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.

Myogenesis in two polyclad platyhelminths with indirect development, Pseudoceros canadensis and Stylostomum sanjuania

“…In addition, X-shaped muscles are present in all three species. The latter are also found in M. crozieri, although the authors do not associate these with the oral hood in this species (Bolan˜os and Litvaitis 2009). In H. inquilina, the oral retractor muscles extend from the apical pole into posterior direction (Reiter et al 1996), similar to the situation found in S. sanjuania (Fig.…”

“…The oral hood of M. crozieri appears to express only circular muscles, whereas in S. sanjuania and P. canadensis diverse retractor muscles, which connect to the stomodeum, are found (Bolan˜os and Litvaitis 2009). In addition, X-shaped muscles are present in all three species.…”

“…The indirect developing polyclad M. crozieri forms secondary circular fibers only by branching, which is in contrast to S. sanjuania and P. canadensis, which form circular muscles mainly via double stranded fiber zones (Bolan˜os and Litvaitis 2009). By contrast, these two species form secondary longitudinal and diagonal muscles by branching from existing muscles.…”

“…Adult myoanatomy of several rhabditophoran taxa including the free-living M. h. marinum, Jensenia sp., as well as the parasitic Urastoma cyprinae, Fasciola hepatica, and Schistosoma mansoni were studied by applying fluorescent-conjugated phalloidin (Stitt et al 1992;Rieger et al 1994;Mair et al 1998Mair et al , 2000Hooge and Tyler 1999;Tyler and Hooge 2004). In addition, the distribution of F-actin was investigated in several life cycle stages of the cestodes Diphyllobothrium dendriticum and Hymenolepis diminuta (Wahlberg 1998;Hartenstein and Jones 2003) as well as the direct developing polyclad M. ferruginea, the indirect developing polyclads M. crozieri and H. inquilina, and the macrostomid M. h. marinum (Reiter et al 1996;Bolan˜os and Litvaitis 2009). These studies point out several differences in the ontogenetic patterns of muscle development, but the potential plasticity of these patterns including the overall larval myoanatomy of indirect developing polyclads remains largely unknown.…”

Platyhelminthes

“…This situation is thus more similar to other studied spiralians and can be considered to be the ancestral state to all Platyhelminthes. Studies are, however, scarce and have mostly focused on the formation of the larval neuromuscular system (Younossi- Hartenstein and Hartenstein 2000 ;Bolaños and Litvaitis 2009 ;Rawlinson 2010Rawlinson , 2014Semmler and Wanninger 2010 ). In indirect developing polyclads, an initial apical helicoid muscle develops into an orthogonal grid of longitudinal and circular muscles, which corresponds to the adult body wall musculature.…”