niiicholas
Posts: 319
Joined: May 2002
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Another classic case is the evolution of antifreeze genes from proteases in arctic & subarctic fish, which has happened independently at least a couple of times:
I believe this article is freely available online from PNAS:
Proc. Natl. Acad. Sci. USA
Vol. 94, pp. 3485-3487, April 1997
Origin of antifreeze protein genes: A cool tale in molecular evolution
John M. Logsdon Jr. and W. Ford Doolittle
http://www.pnas.org/cgi/content/full/94/8/3485
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Where do new genes come from? Duplication, divergence, and exon shuffling are the expected answers, so it is especially exciting when new genes are cobbled together from DNA of no related function (or no function at all). In this issue, Chen et al. (1) describe an antifreeze glycoprotein (AFGP) gene in an Antarctic fish that has arisen (in part) from noncoding DNA. Further, they show that a very similar AFGP from an Arctic fish is the product of some completely unrelated molecular processes (2). Together, these papers shed light on a number of key issues in molecular evolution.
In the late 1960s Arthur DeVries showed that freezing resistance in Antarctic fish was due to blood serum glycoproteins that lowered their freezing temperature below that of the subzero sea surrounding them (3, 4). The ensuing years have witnessed a great deal of work on AFPs (antifreeze proteins; not all are glycoproteins) in a number of phylogenetically diverse fish species, much of it by DeVries and his colleagues (5-7), revealing a number of types differing in their structure and amino-acid composition. These proteins, despite their diversity, function in similar ways to deter ice crystal growth (7, 8). But where did they come from, and how did they arise?
Birth of a Gene
In the first of the two papers, Chen et al. (1) demonstrate that an AFGP gene from the Antarctic notothenioid Dissostichus mawsoni derives from a gene encoding a pancreatic trypsinogen. The relationship of these two genes is not simply one of duplication and divergence (9), co-option/recruitment (10), or exon shuffling (11), processes that have been appreciated by molecular evolutionists for some time now. Instead, the novel portion of the AFGP gene (encoding the ice-binding function) derives from the recruitment and iteration of a small region spanning the boundary between the first intron and second exon of the trypsinogen gene (Fig. 1). This newborn segment was expanded and then iteratively duplicated (perhaps by replication slippage or unequal crossing-over) to produce 41 tandemly repeated segments. Nonetheless, the contemporary AFGP gene retains, as its birthmark, sequences at both ends which are nearly identical to trypsinogen. Retention of the 5 end of the trypsinogen gene may be significant, since this region encodes a signal peptide used for secretion from the pancreas into the digestive tract. Chen et al. (1) hypothesize that an early version of the notothenioid AFGP gene may have had its first function preventing freezing in the intestinal fluid, with this function later expanded into the circulatory system by way of its expression in the liver.
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Here is Figure 1:
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Figure 1. Comparison of gene structures and their sequence similarities. The regions shown represent genomic regions encompassed by sequenced cDNAs, and are not to scale. Exons are shown as large boxes; introns are shown as thinner boxes; inferred initiation and termination codons are indicated. Untranslated regions are hatched, and regions encoding putative signal peptides are stippled. Regions in different genes that are the same color share sequence similarity, but only regions of the same color shade are homologous; dotted lines delineate regions of clear homology between Dissostichus trypsinogen and AFGP genes. The open region of the trypsinogen gene is absent in AFGP. The segment below the double-headed arrow represents expansion of a sequence element present in the Dissostichus trypsinogen gene that appears to have given rise to the canonical AFGP repeat; its subsequent tandem iteration is shown by thin dashed lines. AFGP repeats are numbered and discontinuities are indicated for presentation. Regions between the AFGP repeats (spacers; indicated as either yellow or black) are the presumed sites of posttranslational cleavage. A discontinuity in the intron Dissostichus AFGP gene is shown to represent an internal segment not present in the homologous trypsinogen gene intron.
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(source)
Thanks, nic
Edited by niiicholas on May 31 2002,01:32
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