Question

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. miniatas entire body and some posterior genes limited to the edges, suggesting that ________. which choice most logically completes the text? a) despite the greater prevalence of anterior genes in sea stars genetic makeup, posterior genes active at the bodys perimeter are primarily responsible for the starlike layout that distinguishes sea stars radial symmetry from that of other echinoderms. b) contrary to the belief that they evolved from early ancestors with the bilateral form typical of many other animals, sea stars instead originated with an atypical body layout that was neither bilaterally nor radially symmetrical. c) although the two species are closely related, there is only minimal correspondence in the genetic markers for head, tail, and trunk region development in p. miniata sea stars and s. kowalevskii acorn worms. d) 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.

Explanation:

The text discusses sea - stars evolving from a bilateral origin and then mentions gene activity in Patiria miniata sea stars. The key is to find an option that makes sense based on the gene - activity observation. Option A connects the gene activity (anterior genes across the body and posterior genes at the edges) to the star - like layout that distinguishes sea stars' radial symmetry. Other options either go against the information in the text (like B suggesting a non - bilateral and non - radial origin) or are not well - supported by the gene - activity evidence (C about minimal genetic marker correspondence and D about sea stars evolving to lack a trunk and mainly have a head region).

Answer:

A. Despite the greater prevalence of anterior genes in sea stars' genetic makeup, posterior genes active at the body's perimeter are primarily responsible for the star - like layout that distinguishes sea stars' radial symmetry from that of other echinoderms.