Heterochrony and the evolution of poecilogony: generating larval diversity.

Poecilogony is the production of more than one type of young within a single species of marine invertebrate. We chose a poecilogonous polychaete to investigate potential differences in morphogenesis among offspring that are polymorphic in dispersal potentials (planktonic, benthic) and trophic modes (planktotrophy, adelphophagy). Differences in morphogenesis occur and are strongly influenced by maternal type. Females that provide extra-embryonic nutrition (as nurse eggs; type III females) also produce offspring with an accelerated onset of juvenile traits, relative to planktotrophic offspring of females that do not provide extra-embryonic nutrition (type I females). Thus, progeny of some females appear morphologically preadapted for a benthic lifestyle. Surprisingly, differences in phenotype among offspring do not parallel offspring ecotype, as offspring with early onset of juvenile traits (III) are ecologically bimodal. Some Type III offspring eat the nurse eggs (adelphophagy), have accelerated development, and hatch as benthic juveniles. In contrast, their siblings hatch as small, planktotrophic, dispersive larvae that are morphologically similar to their type III siblings, but ecologically similar to Type I planktotrophic larvae. We propose that poecilogony evolved through sequence heterochrony in morphogenesis with accelerated onset of juvenile traits in type III offspring. In addition, we suggest that heterochrony in life-history events (hatching, metamorphosis) also occurs, thereby generating offspring that are dimorphic in both phenotype and ecotype. Over time, selection acting on different levels of ontogeny (morphogenesis vs. dispersal) may balance this polymorphism and allow poecilogony to persist.