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.

Question

Quizplus · Accepted Answer

B, DThe observation that the last common ancestor of all bilaterians likely had four Hox genes, while most extant cnidarians have two (except Nematostella with three), suggests a loss of Hox genes in cnidarians if they were originally bilateral. This implies that the radial symmetry seen in cnidarians today is a secondary trait (B), evolved from an ancestral bilateral symmetry. However, this does not necessarily mean Cnidaria will replace Acoela as the basal bilaterians (D), as this statement is more speculative and not directly supported by the information given about Hox genes and symmetry.

The Last Common Ancestor of All Bilaterians Is Thought to Have