The term teleostei is derived from the Greek words teleios (meaning “complete”) and osteon (meaning “bone”). This infraclass of ray-finned fishes can roughly be identified by having true bony structures, a homocercal tail, a protrusible jaw and a spine that terminates at the caudal peduncle. In some estimates, 30,000 species of teleosts are known to exist. Other estimates place half the known vertebrate species to be members of teleostei. This easily raises the questions: why have teleosts become more successful than other vertebrates, and why are they so diverse? Theory suggests the answer lies in the genome.
An organism’s genome is made up of its genes or genetic material. It is what codes for the uniqueness of a species. Occasionally, mistakes are made when cellular machinery duplicates the DNA making up the genes. These mistakes are incredibly important as they sometimes provide an advantage to the organism which increases its overall fitness. Although these mutations can be advantageous, it is not necessarily an efficient vector or mechanism for evolutionary progress because the function of the mutated gene comes at the cost of the function of the old gene. In 1951, S. G. Stephens hypothesized that an increase of genetic loci would be the only way to overcome this genetic stalemate and promote evolutionary progress. He goes on to suggest a duplication of the entire genome would be one viable source of increasing loci. As technology has advanced, this hypothesis has been tested and confirmed. For example, in 1997 evidence was found that at some point in its evolutionary history the entire yeast (Saccharomyces cerevisiae) genome was duplicated. Furthermore, in 2002 it was determined that the entire genome of rice (Oryza sativa japonica) was duplicated between 40 and 50 million years ago. Perhaps even more interesting is the evidence suggesting the entire human genome appears to have been duplicated at least twice.
As genomes are duplicated, the event creates genetic redundancy. Each gene has a second copy that can be mutated and have no deleterious effect on the original function of the gene and to the host organism. Because of this genetic blank canvas, the mutation of the second gene copy can create a new function without hindering the function of the original copy. Both genes/functions can be kept ultimately promoting a much faster rate of speciation.
The scientific literature discovering genome duplication events within fishes is growing. A. Amores and others have suggested such an event took place after actinopterygians (ray-finned fish) and sarcopterygians (lobed-finned fish) diverged. This particular event might account for the greater species diversity within actinopterygians versus the diversity found within sarcopterygians. On a smaller scale, the teleost lineage is hypothesized to have risen between the mid Cretaceous and late Triassic periods of the Mesozoic era (ca 100 to 200 million years ago). A genome duplication event is thought to have occurred around the same time and facilitated the rapid radiation of the teleost group of fishes. With the duplicate genetic material available, the teleost group was able to keep and pass on the advantageous gene mutations without giving up the function of the original gene aiding in their rapid ascent to diversity. Without doubling the genome, there could not have been the genetic playground for teleosts to radiate into a brilliant display of diversity. As more of these genomic secrets are discovered in fishes, it becomes clearer that they are the culprit for the vast diversity we see today.
References and Photo Credits
Taylor, J. S., Y. Van de Peer, I. Braasch and A. Meyer. 2001. Comparative genomics provides evidence for an ancient genome duplication event in fish. Philosophical Transactions of the Royal Society of London 356:1661-1679.
Amores, A. et al. 1998. Zebrafish hox clusters and vertebrate genome evolution. Science 282:1711-1714.
Meyer, A. and M. Schartl. 1999. Gene and genome duplications in vertebrates: the one to four rule and the evolution of novel gene functions. Current Opinions of Cell Biology 11:699-704
Carroll, R. L. 1997. Patterns and processes of vertebrate evolution. Cambridge University Press.
Lydeard, C. and K. J. Roe. 1997. The phylogenetic utility of the mitochondrial cutochrome b gene for inferring relationships among actinopterygian fishes. Molecular Systematics of Fishes. pp. 285 – 303. San Diego, CA: Academic Press.
Taylor, J. S., I. Braasch, T. Frickey, A. Meyer and Y. Van de Peer. 2003. Genome duplication, a trait shared by 22,000 species of ray-finned fish. Genome Research 13:382-390.
Stephens, S., G. 1951. Possible significance of duplication in evolution. Advances in Genetics 4:247-265.
Wolfe, K., H. and D.C. Shields. 1997. Molecular evidence for an ancient duplication of the entire yeast genome. Nature 387:708-713.
Goff, S. A., et al. 2002. A draft sequence of the rice genome (Oryza sativa L. ssp. Japonica). Science 296:92-100.
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