Are all fishes ancient polyploids?

Euteleost fishes seem to have more copies of many genes than their tetrapod relatives. Three different mechanisms could explain the origin of these 'extra' fish genes. The duplicates may have been produced during a fish-specific genome duplication event. A second explanation is an increased rate of independent gene duplications in fish. A third possibility is that after gene or genome duplication events in the common ancestor of fish and tetrapods, the latter lost more genes. These three hypotheses have been tested by phylogenetic tree reconstruction. Phylogenetic analyses of sequences from human, mouse, chicken, frog (Xenopus laevis), zebrafish (Danio rerio) and pufferfish (Takifugu rubripes) suggest that ray-finned fishes are likely to have undergone a whole genome duplication event between 200 and 450 million years ago. We also comment here on the evolutionary consequences of this ancient genome duplication.