of canalization with Hsp90 functioning as the Waddington’s widget (see Semin Cell Dev Biol. 2003 Oct;14(5):301-10). This is discussed further under the next bulletpoint, the second question…..

2. Does these aspects of Hsp90-function fit into current models of evolution ?

Yes, although some of these theories are controversial. First we have the idea of punctuated equilibrium and hopeful monsters discussed in my previous post. To expand on these ideas let’s also include the theory of canalization. Canalization explains punctuated equilibrium by referring to an organisms buffering capacity (to counter the potential deleterious effects of mutations). The theory was put forward by C. H. Waddington more than 50 years ago, but is still controversial it seems. Hsp90 is a molecular explanation of the canalization concept in that organisms with different genotypes express the same phenotype until times of stress. There are also indications that other heat shock proteins or other “signaling hub”-proteins or even miRNA can serve such buffering functions (see references within this review).

Taken together, these controversial evolutionary theories and the experimental evidence on Hsp90 supports one another, and a paradigm shift in evolutionary biology is in place. Darwins theories are correct up to the point of gradual and constant evolution of traits. Evolution instead, occurs in bursts. This series of blogposts have conveyed the molecular evidence for such punctuated equlibria and canalization, which comes from studies on the molecular chaperone Hsp90. I hope I have enlightened and convinced at least some evolution biologists into believing that Darwins theories can be expanded to include these (no longer controversial) theories.

There are however, a lot to work out in terms of the underlying molecular mechanisms for Hsp90 (and/or other buffering bioactive molecules ?) in canalization. To end this blogpost-series I will therefore quote the closing remarks from Salathia N and Queitsch C‘s review in 2007:

“Clearly, organisms have succeeded in integrating multiple canalization mechanisms into robust wild-type phenotypes which can respond appropriately to environmental perturbations and evolve new shapes and functions over time. Now it is up to us to determine how molecules as diverse as a molecular chaperone, chromatin remodeling proteins, and the RNAi machinery interact coherently to achieve such synergy, a truly fascinating and worthy field of future inquiry.”