Figure 1

Hox gene expression in molluscan developmental stages. Dorsal (d)–ventral (v) and anterior (a)–posterior (P) axes indicate the orientation (all lateral views). Staggered expression is present in polyplacophorans (A) and gastropods (D). The decapod cephalopod Euprymna scolopes (B)¹³ and the gastropod Gibbula varia depicted herein (D) show traces of staggered Hox expression³², however recently, Huan et al.³⁵ evidenced staggered Hox expression in the gastropod Lottia goshimai (not shown). The early mid-stage trochophore of the scaphopod Antalis entalis exhibits near-to-staggered Hox expression (C)³⁴, while no staggered expression was observed in bivalve embryos so far (E)³⁶. The above-mentioned data and the presence of staggered Hox expression in other bilaterians suggests staggered Hox expression in the last common ancestor of all mollusks (F). The shell/shell plates (dashed black lines), the prototroch/velum (shaded in dark gray), the digestive tract (stippled blue lines), and the blastopore/mouth (asterisks) are outlined. a anterior, ar arm, d dorsal, ep episphere, f foot, fu funnel, m mantle, v ventral, p posterior, pg pedal ganglion, pvg palliovisceral ganglion, sf shell field, ys yolk sac. Sketch modified from Ref.²⁶.