Abstract

Timing is crucial, especially during early development of animals. The theoretical evolutionary process termed progenesis is accepted as the prevailing evolutionary route to underdeveloped (paedomorphic) life forms with retained larval appearance, e.g., salamanders, most meiofaunal groups, and many dwarf males. Progenesis proposedly starts with an accelerated sexual maturation (compared to the ancestor) leading to an early and synchronous offset of somatic development (in the descendant). This evolutionary process is challenging to investigate since most microscopic metazoan lineages are old and lack close macroscopic relatives for comparison. However, the dimorphic bone devouring worms of Osedax (Siboglinidae, Annelida) have both macroscopic females and microscopic dwarf males; the latter potentially being an outcome of progenesis. The embryos of Osedax are genetically similar and sexual differentiation of the competent larvae seemingly depend on environmental cues. Whereas adequate bone substrate & exposure to bacterial symbionts seem to trigger metamorphosis into females, exposure to cospecific female hormones seem to trigger metamorphosis into dwarf males. The life cycle and development of Osedax therefore offer a unique experimental insight into the detailed morphogenesis and epigenetic regulation underlying their symbiotic relationship (in females) and progenesis (in males). Using traditional immunochemistry labelling and CLSM we have reconstructed distinct anatomical stages during the larval and juvenile development of Osedax japonicus to serve as a basis for interpretation of molecular expression patterns. We found striking similarities in the nervous system, musculature, ciliation and chaetae between late larval and adult male stages, which support male dwarfism to be the outcome of an early offset of somatic development (=progenesis). In our ongoing research we now seek to align and couple genetic expressions with anatomical changes during development in order to characterize genetic key players in Osedax development and life cycle.