The morphological novelty of echinoderms—marine invertebrates with radial symmetry, usually starlike, around a central point—impedes comparisons with most other animals, in which bilateral symmetry on an anterior-posterior (head to tail) axis through a trunk is typical. Particularly puzzling are sea stars, thought to have evolved a headless layout from a known bilateral origin. Applying genomic knowledge of Saccoglossus kowalevskii acorn worms (close relatives of sea stars, and thus expected to have similar markers for corresponding anatomical regions) to the body patterning genes of Patiria miniata sea stars, Laurent Formery et al. observed activity only in anterior genes across P. miniata's entire body and some posterior genes limited to the edges, suggesting that ______
Which choice most logically completes the text?
Explanation
Context: Sea stars have radial symmetry but likely evolved from bilateral ancestors (head-to-tail axis). Applying genomic knowledge of acorn worms (close relatives), researchers found anterior genes (head-region genes) are highly expressed across sea stars' bodies. The blank states what this finding reveals about sea star morphology.
Answer: C) rather than undergoing changes resulting in the eventual elimination of a head region in their radial body plan, as previously assumed, sea stars' morphology evolved to completely lack a trunk and consist primarily of a head region.
Why? Anterior genes dominating the sea star body plan suggests sea stars didn't lose their head region — instead, they evolved to be essentially all head with no trunk, a reversal of the previous assumption. C states this counterintuitive finding.
Traps:
- A) despite greater prevalence of anterior genes → acknowledges the finding but draws no conclusion
- B) only limited genomic overlap with acorn worms → contradicts the close genetic relationship established
- D) contrary to belief they evolved from early ancestors → makes a vague ancestral claim that misses the specific head/trunk insight