Heat Tolerance in an Aphid...

This forum is for the discussion of the evidence for evolution. Anyone is welcome to post, however, scripture is not allowed. As the title says, Science Only please!

Moderator: Moderators

Heat Tolerance in an Aphid...

Postby Timothy Chase » Wed Apr 11, 2007 5:16 pm

One of the more interesting stories of late: an aphid's tolerance to heat is determined by a single point mutation (the absence of a nucleotide in a promoter) in a bacterial symbiont which supplies it with a heat shock protein. The shorter form of the promoter consists of a simple sequence repeat of twelve adenines whereas the typical, longer form consists of thirteen adenines. The longer form results in the expression of a heat shock protein which confers resistance to higher temperatures, leaving the aphid fertile whereas the shorter form results in increased fertility at lower temperatures.

Evolution of symbiosis (Non-Tech)
http://www.eurekalert.org/pub_releases/ ... 040307.php

Aphid Thermal Tolerance Is Governed by a Point Mutation in Bacterial Symbionts (Tech)
Helen E. Dunbar, Alex C. C. Wilson¤, Nicole R. Ferguson, Nancy A. Moran
PLoS Biol 5(5): e96 doi:10.1371/journal.pbio.0050096
http://biology.plosjournals.org/perlser ... io.0050096

*

The Broader Picture...

Simple sequence repeats tend to be subject to a form of hypermutation, with some stretches being subject to a rate of mutation perhaps as much as a hundred thousand times above the normal background rate. However, the prevailence of such hypermutation will depend in part upon the error-correcting mechanisms in use by a given species and in part the specific motif (GT in a GTGTGT... stretch), and the number of times the motif has already been repeated. Some repair enzymes themselves have simple sequence repeats - and their consequent mutation will determine their sensitivity to repeats during the duplication of the DNA sequence. Mononucleotide stretches are especially prone to mutation.

Non-trinucleotide repeats are almost entirely absent in coding regions as they would be selected against due to the frameshift errors they would give rise to, prematurely stopping the transcription of a polypeptide and consequently the protein that the gene expresses - which would be selected against. Trinucleotide repeats are quite common in the coding regions for metabolic proteins. Thus, for example, trinucleotide repeats found in coding regions were implicated in the variation between different breeds of dog. However, mononucleotide and dinucleotide repeats are quite common in promoters, and consequently determine the extent to which transcription is dependent upon the presence of transcription factors (i.e., regulatory proteins). Mononucleotide and dinucleotide repeats are also common in introns, consequently giving rise to variations in alternate splicing, altering which proteins (isoforms) get expressed by a given gene.

In the case of a polyadenine stretch, the tract curves towards the minor groove of the DNA structure, but the curvature is relaxed at higher temperatures. This suggests that the promoter's binding properties are due to its shape rather than specific chemical bonds.

In eukaryotes, the origin of a polyadenine stretch (or other simple sequence repeat) would likely be attributed to template switching by either a LINE (long interspersed nuclear element) retrotransposon or the SINEs (short interspersed nuclear element) which they generate and lend their retrotransposase enzyme to - resulting in the retrotransposition of the SINEs, where the retrotransposon itself would be a decayed copy of an endogenised retrotrovirus. Retroviruses are absent bacteria. However, type I and type II introns are present and are capable of retrotransposition. Moreover, type II (which are most closely related to the spliceosomal introns found in eukaryotes) will have poly-tails similar to line-1 retrotransposons and may consequently give rise to mono- and polynucleotide simple sequence repeats.

It widely believed that retroviruses descended from type II introns - evolving in response to the evolution of multicellular life.
User avatar
Timothy Chase
 
Posts: 532
Joined: Sat Oct 07, 2006 4:59 pm

Postby Timothy Chase » Wed Apr 11, 2007 6:54 pm

PS

Sorry for the edits. And yes, in case any one suspected, I was showing off a bit - going off memory. But I like the big picture. It is something which has taken me a while to come by, although I hadn't touched on it for a month or two. Anyway, I hope to do a piece or two along these LINEs in a little bit which will be a SINE of our increasing understanding of the process of evolution.

PPS

The last sentence was retroelement humour...
User avatar
Timothy Chase
 
Posts: 532
Joined: Sat Oct 07, 2006 4:59 pm

Re: Heat Tolerance in an Aphid...

Postby marc » Sat Apr 21, 2007 11:07 am

Timothy Chase wrote:One of the more interesting stories of late: an aphid's tolerance to heat is determined by a single point mutation (the absence of a nucleotide in a promoter) in a bacterial symbiont which supplies it with a heat shock protein. The shorter form of the promoter consists of a simple sequence repeat of twelve adenines whereas the typical, longer form consists of thirteen adenines. The longer form results in the expression of a heat shock protein which confers resistance to higher temperatures, leaving the aphid fertile whereas the shorter form results in increased fertility at lower temperatures.

:<snip

It widely believed that retroviruses descended from type II introns - evolving in response to the evolution of multicellular life.


Timothy,
Thanks for posting this. It is a great example of the genetic basis for adaptability. Of course as a creationist biologist I understand this to be an example of how the genome is designed to enable organisms to respond to environmental changes.
Marc
marc
 
Posts: 5
Joined: Wed Dec 06, 2006 2:47 pm

Re: Heat Tolerance in an Aphid...

Postby Roger Stanyard » Sat Apr 21, 2007 11:48 am

marc wrote:Timothy,
Thanks for posting this. It is a great example of the genetic basis for adaptability. Of course as a creationist biologist I understand this to be an example of how the genome is designed to enable organisms to respond to environmental changes.


But you are not a creationist biologist. You gave up on science years ago and you run a shop in Edinburgh.
User avatar
Roger Stanyard
Forum Admin
 
Posts: 6162
Joined: Sat Oct 07, 2006 4:59 pm


Return to Science Only

Who is online

Users browsing this forum: No registered users and 5 guests