The cartilaginous elephant shark has a basal phylogenetic position useful for understanding jawed vertebrate evolution. Survey sequencing of its genome identified four Hox clusters, suggesting that, unlike for teleost fishes, no additional whole-genome duplication has occurred.
Cartilaginous fishes (sharks, rays, skates, and chimaeras) are the phylogenetically oldest group of living jawed vertebrates. They are also an important outgroup for understanding the evolution of bony vertebrates such as human and teleost fishes. In a new study published online this week in the open access journal PLoS Biology, Byrappa Venkatesh, Sydney Brenner, and colleagues performed survey sequencing (1.4× coverage) of a chimaera, the elephant shark (Callorhinchus milii).
The elephant shark genome, estimated to be about 910 Mb long, comprises about 28% repetitive elements. Comparative analysis of approximately 15,000 elephant shark gene fragments revealed examples of several ancient genes that have been lost differentially during the evolution of human and teleost fish lineages. Interestingly, the human and elephant shark genomes exhibit a higher degree of synteny and sequence conservation than human and teleost fish (zebrafish and fugu) genomes, even though humans are more closely related to teleost fishes than to the elephant shark.
Unlike teleost fish genomes, the elephant shark genome does not seem to have experienced an additional round of whole-genome duplication. These findings underscore the importance of the elephant shark as a useful "model" cartilaginous fish genome for understanding vertebrate genome evolution.
Citation: Venkatesh B, Kirkness EF, Loh YH, Halpern AL, Lee AP, et al. (2007) Survey sequencing and comparative analysis of the elephant shark (Callorhinchus milii) genome.
PLoS Biol 5(4): e101. doi:10.1371/journal.pbio.0050101. (http://www.plosbiology.org)
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