Molluscan Shells, Spicules, and Gladii Are Evolutionarily Deeply Conserved

Shells, spicules, and chaetae are diverse among extant and extinct spiralians such as mollusks, annelids, or brachiopods. These hard parts serve different functions, but their formation process and evolutionary interrelationships are still contentious. We investigated the expression of evolutionarily conserved transcription factor encoding genes as well as the structural genes chitin synthase and ferritin in cells giving rise to shells and spicules of aculiferans, i.e. the polyplacophoran Acanthochitona fascicularis and the neomeniomorph Wirenia argentea, as well as the conchiferan cephalopod Xipholeptos notoides and the scaphopod Antalis entalis. Polyplacophorans and neomeniomorphs express hox1 (only neomeniomorphs), goosecoid, grainyhead, and chitin-synthase in their spicules. Grainyhead, notch, delta, and zic are expressed in the polyplacophoran shell fields and spicule-bearing cells. In conchiferans, hox1 (scaphopods and cephalopods), goosecoid, and grainyhead (scaphopods) are expressed in the shell fields. Ferritin, is a gene that has been shown to be expressed in the gastropod shell field; however, it is not expressed in the shell fields or by the spicule-bearing cells of the studied species. Our study shows that all candidate genes are expressed in epithelia that give rise to spicules and shells, revealing a close relationship between spicule-bearing cells and shell fields. In contrast, ferritin expression in the shell field appears to be a gastropod innovation. Building on previous research involving brachiopod and annelid chaetal sacs, our results suggest that spicules may have predated molluscan shells and may be homologous to brachiopod and annelid chaetae. If this were true, then conchiferan mollusks would have secondarily lost spicules.