Abstract
AbstractBackgroundSpiral cleavage is a conserved early developmental mode found in several phyla of Lophotrochozoans with highly diverse adult body plans. While the cleavage pattern has clearly been broadly conserved, it has also undergone many modifications in various taxa. The precise mechanisms of how different adaptations have altered the ancestral spiral cleavage pattern is an important ongoing evolutionary question and adequately answering this question requires obtaining a broad developmental knowledge of different spirally cleaving taxa.In flatworms (Platyhelminthes), the spiral cleavage program has been lost or severely modified in most taxa. Polyclad flatworms, however, have retained the pattern up to the 32-cell stage. Here we study early embryogenesis of the cotylean polyclad flatwormMaritigrella crozierito investigate how closely this species follows the canonical spiral cleavage pattern and to discover any potential deviations from it.ResultsUsing live imaging recordings and 3D reconstructions of embryos, we give a detailed picture of the events that occur during spiral cleavage inM. crozieri. We suggest, contrary to previous observations, that the 4-cell stage is a product of unequal cleavages. We show that that the formation of third and fourth micromere quartets are accompanied by strong blebbing events; blebbing also accompanies the formation of micromere 4d. We find an important deviation from the canonical pattern of cleavages with clear evidence that micromere 4d follows an atypical cleavage pattern, so far exclusively found in polyclad flatworms.ConclusionsOur findings highlight that early development inM. crozierideviates in several important aspects from the canonical spiral cleavage pattern. We suggest that some of our observations extend to polyclad flatworms in general as they have been described in both suborders of the Polycladida, the Cotylea and Acotylea.
Publisher
Cold Spring Harbor Laboratory