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  Topic: Blood-clotting, evolution, and Behe, References, links, and material on above< Next Oldest | Next Newest >  

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Joined: May 2002

(Permalink) Posted: May 24 2002,23:51   

A somewhat interesting but mildly confused article in Geology News, "Geoscience at the BA: Clots have been with us for 400 million years", reporting on a talk by Colin Davidson (Imperial College School of Medicine) on the evolution of blood-clotting, especially regarding the role of large (genome or chromosome) duplications.  I say the article is confused as punctuated equilibrium is invoked as having something to do with the situation, which AFAICT it doesn't, and as none of the connections of blood-clotting to more ancient protease cascade systems are discussed.  The interesting quotes concern gene duplication:


Evolution at molecular level

The basic tenets of molecular biology make understanding evolution at the molecular level possible. Genes are encoded by DNA, also known as nucleic acid, and composed of long strings of four biochemical units termed bases (shortened to A, G, C, and T). The linear order or sequence of these bases encodes special signals for starting and stopping as well code-script read in triplets for amino acids.

These chains of amino acids - of which there are 20 - form proteins and are the products of most (but not all) genes. We can, therefore, determine the determine the protein amino acid sequence of genes by decoding the triplet DNA sequences and compare it to any other gene. The more evolutionarily similar two organisms, the greater the similarity of amino acid sequences when comparing homologous genes. This is because there has been less time for the natural mutation process - such as inborn errors in DNA replication - to act on a gene's DNA sequence. When comparing most human and chimpanzee proteins one finds 98 to 100% identity, but the same proteins shared with bacteria the identity drops between 30 to 60% or less.

Proteins also exist in the genomes of most organisms that share a high degree of identity, and who share a common ancestor. The best understood mechanism of gene duplication occurs during the production of gametes in sexual organisms. This process is called meiosis in which the two sets of chromosomes - one from each parent - line up and the threads of DNA cross, break and rejoin so that DNA is exchanged between parental chromosomes - so generating diversity.

Usually this is an equal and reciprocal exchange, but occasionally the result is unequal and one of the chromosomes acquires a little more DNA containing an extra gene,or part of one. If the gamete with extra DNA acquires an entire gene it is referred to as "gene duplication by unequal crossover".

In blood clotting, two of the similar proteases, Factor VII (FVII) and X (FX), are within close proximity on chromosome 13 in humans indicating that unequal crossover played a role in the evolution of blood clotting.

Complete genome duplication

Another mechanism of gene duplication is through complete genome duplication. The entire genome of brewer's yeast Saccharomyces cerevisiae has been sequenced, and provides evidence of complete genome duplication by the genetic redundancy and conserved arrangement of the remaining duplicated genes. One way that genome duplication can occur is through polyploidy and subsequent degeneration back to a lower ploidy state. (Polyploidy is where the cell nucleus contains multiple sets of chromosomes.  Humans are diploid meaning we have two copies of each chromosome but the toad Xenopus laevis has four copies of each chromosome and therefore an additional two copies of each gene.)

Such duplicated genes can take one of two career paths. The majority of gene duplicates enter "early retirement" as the constant mutational pressure results in amino acid substitutions that render the protein non-functional. In the rare event that the amino acid substitution is beneficial, the duplicated gene can participate in natural selection and acquire a new function or interaction within an existing pathway - such as blood clotting.

" It appears that the physiologically important 'extrinsic' pathway as we know it mammals evolved by two rounds of genome or chromosomal duplication and one tandem duplication prior to a last common ancestor with fish ~400 MYA and after the origin of the vertebrates ~520 MYA. This brief window of evolutionary time indicates that a flurry of duplication events occurred to generate the clotting pathway and supports the theories of punctuated evolution.

"The evolutionary time frame of blood clotting evolution is supported by investigations into other gene families such as Hox genes and the genes of the MHC. Investigations in other systems support the two large-scale duplication of the ancestral vertebrate genome as well. The debatable issue is when both duplications occurred." says Davidson.

However, in the conclusion Davidson is also quoted as saying:


Fundamental evolutionary event

"Vertebrates are one of the planet's most diverse and successful group of animals colonizing both the water and land. Vertebrates share common developmental physiology, and anatomy and are distinguished from invertebrates by complex systems such as immune, nervous, endocrine and blood clotting systems.

"Since blood transports and mediates many of the above vertebrate innovations it seems certain that the protective function of blood clotting was a fundamental evolutionary event. A more primitive state of blood clotting than that found in jawless vertebrates is likely not present in any living creature and died with the primitive vertebrates in the Cambrian.

"The origin of vertebrate blood clotting, like the origin of life must have been a rare event, yet without protection from bleeding it would not have been possible for the radiation of vertebrates from fish to dinosaurs and ultimately humans."

I think it is a mistake to compare the origin of blood-clotting to the origin of life, or even to imply that it is rare: it is apparent that a similar system has arisen in arthropods at least (see Miller's (1999) discussion of the evolution of decapod blood-clotting), and it seems quite likely various clotting systems will be discovered in other complex metazoans (perhaps descended from a primitive ancestral system, but still with considerable independent evolution in each lineage, as e.g. with eyes).  One can even argue that the pitch of trees is a clotting system. Perhaps Davidson is a wee bit vertebrate-biased.


  24 replies since May 17 2002,22:00 < Next Oldest | Next Newest >  


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