Postembryonic Polarity Modification in the Acoel Convolutriloba longifissura

Metazoans establish the bilateral body plan early in embryogenesis by patterning orthogonal body axes with polarity that is unaltered during the lifetime of most animals. While some organisms re-establish/modify body axes during regeneration and asexual reproduction, the acoel flatworm Convolutriloba longifissura is unusual in its ability to modify left-right (L-R) axis polarity during longitudinal fission. We have developed C. longifissura as a model for studying the mechanisms of L-R polarity modification during postembryonic development. Regeneration experiments have elucidated the temporal dynamics of midline re-specification, suggesting that parallel L-R axes replace the pre-existing midline prior to longitudinal fission. We have characterized the spatiotemporal expression of genes encoding ligands and receptors of signaling pathways with conserved functions in polarity specification and axial patterning. Expression domains of BMP, Notch, and Slit/Robo signaling components are dynamic prior to and during longitudinal fission. RNAi-mediated gene knockdown of Notch and Slit/Robo signals disrupt longitudinal fission while BMP disrupted normal midline patterning suggesting a role in modulating changes in L-R axis polarity

Mechanisms of axial polarity modification during postembryonic development of the basal bilaterian Convolutriloba macropyga

Acoel flatworms have varied modes of asexual reproduction that involve dramatic postembryonic modification of their anterior-posterior (AP) axis. The acoel species Convolutriloba macropyga reproduces through a reversed polarity budding process in which offspring develop from two posterior budding sites with a complete reversal of the AP axis compared to the parent. Reversed polarity budding is preceded by the development of a zone of tissue with disorganized musculature that is incapable of regeneration, suggesting a transient loss of axis polarity at each budding site. For this reason, these tissues are titled the polarity transition zone (PTZ). While this alteration of existing axial polarity seems to be required for subsequent reversal of the AP axis in the budding progeny, the mechanisms that allow for temporary axis modification and reversal are not known. Wnt, Hedgehog, and other signal transduction pathways have conserved roles in AP axis development and reestablishment during both metazoan embryogenesis and regeneration, suggesting these signals may function in mediating changes in axis polarity during budding in C. macropyga. Here, we have used a pharmacological screen to perturb conserved signaling pathways in Convolutriloba tissues and observed loss of axial polarity in tissues exposed to inhibitors of the Hedgehog pathway. Given the putative role of Hedgehog signal transduction in mediating alterations in axial polarity during budding, we have quantified changes in the expression of Hedgehog signaling components and regulators in budding tissues using qPCR. Hedgehog signaling was downregulated within the PTZ when compared to neighboring polarized tissues. RNAi mediated knockdown of Hedgehog gene products resulted in phenotypes of delayed bud detachment and failure to initiate future budding events. These data support Hedgehog as a key signaling pathway involved in the modification of AP axis polarity during asexual reproduction and may provide key insights towards better understanding the evolution of asexual reproduction strategies in other taxa

Acoel and Platyhelminth Models for Stem-cell Research

Acoel and platyhelminth worms are particularly attractive invertebrate models for stem-cell research because their bodies are continually renewed from large pools of somatic stem cells. Several recent studies, including one in BMC Developmental Biology, are beginning to reveal the cellular dynamics and molecular basis of stem-cell function in these animals. See research article http://www.biomedcentral.com/1471-213X/9/69. Adult somatic stem cells can play critical roles in postembryonic developmental processes such as tissue renewal, growth, repair, and regeneration [1]. Understanding how such cells are maintained and produce differentiated progeny is thus of general interest in developmental biology, in addition to being of clear biomedical relevance. Invertebrate models have great potential for elucidating the cellular and molecular basis of stem-cell function. However, in the main invertebrate models used for dissecting the details of animal development, including Drosophila and Caenorhabditis, adult somatic tissues are primarily post-mitotic and are largely or entirely devoid of adult stem cells, which limits the use of these established models for stem-cell research. Representatives of two groups of soft-bodied worms, the Acoela and the Platyhelminthes, possess large pools of adult somatic stem cells, making them useful invertebrate models for stem-cell biology. These organisms are now beginning to provide new insights into the cellular and molecular basis of adult stem-cell function

Regeneration in Spiralians: Evolutionary Patterns and Developmental Processes

Animals differ markedly in their ability to regenerate, yet still little is known about how regeneration evolves. In recent years, important advances have been made in our understanding of animal phylogeny and these provide new insights into the phylogenetic distribution of regeneration. The developmental basis of regeneration is also being investigated in an increasing number of groups, allowing commonalities and differences across groups to become evident. Here, we focus on regeneration in the Spiralia, a group that includes several champions of animal regeneration, as well as many groups with more limited abilities. We review the phylogenetic distribution and developmental processes of regeneration in four major spiralian groups: annelids, nemerteans, platyhelminths, and molluscs. Although comparative data are still limited, this review highlights phylogenetic and developmental patterns that are emerging regarding regeneration in spiralians and identifies important avenues for future research