The Last Common Ancestor of All Bilaterians Is Thought to Have

question 35

Multiple Choice

The last common ancestor of all bilaterians is thought to have had four Hox genes. Most extant cnidarians have two Hox genes, except Nematostella (of β-catenin fame) , which has three Hox genes. On the basis of these observations, some have proposed that the ancestral cnidarians were originally bilateral and, in stages, lost Hox genes from their genomes. If true, this would mean that

• A) Radiata should be a true clade.
• B) the radial symmetry of extant cnidarians is secondarily derived, rather than being an ancestral trait.
• C) Hox genes play little actual role in coding for an animal's "body plan."
• D) Cnidaria may someday replace Acoela as the basal bilaterians.
• E) Two of the responses above are correct.

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