Myogenesis in the basal bilaterian Symsagittifera roscoffensis (Acoela)

Henrike Semmler,Andreas Wanninger,Xavier Bailly

doi:10.1186/1742-9994-5-14

Henrike Semmler, Andreas Wanninger + Show 1 more

Open Access

https://doi.org/10.1186/1742-9994-5-14

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Abstract

BackgroundIn order to increase the weak database concerning the organogenesis of Acoela – a clade regarded by many as the earliest extant offshoot of Bilateria and thus of particular interest for studies concerning the evolution of animal bodyplans – we analyzed the development of the musculature of Symsagittifera roscoffensis using F-actin labelling, confocal laserscanning microscopy, and 3D reconstruction software.ResultsAt 40% of development between egg deposition and hatching short subepidermal fibres form. Muscle fibre development in the anterior body half precedes myogenesis in the posterior half. At 42% of development a grid of outer circular and inner longitudinal muscles is present in the bodywall. New circular muscles either branch off from present fibres or form adjacent to existing ones. The number of circular muscles is higher than that of the longitudinal muscles throughout all life cycle stages. Diagonal, circular and longitudinal muscles are initially rare but their number increases with time. The ventral side bears U-shaped muscles around the mouth, which in addition is surrounded by a sphincter muscle. With the exception of the region of the statocyst, dorsoventral muscles are present along the entire body of juveniles and adults, while adults additionally exhibit radially oriented internal muscles in the anterior tip. Outer diagonal muscles are present at the dorsal anterior tip of the adult. In adult animals, the male gonopore with its associated sexual organs expresses distinct muscles. No specific statocyst muscles were found. The muscle mantles of the needle-shaped sagittocysts are situated along the lateral edges of the animal and in the posterior end close to the male gonopore. In both juveniles and adults, non-muscular filaments, which stain positively for F-actin, are associated with certain sensory cells outside the bodywall musculature.ConclusionCompared to the myoanatomy of other acoel taxa, Symsagittifera roscoffensis shows a very complex musculature. Although data on presumably basal acoel clades are still scarce, the information currently available suggests an elaborated musculature with longitudinal, circular and U-shaped muscles as being part of the ancestral acoel bodyplan, thus increasing the possibility that Urbilateria likewise had a relatively complicated muscular ground pattern.

Highlights

In order to increase the weak database concerning the organogenesis of Acoela – a clade regarded by many as the earliest extant offshoot of Bilateria and of particular interest for studies concerning the evolution of animal bodyplans – we analyzed the development of the musculature of Symsagittifera roscoffensis using F-actin labelling, confocal laserscanning microscopy, and 3D reconstruction software
The specimens were fixed in 4% paraformaldehyde in 0.1 M phosphate buffer (PB) for 0.5–2 hours at room temperature
S. roscoffensis is a hermaphroditic species with internal fertilization

Summary

Introduction

In order to increase the weak database concerning the organogenesis of Acoela – a clade regarded by many as the earliest extant offshoot of Bilateria and of particular interest for studies concerning the evolution of animal bodyplans – we analyzed the development of the musculature of Symsagittifera roscoffensis using F-actin labelling, confocal laserscanning microscopy, and 3D reconstruction software. Supposedly the earliest recent bilaterian offshoot, show only few morphological characters to infer their evolution and their phylogenetic relationships [1]. They are acoelomate, gutless ciliated worms mainly from marine habitats and have traditionally been assigned to the taxon Turbellaria within Platyhelminthes [2]. Within the Acoela, the interrelationship of taxa still remains unclear due to the lack of morphological characters [9]. Morphological characters used to infer acoel interrelationships focus mainly on the male genital organs, but female genital organs, the structure of the epithelium, the nervous system, the pharynx and the bodywall musculature are included in these analyses [11]

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