Muscular Development in Urechis unicinctus (Echiura, Annelida)

Abstract

Echiura is one of the most intriguing major subgroups of phylum Annelida because, unlike most other annelids, echiuran adults lack metameric body segmentation. Urechis unicinctus lives in U-shape burrows of soft sediments. Little is known about the molecular mechanisms underlying the development of U. unicinctus. Herein, we overviewed the developmental process from zygote to juvenile U. unicinctus using immunohistochemistry and F-actin staining for the nervous and muscular systems, respectively. Through F-actin staining, we found that muscle fibers began to form in the trochophore phase and that muscles for feeding were produced first. Subsequently, in the segmentation larval stage, the transversal muscle was formed in the shape of a ring in an anterior-to-posterior direction with segment formation, as well as a ventromedian muscle for the formation of a ventral nerve cord. After that, many muscle fibers were produced along the entire body and formed the worm-shaped larva. Finally, we investigated the spatiotemporal expression of Uun_st-mhc, Uun_troponin I, Uun_calponin, and Uun_twist genes found in U. unicinctus. During embryonic development, the striated and smooth muscle genes were co-expressed in the same region. However, the adult body wall muscles showed differential gene expression of each muscle layer. The results of this study will provide the basis for the understanding of muscle differentiation in Echiura.

Recent Light on the Developmental Perspectives of Regeneration in Annelids

“…The mature body wall muscles, however, had distinct gene expression for every muscle layer. The knowledge of muscle differentiation in Echiura will be based on the findings of this study [Han et al, 2020].…”

Radial Expansion Favors the Burrowing Behavior of Urechis unicinctus

“…Urechis unicinctus (Echiura, Annelida), commonly known as sea intestine, can advance in sands by a unique peristaltic motion, i.e., huge radial expansion and shrink. U. unicinctus lives in U-shape burrows in the subtidal zone and has high nutritional and medicinal values, rapidly promoting its aquaculture [ 15 , 16 ]. Existing studies mainly focus on its nutritional components, medical value, and environmental tolerance [ 17 , 18 ].…”

Behavioral variation according to feeding organ diversification in glossiphoniid leeches (Phylum: Annelida)

“…The proboscis is comprised of longitudinal, radial and circular muscles, which it uses to extend forward and backward to penetrate the body wall of the host and ingest body fluids 13 . In the Lophotrochozoans, the striated muscle specific gene st-mhc and troponin complex are known to be involved in the development of foregut muscles as well as somatic muscles 24 , 25 . These studies hint at the existence of foregut specific factors, which further implies the potential for visualization of these factors within the foregut muscle structure.…”