Muscular remodeling and anteroposterior patterning during tapeworm segmentation.

Abstract

Tapeworms are parasitic flatworms that independently evolved a segmented body plan, historically confounding comparisons with other animals. Anteroposterior (AP) patterning in free-living flatworms and in tapeworm larvae is associated with canonical Wnt signaling and positional control genes (PCGs) are expressed by their musculature in regionalized domains along the AP axis. Here, we extend investigations of PCG expression to the adult of the mouse bile-duct tapeworm Hymenolepis microstoma, focusing on the growth zone of the neck region and the initial establishment of segmental patterning. We show that the adult musculature includes new, segmental elements that first appear in the neck and that the spatial patterns of Wnt factors are consistent with expression by muscle cells. Wnt factor expression is highly regionalized and becomes AP-polarized in segments, marking them with axes in agreement with the polarity of the main body axis, while the transition between the neck and strobila is specifically demarcated by the expression domain of a Wnt11 paralog. We suggest that segmentation could originate in the muscular system and participate in patterning the AP axis through regional and polarized expression of PCGs, akin to the gene regulatory networks employed by free-living flatworms and other animals.