Shell field morphogenesis in the polyplacophoran mollusk Acanthochitona rubrolineata

The polyplacophoran mollusks (chitons) possess serially arranged shell plates. This feature is unique among mollusks and believed to be essential to explore the evolution of mollusks as well as their shells. Previous studies revealed several cell populations in the dorsal epithelium (shell field) of polyplacophoran larvae and their roles in the formation of shell plates. Nevertheless, they provide limited molecular information, and shell field morphogenesis remains largely uninvestigated.

In the present study, we investigated shell field development in the chiton Acanthochitona rubrolineata based on morphological characteristics and molecular patterns. A total of four types of tissue could be recognized from the shell field of A. rubrolineata. The shell field comprised not only the centrally located, alternatively arranged plate fields and ridges, but also the tissues surrounding them, which were the precursors of the girdle and we termed as the girdle field. The girdle field exhibited a concentric organization composed of two circularly arranged tissues, and spicules were only developed in the outer circle. Dynamic engrailed expression and F-actin (filamentous actin) distributions revealed relatively complicated morphogenesis of the shell field. The repeated units (plate fields and ridges) were gradually established in the shell field, seemingly different from the manners used in the segmentation of Drosophila or vertebrates. The seven repeated ridges also experienced different modes of ontogenesis from each other. In the girdle field, the presumptive spicule-formation cells exhibited different patterns of F-actin aggregations as they differentiate.

These results reveal the details concerning the structure of polyplacophoran shell field as well as its morphogenesis. They would contribute to exploring the mechanisms of polyplacophoran shell development and molluscan shell evolution.