Meeting Abstract
Sipuncula is an ancient clade of marine annelid worms with an unsegmented body plan. Development exhibits unequal quartet spiral cleavage, conspicuously large prototroch cells, a postoral metatroch, paired introvert retractor muscles, and a U-shaped digestive organ system. There are four distinct life history patterns, and they have evolved a unique metazoan larval type, the pelagosphera. Evidence of morphological segmentation within organ systems along the anteroposterior axis during development is questionable, and under investigation. Using CLSM and gene expression, we highlight developmental differences between two species with contrasting life histories: Phascolion cryptum develops directly without ciliated trochal bands, buccal organ or terminal organ; Nephasoma pellucidum develops indirectly through lecithotrophic trochophore and planktotrophic pelagosphera larvae with a buccal organ, lip gland, functional gut and retractable terminal organ. Based on recent phylogenetic hypotheses, planktotrophy is part of the ancestral life history pattern in Sipuncula. We also introduce preliminary results from de novo developmental transcriptomes of both species, which generated an average of 22,106,986 reads per sample, assembled into an average of 42,104 annotated transcripts per sample with an assembly N50 of 3,432 bp. We find genetic evidence for many critical developmental programs within and between species and life history patterns. Thus, with unique developmental morphology, new comparative genomic data for evo-devo, and well-characterized life history patterns, sipunculans are emerging as valuable research models, and will provide a deeper understanding of the evolutionary history of Annelida, Spiralia and Metazoa.
