MODULARITY OF THE ANGIOSPERM FEMALE GAMETOPHYTE AND ITS BEARING ON THE EARLY EVOLUTION OF ENDOSPERM IN FLOWERING PLANTS

The gastropod foregut — evolution viewed through a developmental lens

Comparative data on the developing gastropod foregut suggest that this multicomponent feeding complex consists of two developmental modules. Modularity is revealed by delayed development of the buccal cavity and radular sac (“ventral module”) relative to the dorsal food channel (“dorsal module”) in gastropods with feeding larvae compared with those that may have never had a feeding larval stage. If nonfeeding larvae like those of extant patellogastropods and vetigastropods are ancestral for gastropods, then the uncoupling and heterochronic offset of dorsal and ventral foregut modules allowed the post-metamorphic dorsal food channel to be co-opted as a simple but functional esophagus for feeding larvae. Furthermore, by reducing energy cost per ovum, the heterochronic offset may have given mothers the evolutionary option of increasing fecundity or investing in protective egg encapsulation material. A second developmental innovation was spatial separation of the dorsal and ventral foregut modules, as illustrated by distal foregut development in buccinid neogastropods and venom gland development in cone snails. Spatial uncoupling may have enhanced the evolvability of gastropod foreguts by allowing phenotypic variants of ventral module components to be selected within post-metamorphic ecological settings, without needing to be first tested for compatibility with larval feeding. Finally, we describe a case in which foregut modularity has helped facilitate a highly derived life history in which encapsulated embryos ingest nurse eggs.