European eel (Anguilla anguilla) and Japanese eel (Anguilla japonica), two of the species with sequenced genomes that you can get from my collection of illustrations. Hi-res and web-ready files available here: Illustrations - Shared folder.
Like the arowanas, eels diverged early in teleost fish evolution, and can therefore offer a “snapshot” into the processes that occurred during this time. Teleost fishes are characterized by an early whole genome duplication that happened in a common ancestor approximately 350 million years ago. This was followed by a period of approximately 80-100 million years where teleost genomes were intensely rearranged, with pieces of chromosomes fusing, breaking apart and changing places, perhaps as a direct consequence of the genome duplication. By studying eel genomes, we have a much clearer view of this process (1).
Eels have also kept and found uses for some remnants of this early evolution that other teleost fishes have lost. Hox genes guide the development of the body plan in most animals, whether they are fruit flies, jellyfishes, humans or teleost fishes. All teleosts have kept some duplicated Hox genes that resulted from their ancient whole genome duplication, but eels kept all the duplicated Hox genes (2). This might be part in explaining why eels have such a different body plan and development compared with other teleosts. Eels go through a complex process of metamorphosis during their lives, including several larval stages and, in many species, a long migration from the oceanic spawning grounds up to rivers and streams and then back out into the sea.
This complex development has also made eels extra sensitive to climate change and the disruption or disappearence of their environments. It doesn’t help that many of them also are traditional food sources. The Japanese and American eels are classified as endangered by the IUCN, while the European eel is considered critically endangered.
1) Kai, W. et al. (2014) A ddRAD-based genetic map and its integration with the genome assembly of Japanese eel (Anguilla japonica) provides insights into genome evolution after the teleost-specific genome duplication. BMC Genomics, 15(1), 233. DOI: 10.1186/1471-2164-15-233
2) Henkel, C. et al. (2012) Primitive Duplicate Hox Clusters in the European Eel’s Genome. PLoS ONE, 7(2), e32231. DOI: 10.1371/journal.pone.0032231
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