title: Embryonic muscle development in direct and indirect developing marine flatworms (Platyhelminthes, Polycladida).
abstract: We compared embryonic myogenesis of the direct-developing acotylean polyclad Melloplana ferruginea with that of Maritigrella crozieri, a cotylean that develops via a larval stage. Fluorescently labeled F-actin was visualized with laser confocal microscopy. Developmental times are reported as percentages of the time from oviposition to hatching: 7 days for M. crozieri and 22 days for M. ferruginea. The epithelium began to form at 30% development in M. crozieri and at 15% development in M. ferruginea. Random myoblasts appeared in peripheral areas of the embryo at 36% and 22-30% development in M. crozeri and M. ferruginea, respectively. Circular and longitudinal muscle bands formed synchronously at 37-44% development in M. crozieri; yolk obscured observations of early myogenesis in M. ferruginea. An orthogonal muscle grid was established by 45-50% development in both species. Diagonal muscles developed in M. ferruginea at 60-71% development. Hence, juveniles of this species hatch with the same basic body-wall musculature as adults. Larvae of M. crozieri did not hatch with diagonal muscles; these muscles are acquired postmetamorphosis. Additionally, a specialized musculature developed in the larval lobes of M. crozieri. Oral musculature was complex and established by 72% development in both species. Our results are comparable to the muscle differentiation reported for other indirect-developing polyclads and for direct-developing species of macrostomid flatworms. Furthermore, they provide additional support that the orthogonal muscle pattern of circular and longitudinal muscles is a symplesiomorphy of Spiralia.
authors.affiliation: Department of Zoology and Natural Resources, University of New Hampshire, Durham, NH 03824, USA.
authors.forename: D Marcela
authors.initials: DM
authors.lastname: Bolaños
authors.forename: Marian K
authors.initials: MK
authors.lastname: Litvaitis
mesh_terms: D000818:Animals; D047108:Embryonic Development; D005456:Fluorescent Dyes; D018613:Microscopy, Confocal; D009132:Muscles / Q000196:embryology*; D010986:Platyhelminths / Q000196:embryology*
chemical_list: D005456:Fluorescent Dyes