| The starlet sea anemone Nematostella vectensis differs from major developmental models (e.g., zebrafish, fruitfly) in that its adult body plan may arise via four distinct developmental trajectories, (1) embryogenesis, (2) regeneration, (3) polarity reversal and (4) physal pinching. Each trajectory begins with a distinct phenotype and leads to an identical adult morphology. The evolution of multiple reproductive strategies has profound consequences for the ecology and evolution of populations. Asexual reproductive strategies preserve favorable genetic combinations, allowing immediate exploitation of current environments. Sexual reproduction generates new genetic combinations, providing an avenue for adaptation and additional niche exploitation.; Considering the potential evolutionary and ecological consequences of reproductive plasticity, it is important to understand its developmental basis. This was addressed by comparing embryogenesis, fission, and regeneration at the morphological and molecular level. During embryogenesis, the structures of Nematostella form in a predictable order. The mouth develops first, followed by synchronous development of the pharynx, tentacles, mesenteries, and physa. In contrast, observations of regeneration and transverse fission indicate that these structures vary in their spatial and temporal relationships. The variation observed indicates that the genetic architecture governing development of different structures is semi-autonomous. If so, the development of one structure may be de-coupled from another. Identical gene networks, however, should be redeployed for identical structures during each trajectory.; Based on embryonic expression, Nv-fox and Nv-otx were selected as markers for oral and aboral development, respectively. Five Hox genes were selected as markers for primary axis development. The regulatory genes are expressed in comparable manners (i.e., the same body region and tissue layer) across trajectories. Nv-otx is expressed in the developing physa, Nv-fox in the invaginating mouth, and Hox genes along the primary axis. During embryogenesis, Hox genes are also expressed asymmetrically along the secondary axis, suggesting a role in patterning this axis. This potential alternate role for Hox genes is not observed during alternate trajectories, supporting the discrete and separable nature of these gene networks. Given that identical structures result from expression of identical genes in similar contexts, it is likely that these genes are participating in conserved genetic pathways across trajectories. |
