Foregut Development and Metamorphosis in a Pyramidellid Gastropod: Modularity and Constraint within a Complex Life Cycle

Pyramidellids are tiny ectoparasitic gastropods with highly derived feeding structures for piercing and sucking. We attempted to resolve homology controversies about unique pyramidellid feeding structures by examining foregut development through larval and metamorphic stages, using sections for light and electron microscopy. We anticipated that, like many marine invertebrate larvae, post-metamorphic structures would differentiate extensively in late larvae to speed metamorphic transition. Previous studies of gastropods suggested that development of juvenile feeding structures in larvae was facilitated by foregut subdivision into dorsal and ventral developmental modules, and spatial uncoupling of these modules may have facilitated adaptive radiation in neogastropods. Observations of Odostomia tenuisculpta suggested that the stylet may be derived from cuticle-secreting buccal epithelium surrounding the proximal end of the salivary duct, whereas the stylet sheath could be either a derived jaw or a radular tooth. The anterior half of the remarkable buccal pump of these euthyneuran gastropods develops from the larval esophagus, which is unorthodox compared to caenogastropods, where extensive post-metamorphic specialization of a dorsal module component has not been previously described. The introvert tube develops from pouches of the distal larval esophagus and may actually be an eversible oral tube rather than an acrembolic proboscis. Minimal differentiation of presumptive juvenile foregut structures occurred during the larval stage of O. tenuisculpta, when compared to other gastropods. The stylet, stylet sheath, and buccal pump may be incompatible with functioning of the larval esophagus; thus, an explosive period of morphogenesis is necessary at metamorphosis. Although dorsal and ventral modules were recognizable during the development of O. tenuisculpta, we failed to find evidence that this modularity facilitated the extreme evolutionary remodeling of post-metamorphic feeding structures.

Novel Genes, Ancient Genes, and Gene Co-Option Contributed to the Genetic Basis of the Radula, a Molluscan Innovation

The radula is the central foraging organ and apomorphy of the Mollusca. However, in contrast to other innovations, including the mollusk shell, genetic underpinnings of radula formation remain virtually unknown. Here, we present the first radula formative tissue transcriptome using the viviparous freshwater snail Tylomelania sarasinorum and compare it to foot tissue and the shell-building mantle of the same species. We combine differential expression, functional enrichment, and phylostratigraphic analyses to identify both specific and shared genetic underpinnings of the three tissues as well as their dominant functions and evolutionary origins. Gene expression of radula formative tissue is very distinct, but nevertheless more similar to mantle than to foot. Generally, the genetic bases of both radula and shell formation were shaped by novel orchestration of preexisting genes and continuous evolution of novel genes. A significantly increased proportion of radula-specific genes originated since the origin of stem-mollusks, indicating that novel genes were especially important for radula evolution. Genes with radula-specific expression in our study are frequently also expressed during the formation of other lophotrochozoan hard structures, like chaetae (hes1, arx), spicules (gbx), and shells of mollusks (gbx, heph) and brachiopods (heph), suggesting gene co-option for hard structure formation. Finally, a Lophotrochozoa-specific chitin synthase with a myosin motor domain (CS-MD), which is expressed during mollusk and brachiopod shell formation, had radula-specific expression in our study. CS-MD potentially facilitated the construction of complex chitinous structures and points at the potential of molecular novelties to promote the evolution of different morphological innovations.

Contrasting Metatrochal Behavior of Mollusc and Annelid Larvae and the Regulation of Feeding While Swimming

Molluscan veliger larvae and some annelid larvae capture particulate food between a preoral prototrochal band of long cilia that create a current for both swimming and feeding and a postoral metatrochal band of shorter cilia that beat toward the prototroch. Larvae encountering satiating or noxious particles must somehow swim without capturing particles or else reject large numbers of captured particles. Because high rates of particle capture are inferred to depend on the beat of both ciliary bands, arrest of the metatroch could be one way to swim while reducing captures. Larvae in eight families of annelids arrest metatrochal cilia frequently during prototrochal beat, often over a large part of the metatrochal band and with the arrested cilia aligned near the beginning of the effective stroke. In contrast, metatrochs of veligers of gastropods and bivalves rarely arrested while the prototroch beat, and those arrests were more localized and variable in position. This difference in metatrochal arrest was unexpected under hypotheses of either a single origin of this feeding mechanism or multiple origins within each phylum. Although different in metatrochal arrests, larvae of both phyla can separate swimming from feeding while both prototroch and metatroch beat. One hypothesis explaining low rates of capture per encounter, without metatrochal arrest, is a change in adhesion of prototrochal cilia with algae. In a few observations, part of the velar edge was retained within the veliger's shell so that exposed prototrochal cilia contributed to swimming while the adjacent metatroch and food groove were sequestered.

Insights into the Evolution of Shells and Love Darts of Land Snails Revealed from Their Matrix Proteins

Over the past decade, many skeletal matrix proteins that are possibly related to calcification have been reported in various calcifying animals. Molluscs are among the most diverse calcifying animals and some gastropods have adapted to terrestrial ecological niches. Although many shell matrix proteins (SMPs) have already been reported in molluscs, most reports have focused on marine molluscs, and the SMPs of terrestrial snails remain unclear. In addition, some terrestrial stylommatophoran snails have evolved an additional unique calcified character, called a "love dart," used for mating behavior. We identified 54 SMPs in the terrestrial snail Euhadra quaesita, and found that they contain specific domains that are widely conserved in molluscan SMPs. However, our results also suggest that some of them possibly have evolved independently by domain shuffling, domain recruitment, or gene co-option. We then identified four dart matrix proteins, and found that two of them are the same proteins as those identified as SMPs. Our results suggest that some dart matrix proteins possibly have evolved by independent gene co-option from SMPs during dart evolution events. These results provide a new perspective on the evolution of SMPs and "love darts" in land snails.