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Author Topic:   The origin of new genes
bernd
Member (Idle past 3981 days)
Posts: 95
From: Munich,Germany
Joined: 07-10-2005


Message 1 of 2 (351140)
09-21-2006 5:32 PM


In recent threads the role of mutation for evolutionary processes has been the point of issue. One side seems to claim that the phenotypic and genetic diversity of sexual reproducing organism can be explained without mutation - all observed “novel” traits are viewed simply as the result of a new combination of pre-existing alleles.
It might therefore be interesting to discuss the molecular processes which generate new genes.
A good introduction offers the following review published in Nature Reviews Genetics 4, 865-875 (2003), which describes the currently known mechanism of novel gene formation:
  • Exon shuffling
  • Gene duplication
  • Retroposition
  • Mobile elements
  • Lateral gene transfer
  • Gene fusion/fision
  • De novo origination
  • Combined Mechanism
For each mechanism several examples are provided, including references to the primary literature (see table 1).
In order to keep the discussion focused I would propose to concentrate on “jingwei” a recently (~ 2.5 million years BP) evolved gene in Drosophila, which has been formed by the combination of exon shuffling, retroposition and gene duplication. The article provides a short overview:
In the early 1990s, the first young gene to be described was jingwei in a group of African Drosophila species32. It provided enough details for the molecular mechanism underlying its origination to be deduced. A portion of jingwei was found to be a homologue of the Adh gene that encodes alcohol dehydrogenase98 and was later characterized as a retrosequence of Adh99. Further population genetic, molecular biological and comparative phylogenetic analyses showed it to be a new processed functional gene that originated around 2 million years ago in the common ancestor of two African Drosophila species, Drosophila yakuba and Drosophila teissieri.
In the ancestral species, there were two single copy genes, yellow-emperor (ymp) and Adh. yellow-emperor was duplicated into two copies: one also called yellow-emperor and the other called yande (ynd)100,101.Whereas yellow-emperor maintained its original functions, yande was further involved in the origin of jingwei. In the short time before the speciation event, Adh mRNA retroposed into the third intron of yande as a fused exon and recombined with the first three yande exons. This formed jingwei, which is a gene that is translated into a chimeric protein.
The funtion of jingwei has been described in the following article . The abstract reads:
The mechanism by which protein functional diversity expands is an important evolutionary issue. Studies of recently evolved chimeric genes permit direct investigation of the origin of new protein functions before they become obscured by subsequent evolution. Found in several African Drosophila species, jingwei (jgw), a recently evolved gene with a domain derived from the still extant short-chain alcohol dehydrogenase (ADH) through retroposition, provides an opportunity to examine this previously undescribed process directly. We expressed JGW proteins in a microbial expression system and, after purification, investigated their enzymatic properties. We found that, unexpectedly, positive Darwinian selection for amino acid replacements outside the active site of JGW produced a novel dehydrogenase with altered substrate specificity compared with the ancestral ADH. Instead of detoxifying and assimilating ethanol like its Adh parental gene, we observe that JGW efficiently utilizes long-chain primary alcohols found in hormone and pheromone metabolism. These data suggest that protein functional diversity can expand rapidly under the joint forces of exon shuffling, gene duplication, and natural selection.
Taken together I claim that we look at an example that mutation created a novel, useful gene - which seems to me difficult to reconcile with the above mentioned view which implies that no natural process is able to create new alleles, let alone new genes.
-Bernd
Edited by bernd, : Spelling correction
Edited by bernd, : insertion of "which implies" in last sentence

AdminNosy
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Message 2 of 2 (351153)
09-21-2006 6:43 PM


Thread copied to the The origin of new genes thread in the Biological Evolution forum, this copy of the thread has been closed.

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